subsys: build+install (header-only library)

CMakeLists.txt

@@ -0,0 +1,93 @@
+cmake_minimum_required(VERSION 3.10)
+
+project(subsys VERSION 0.1)
+enable_language(CXX)
+
+include(cmake/cxx.cmake)
+include(cmake/code-coverage.cmake)
+
+enable_testing()
+# activate code coverage for all executables + libraries (when -DCODE_COVERAGE=ON)
+add_code_coverage()
+
+# 1. assuming that /nix/store/ prefixes .hpp files belonging to gcc, catch2 etc.
+#    we're not interested in code coverage for these sources.
+# 2. exclude the utest/ subdir, we don't need coverage on the unit tests themselves;
+#    rather,  want coverage on the code that the unit tests exercise.
+#
+add_code_coverage_all_targets(EXCLUDE /nix/store/* utest/*)
+
+set(XO_PROJECT_NAME subsys)
+
+if(NOT CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_STANDARD 20)
+endif()
+
+set(CMAKE_CXX_STANDARD_REQUIRED True)
+
+# always write compile_commands.json
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON CACHE INTERNAL "")
+
+# ----------------------------------------------------------------
+# - author's convenience:  default install prefix to /home/$USER/local
+# - otherwise use -DCMAKE_INSTALL_PREFIX=/path/to/somewhere
+
+if(NOT USER)
+    set(USER $ENV{USER})
+endif()
+
+# hmm. this works if explicitly given with cmake:
+#    cmake -DCMAKE_INSTALL_PREFIX=/home/roland/local path/to/source
+# but not as default
+if(NOT CMAKE_INSTALL_PREFIX)
+    set(CMAKE_INSTALL_PREFIX /home/${USER}/local CACHE STRING "install directory")
+endif()
+if(NOT CMAKE_INSTALL_RPATH)
+    set(CMAKE_INSTALL_RPATH ${CMAKE_INSTALL_PREFIX}/lib CACHE STRING "runpath in installed libraries/executables")
+endif()
+
+#add_subdirectory(example)
+#add_subdirectory(utest)
+
+# ----------------------------------------------------------------
+# installing header-only library
+
+add_library(subsys INTERFACE)
+target_include_directories(subsys INTERFACE
+  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
+  $<INSTALL_INTERFACE:include>
+)
+xo_install_library(subsys)
+
+# ----------------------------------------------------------------
+# cmake export
+# (so this library works with cmake's find_package())
+
+set(XO_PROJECT_CONFIG_VERSION "${XO_PROJECT_NAME}ConfigVersion.cmake")
+set(XO_PROJECT_CONFIG "${XO_PROJECT_NAME}Config.cmake")
+
+include(CMakePackageConfigHelpers)
+write_basic_package_version_file("${PROJECT_BINARY_DIR}/${XO_PROJECT_CONFIG_VERSION}"
+    VERSION 0.1
+    COMPATIBILITY AnyNewerVersion
+)
+
+configure_package_config_file(
+    "${PROJECT_SOURCE_DIR}/cmake/${XO_PROJECT_NAME}Config.cmake.in"
+    "${PROJECT_BINARY_DIR}/${XO_PROJECT_CONFIG}"
+    INSTALL_DESTINATION lib/cmake/${XO_PROJECT_NAME}
+    )
+
+install(EXPORT ${XO_PROJECT_NAME}Targets DESTINATION lib/cmake/${XO_PROJECT_NAME})
+install(
+    FILES
+    "${PROJECT_BINARY_DIR}/${XO_PROJECT_CONFIG_VERSION}"
+    "${PROJECT_BINARY_DIR}/${XO_PROJECT_CONFIG}"
+    DESTINATION lib/cmake/${XO_PROJECT_NAME})
+
+# ----------------------------------------------------------------
+# install .hpp
+
+install(DIRECTORY ${PROJECT_SOURCE_DIR}/include/ DESTINATION include)
+
+# end CMakeLists.txt

cmake/code-coverage.cmake

@@ -0,0 +1,678 @@
+#
+# Copyright (C) 2018-2020 by George Cave - gcave@stablecoder.ca
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+# USAGE: To enable any code coverage instrumentation/targets, the single CMake
+# option of `CODE_COVERAGE` needs to be set to 'ON', either by GUI, ccmake, or
+# on the command line.
+#
+# From this point, there are two primary methods for adding instrumentation to
+# targets:
+#
+# 1 - A blanket instrumentation by calling `add_code_coverage()`, where
+# all targets in that directory and all subdirectories are automatically
+# instrumented.
+#
+# 2 - Per-target instrumentation by calling
+# `target_code_coverage(<TARGET_NAME>)`, where the target is given and thus only
+# that target is instrumented. This applies to both libraries and executables.
+#
+# To add coverage targets, such as calling `make ccov` to generate the actual
+# coverage information for perusal or consumption, call
+# `target_code_coverage(<TARGET_NAME>)` on an *executable* target.
+#
+# Example 1: All targets instrumented
+#
+# In this case, the coverage information reported will will be that of the
+# `theLib` library target and `theExe` executable.
+#
+# 1a: Via global command
+#
+# ~~~
+# add_code_coverage() # Adds instrumentation to all targets
+#
+# add_library(theLib lib.cpp)
+#
+# add_executable(theExe main.cpp)
+# target_link_libraries(theExe PRIVATE theLib)
+# target_code_coverage(theExe) # As an executable target, adds the 'ccov-theExe' target
+#                              # (instrumentation already added via global anyways)
+#                              # for generating code coverage reports.
+# ~~~
+#
+# 1b: Via target commands
+#
+# ~~~
+# add_library(theLib lib.cpp)
+# target_code_coverage(theLib) # As a library target, adds coverage instrumentation but no targets.
+#
+# add_executable(theExe main.cpp)
+# target_link_libraries(theExe PRIVATE theLib)
+# target_code_coverage(theExe) # As an executable target, adds the 'ccov-theExe' target and instrumentation for generating code coverage reports.
+# ~~~
+#
+# Example 2: Target instrumented, but with regex pattern of files to be excluded
+# from report
+#
+# ~~~
+# add_executable(theExe main.cpp non_covered.cpp)
+# target_code_coverage(theExe EXCLUDE non_covered.cpp test/*) # As an executable target, the reports will exclude the non-covered.cpp file, and any files in a test/ folder.
+# ~~~
+#
+# Example 3: Target added to the 'ccov' and 'ccov-all' targets
+#
+# ~~~
+# add_code_coverage_all_targets(EXCLUDE test/*) # Adds the 'ccov-all' target set and sets it to exclude all files in test/ folders.
+#
+# add_executable(theExe main.cpp non_covered.cpp)
+# target_code_coverage(theExe AUTO ALL EXCLUDE non_covered.cpp test/*) # As an executable target, adds to the 'ccov' and ccov-all' targets, and the reports will exclude the non-covered.cpp file, and any files in a test/ folder.
+# ~~~
+
+# Options
+option(
+  CODE_COVERAGE
+  "Builds targets with code coverage instrumentation. (Requires GCC or Clang)"
+  OFF)
+
+# Programs
+find_program(LLVM_COV_PATH llvm-cov)
+find_program(LLVM_PROFDATA_PATH llvm-profdata)
+find_program(LCOV_PATH lcov)
+find_program(GENHTML_PATH genhtml)
+# Hide behind the 'advanced' mode flag for GUI/ccmake
+mark_as_advanced(FORCE LLVM_COV_PATH LLVM_PROFDATA_PATH LCOV_PATH GENHTML_PATH)
+
+# Variables
+set(CMAKE_COVERAGE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/ccov)
+set_property(GLOBAL PROPERTY JOB_POOLS ccov_serial_pool=1)
+
+# Common initialization/checks
+if(CODE_COVERAGE AND NOT CODE_COVERAGE_ADDED)
+  set(CODE_COVERAGE_ADDED ON)
+
+  # Common Targets
+  add_custom_target(
+    ccov-preprocessing
+    COMMAND ${CMAKE_COMMAND} -E make_directory
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}
+    DEPENDS ccov-clean)
+
+  if(CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang"
+     OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
+    # Messages
+    message(STATUS "Building with llvm Code Coverage Tools")
+
+    if(NOT LLVM_COV_PATH)
+      message(FATAL_ERROR "llvm-cov not found! Aborting.")
+    else()
+      # Version number checking for 'EXCLUDE' compatibility
+      execute_process(COMMAND ${LLVM_COV_PATH} --version
+                      OUTPUT_VARIABLE LLVM_COV_VERSION_CALL_OUTPUT)
+      string(REGEX MATCH "[0-9]+\\.[0-9]+\\.[0-9]+" LLVM_COV_VERSION
+                   ${LLVM_COV_VERSION_CALL_OUTPUT})
+
+      if(LLVM_COV_VERSION VERSION_LESS "7.0.0")
+        message(
+          WARNING
+            "target_code_coverage()/add_code_coverage_all_targets() 'EXCLUDE' option only available on llvm-cov >= 7.0.0"
+        )
+      endif()
+    endif()
+
+    # Targets
+    if(${CMAKE_VERSION} VERSION_LESS "3.17.0")
+      add_custom_target(
+        ccov-clean
+        COMMAND ${CMAKE_COMMAND} -E remove -f
+                ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list
+        COMMAND ${CMAKE_COMMAND} -E remove -f
+                ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/profraw.list)
+    else()
+      add_custom_target(
+        ccov-clean
+        COMMAND ${CMAKE_COMMAND} -E rm -f
+                ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list
+        COMMAND ${CMAKE_COMMAND} -E rm -f
+                ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/profraw.list)
+    endif()
+
+    # Used to get the shared object file list before doing the main all-
+    # processing
+    add_custom_target(
+      ccov-libs
+      COMMAND ;
+      COMMENT "libs ready for coverage report.")
+
+  elseif(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES
+                                              "GNU")
+    # Messages
+    message(STATUS "Building with lcov Code Coverage Tools")
+
+    if(CMAKE_BUILD_TYPE)
+      string(TOUPPER ${CMAKE_BUILD_TYPE} upper_build_type)
+      if(NOT ${upper_build_type} STREQUAL "DEBUG")
+        message(
+          WARNING
+            "Code coverage results with an optimized (non-Debug) build may be misleading"
+        )
+      endif()
+    else()
+      message(
+        WARNING
+          "Code coverage results with an optimized (non-Debug) build may be misleading"
+      )
+    endif()
+    if(NOT LCOV_PATH)
+      message(FATAL_ERROR "lcov not found! Aborting...")
+    endif()
+    if(NOT GENHTML_PATH)
+      message(FATAL_ERROR "genhtml not found! Aborting...")
+    endif()
+
+    # Targets
+    add_custom_target(ccov-clean COMMAND ${LCOV_PATH} --directory
+                                         ${CMAKE_BINARY_DIR} --zerocounters)
+
+  else()
+    message(FATAL_ERROR "Code coverage requires Clang or GCC. Aborting.")
+  endif()
+endif()
+
+# Adds code coverage instrumentation to a library, or instrumentation/targets
+# for an executable target.
+# ~~~
+# EXECUTABLE ADDED TARGETS:
+# GCOV/LCOV:
+# ccov : Generates HTML code coverage report for every target added with 'AUTO' parameter.
+# ccov-${TARGET_NAME} : Generates HTML code coverage report for the associated named target.
+# ccov-all : Generates HTML code coverage report, merging every target added with 'ALL' parameter into a single detailed report.
+#
+# LLVM-COV:
+# ccov : Generates HTML code coverage report for every target added with 'AUTO' parameter.
+# ccov-report : Generates HTML code coverage report for every target added with 'AUTO' parameter.
+# ccov-${TARGET_NAME} : Generates HTML code coverage report.
+# ccov-report-${TARGET_NAME} : Prints to command line summary per-file coverage information.
+# ccov-export-${TARGET_NAME} : Exports the coverage report to a JSON file.
+# ccov-show-${TARGET_NAME} : Prints to command line detailed per-line coverage information.
+# ccov-all : Generates HTML code coverage report, merging every target added with 'ALL' parameter into a single detailed report.
+# ccov-all-report : Prints summary per-file coverage information for every target added with ALL' parameter to the command line.
+# ccov-all-export : Exports the coverage report to a JSON file.
+#
+# Required:
+# TARGET_NAME - Name of the target to generate code coverage for.
+# Optional:
+# PUBLIC - Sets the visibility for added compile options to targets to PUBLIC instead of the default of PRIVATE.
+# INTERFACE - Sets the visibility for added compile options to targets to INTERFACE instead of the default of PRIVATE.
+# PLAIN - Do not set any target visibility (backward compatibility with old cmake projects)
+# AUTO - Adds the target to the 'ccov' target so that it can be run in a batch with others easily. Effective on executable targets.
+# ALL - Adds the target to the 'ccov-all' and 'ccov-all-report' targets, which merge several executable targets coverage data to a single report. Effective on executable targets.
+# EXTERNAL - For GCC's lcov, allows the profiling of 'external' files from the processing directory
+# COVERAGE_TARGET_NAME - For executables ONLY, changes the outgoing target name so instead of `ccov-${TARGET_NAME}` it becomes `ccov-${COVERAGE_TARGET_NAME}`.
+# EXCLUDE <PATTERNS> - Excludes files of the patterns provided from coverage. Note that GCC/lcov excludes by glob pattern, and clang/LLVM excludes via regex! **These do not copy to the 'all' targets.**
+# OBJECTS <TARGETS> - For executables ONLY, if the provided targets are shared libraries, adds coverage information to the output
+# ARGS <ARGUMENTS> - For executables ONLY, appends the given arguments to the associated ccov-* executable call
+# ~~~
+function(target_code_coverage TARGET_NAME)
+  # Argument parsing
+  set(options AUTO ALL EXTERNAL PUBLIC INTERFACE PLAIN)
+  set(single_value_keywords COVERAGE_TARGET_NAME)
+  set(multi_value_keywords EXCLUDE OBJECTS ARGS)
+  cmake_parse_arguments(
+    target_code_coverage "${options}" "${single_value_keywords}"
+    "${multi_value_keywords}" ${ARGN})
+
+  # Set the visibility of target functions to PUBLIC, INTERFACE or default to
+  # PRIVATE.
+  if(target_code_coverage_PUBLIC)
+    set(TARGET_VISIBILITY PUBLIC)
+    set(TARGET_LINK_VISIBILITY PUBLIC)
+  elseif(target_code_coverage_INTERFACE)
+    set(TARGET_VISIBILITY INTERFACE)
+    set(TARGET_LINK_VISIBILITY INTERFACE)
+  elseif(target_code_coverage_PLAIN)
+    set(TARGET_VISIBILITY PUBLIC)
+    set(TARGET_LINK_VISIBILITY)
+  else()
+    set(TARGET_VISIBILITY PRIVATE)
+    set(TARGET_LINK_VISIBILITY PRIVATE)
+  endif()
+
+  if(NOT target_code_coverage_COVERAGE_TARGET_NAME)
+    # If a specific name was given, use that instead.
+    set(target_code_coverage_COVERAGE_TARGET_NAME ${TARGET_NAME})
+  endif()
+
+  if(CODE_COVERAGE)
+
+    # Add code coverage instrumentation to the target's linker command
+    if(CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang"
+       OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
+      target_compile_options(${TARGET_NAME} ${TARGET_VISIBILITY}
+                             -fprofile-instr-generate -fcoverage-mapping)
+      target_link_options(${TARGET_NAME} ${TARGET_VISIBILITY}
+                          -fprofile-instr-generate -fcoverage-mapping)
+    elseif(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES
+                                                "GNU")
+      target_compile_options(${TARGET_NAME} ${TARGET_VISIBILITY} -fprofile-arcs
+                             -ftest-coverage)
+      target_link_libraries(${TARGET_NAME} ${TARGET_LINK_VISIBILITY} gcov)
+    endif()
+
+    # Targets
+    get_target_property(target_type ${TARGET_NAME} TYPE)
+
+    # Add shared library to processing for 'all' targets
+    if(target_type STREQUAL "SHARED_LIBRARY" AND target_code_coverage_ALL)
+      if(CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang"
+         OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
+        add_custom_target(
+          ccov-run-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${CMAKE_COMMAND} -E echo "-object=$<TARGET_FILE:${TARGET_NAME}>" >>
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list
+          DEPENDS ccov-preprocessing ${TARGET_NAME})
+
+        if(NOT TARGET ccov-libs)
+          message(
+            FATAL_ERROR
+              "Calling target_code_coverage with 'ALL' must be after a call to 'add_code_coverage_all_targets'."
+          )
+        endif()
+
+        add_dependencies(ccov-libs
+                         ccov-run-${target_code_coverage_COVERAGE_TARGET_NAME})
+      endif()
+    endif()
+
+    # For executables add targets to run and produce output
+    if(target_type STREQUAL "EXECUTABLE")
+      if(CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang"
+         OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
+
+        # If there are shared objects to also work with, generate the string to
+        # add them here
+        foreach(SO_TARGET ${target_code_coverage_OBJECTS})
+          # Check to see if the target is a shared object
+          if(TARGET ${SO_TARGET})
+            get_target_property(SO_TARGET_TYPE ${SO_TARGET} TYPE)
+            if(${SO_TARGET_TYPE} STREQUAL "SHARED_LIBRARY")
+              set(SO_OBJECTS ${SO_OBJECTS} -object=$<TARGET_FILE:${SO_TARGET}>)
+            endif()
+          endif()
+        endforeach()
+
+        # Run the executable, generating raw profile data Make the run data
+        # available for further processing. Separated to allow Windows to run
+        # this target serially.
+        add_custom_target(
+          ccov-run-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${CMAKE_COMMAND} -E env
+            LLVM_PROFILE_FILE=${target_code_coverage_COVERAGE_TARGET_NAME}.profraw
+            $<TARGET_FILE:${TARGET_NAME}> ${target_code_coverage_ARGS}
+          COMMAND
+            ${CMAKE_COMMAND} -E echo "-object=$<TARGET_FILE:${TARGET_NAME}>"
+            ${SO_OBJECTS} >> ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list
+          COMMAND
+            ${CMAKE_COMMAND} -E echo
+            "${CMAKE_CURRENT_BINARY_DIR}/${target_code_coverage_COVERAGE_TARGET_NAME}.profraw"
+            >> ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/profraw.list
+          JOB_POOL ccov_serial_pool
+          DEPENDS ccov-preprocessing ccov-libs ${TARGET_NAME})
+
+        # Merge the generated profile data so llvm-cov can process it
+        add_custom_target(
+          ccov-processing-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${LLVM_PROFDATA_PATH} merge -sparse
+            ${target_code_coverage_COVERAGE_TARGET_NAME}.profraw -o
+            ${target_code_coverage_COVERAGE_TARGET_NAME}.profdata
+          DEPENDS ccov-run-${target_code_coverage_COVERAGE_TARGET_NAME})
+
+        # Ignore regex only works on LLVM >= 7
+        if(LLVM_COV_VERSION VERSION_GREATER_EQUAL "7.0.0")
+          foreach(EXCLUDE_ITEM ${target_code_coverage_EXCLUDE})
+            set(EXCLUDE_REGEX ${EXCLUDE_REGEX}
+                              -ignore-filename-regex='${EXCLUDE_ITEM}')
+          endforeach()
+        endif()
+
+        # Print out details of the coverage information to the command line
+        add_custom_target(
+          ccov-show-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${LLVM_COV_PATH} show $<TARGET_FILE:${TARGET_NAME}> ${SO_OBJECTS}
+            -instr-profile=${target_code_coverage_COVERAGE_TARGET_NAME}.profdata
+            -show-line-counts-or-regions ${EXCLUDE_REGEX}
+          DEPENDS ccov-processing-${target_code_coverage_COVERAGE_TARGET_NAME})
+
+        # Print out a summary of the coverage information to the command line
+        add_custom_target(
+          ccov-report-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${LLVM_COV_PATH} report $<TARGET_FILE:${TARGET_NAME}> ${SO_OBJECTS}
+            -instr-profile=${target_code_coverage_COVERAGE_TARGET_NAME}.profdata
+            ${EXCLUDE_REGEX}
+          DEPENDS ccov-processing-${target_code_coverage_COVERAGE_TARGET_NAME})
+
+        # Export coverage information so continuous integration tools (e.g.
+        # Jenkins) can consume it
+        add_custom_target(
+          ccov-export-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${LLVM_COV_PATH} export $<TARGET_FILE:${TARGET_NAME}> ${SO_OBJECTS}
+            -instr-profile=${target_code_coverage_COVERAGE_TARGET_NAME}.profdata
+            -format="text" ${EXCLUDE_REGEX} >
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/${target_code_coverage_COVERAGE_TARGET_NAME}.json
+          DEPENDS ccov-processing-${target_code_coverage_COVERAGE_TARGET_NAME})
+
+        # Generates HTML output of the coverage information for perusal
+        add_custom_target(
+          ccov-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${LLVM_COV_PATH} show $<TARGET_FILE:${TARGET_NAME}> ${SO_OBJECTS}
+            -instr-profile=${target_code_coverage_COVERAGE_TARGET_NAME}.profdata
+            -show-line-counts-or-regions
+            -output-dir=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/${target_code_coverage_COVERAGE_TARGET_NAME}
+            -format="html" ${EXCLUDE_REGEX}
+          DEPENDS ccov-processing-${target_code_coverage_COVERAGE_TARGET_NAME})
+
+      elseif(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES
+                                                  "GNU")
+        set(COVERAGE_INFO
+            "${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/${target_code_coverage_COVERAGE_TARGET_NAME}.info"
+        )
+
+        # Run the executable, generating coverage information
+        add_custom_target(
+          ccov-run-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND $<TARGET_FILE:${TARGET_NAME}> ${target_code_coverage_ARGS}
+          DEPENDS ccov-preprocessing ${TARGET_NAME})
+
+        # Generate exclusion string for use
+        foreach(EXCLUDE_ITEM ${target_code_coverage_EXCLUDE})
+          set(EXCLUDE_REGEX ${EXCLUDE_REGEX} --remove ${COVERAGE_INFO}
+                            '${EXCLUDE_ITEM}')
+        endforeach()
+
+        if(EXCLUDE_REGEX)
+          set(EXCLUDE_COMMAND ${LCOV_PATH} ${EXCLUDE_REGEX} --output-file
+                              ${COVERAGE_INFO})
+        else()
+          set(EXCLUDE_COMMAND ;)
+        endif()
+
+        if(NOT ${target_code_coverage_EXTERNAL})
+          set(EXTERNAL_OPTION --no-external)
+        endif()
+
+        # Capture coverage data
+        if(${CMAKE_VERSION} VERSION_LESS "3.17.0")
+          add_custom_target(
+            ccov-capture-${target_code_coverage_COVERAGE_TARGET_NAME}
+            COMMAND ${CMAKE_COMMAND} -E remove -f ${COVERAGE_INFO}
+            COMMAND ${LCOV_PATH} --directory ${CMAKE_BINARY_DIR} --zerocounters
+            COMMAND $<TARGET_FILE:${TARGET_NAME}> ${target_code_coverage_ARGS}
+            COMMAND
+              ${LCOV_PATH} --directory ${CMAKE_BINARY_DIR} --base-directory
+              ${CMAKE_SOURCE_DIR} --capture ${EXTERNAL_OPTION} --output-file
+              ${COVERAGE_INFO}
+            COMMAND ${EXCLUDE_COMMAND}
+            DEPENDS ccov-preprocessing ${TARGET_NAME})
+        else()
+          add_custom_target(
+            ccov-capture-${target_code_coverage_COVERAGE_TARGET_NAME}
+            COMMAND ${CMAKE_COMMAND} -E rm -f ${COVERAGE_INFO}
+            COMMAND ${LCOV_PATH} --directory ${CMAKE_BINARY_DIR} --zerocounters
+            COMMAND $<TARGET_FILE:${TARGET_NAME}> ${target_code_coverage_ARGS}
+            COMMAND
+              ${LCOV_PATH} --directory ${CMAKE_BINARY_DIR} --base-directory
+              ${CMAKE_SOURCE_DIR} --capture ${EXTERNAL_OPTION} --output-file
+              ${COVERAGE_INFO}
+            COMMAND ${EXCLUDE_COMMAND}
+            DEPENDS ccov-preprocessing ${TARGET_NAME})
+        endif()
+
+        # Generates HTML output of the coverage information for perusal
+        add_custom_target(
+          ccov-${target_code_coverage_COVERAGE_TARGET_NAME}
+          COMMAND
+            ${GENHTML_PATH} -o
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/${target_code_coverage_COVERAGE_TARGET_NAME}
+            ${COVERAGE_INFO}
+          DEPENDS ccov-capture-${target_code_coverage_COVERAGE_TARGET_NAME})
+      endif()
+
+      add_custom_command(
+        TARGET ccov-${target_code_coverage_COVERAGE_TARGET_NAME}
+        POST_BUILD
+        COMMAND ;
+        COMMENT
+          "Open ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/${target_code_coverage_COVERAGE_TARGET_NAME}/index.html in your browser to view the coverage report."
+      )
+
+      # AUTO
+      if(target_code_coverage_AUTO)
+        if(NOT TARGET ccov)
+          add_custom_target(ccov)
+        endif()
+        add_dependencies(ccov ccov-${target_code_coverage_COVERAGE_TARGET_NAME})
+
+        if(NOT CMAKE_C_COMPILER_ID MATCHES "GNU" AND NOT CMAKE_CXX_COMPILER_ID
+                                                     MATCHES "GNU")
+          if(NOT TARGET ccov-report)
+            add_custom_target(ccov-report)
+          endif()
+          add_dependencies(
+            ccov-report
+            ccov-report-${target_code_coverage_COVERAGE_TARGET_NAME})
+        endif()
+      endif()
+
+      # ALL
+      if(target_code_coverage_ALL)
+        if(NOT TARGET ccov-all-processing)
+          message(
+            FATAL_ERROR
+              "Calling target_code_coverage with 'ALL' must be after a call to 'add_code_coverage_all_targets'."
+          )
+        endif()
+
+        add_dependencies(ccov-all-processing
+                         ccov-run-${target_code_coverage_COVERAGE_TARGET_NAME})
+      endif()
+    endif()
+  endif()
+endfunction()
+
+# Adds code coverage instrumentation to all targets in the current directory and
+# any subdirectories. To add coverage instrumentation to only specific targets,
+# use `target_code_coverage`.
+function(add_code_coverage)
+  if(CODE_COVERAGE)
+    if(CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang"
+       OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
+      add_compile_options(-fprofile-instr-generate -fcoverage-mapping)
+      add_link_options(-fprofile-instr-generate -fcoverage-mapping)
+    elseif(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES
+                                                "GNU")
+      add_compile_options(-fprofile-arcs -ftest-coverage)
+      link_libraries(gcov)
+    endif()
+  endif()
+endfunction()
+
+# Adds the 'ccov-all' type targets that calls all targets added via
+# `target_code_coverage` with the `ALL` parameter, but merges all the coverage
+# data from them into a single large report  instead of the numerous smaller
+# reports. Also adds the ccov-all-capture Generates an all-merged.info file, for
+# use with coverage dashboards (e.g. codecov.io, coveralls).
+# ~~~
+# Optional:
+# EXCLUDE <PATTERNS> - Excludes files of the patterns provided from coverage. Note that GCC/lcov excludes by glob pattern, and clang/LLVM excludes via regex!
+# ~~~
+function(add_code_coverage_all_targets)
+  # Argument parsing
+  set(multi_value_keywords EXCLUDE)
+  cmake_parse_arguments(add_code_coverage_all_targets "" ""
+                        "${multi_value_keywords}" ${ARGN})
+
+  if(CODE_COVERAGE)
+    if(CMAKE_C_COMPILER_ID MATCHES "(Apple)?[Cc]lang"
+       OR CMAKE_CXX_COMPILER_ID MATCHES "(Apple)?[Cc]lang")
+
+      # Merge the profile data for all of the run executables
+      if(WIN32)
+        add_custom_target(
+          ccov-all-processing
+          COMMAND
+            powershell -Command $$FILELIST = Get-Content
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/profraw.list\; llvm-profdata.exe
+            merge -o ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata
+            -sparse $$FILELIST)
+      else()
+        add_custom_target(
+          ccov-all-processing
+          COMMAND
+            ${LLVM_PROFDATA_PATH} merge -o
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata -sparse `cat
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/profraw.list`)
+      endif()
+
+      # Regex exclude only available for LLVM >= 7
+      if(LLVM_COV_VERSION VERSION_GREATER_EQUAL "7.0.0")
+        foreach(EXCLUDE_ITEM ${add_code_coverage_all_targets_EXCLUDE})
+          set(EXCLUDE_REGEX ${EXCLUDE_REGEX}
+                            -ignore-filename-regex='${EXCLUDE_ITEM}')
+        endforeach()
+      endif()
+
+      # Print summary of the code coverage information to the command line
+      if(WIN32)
+        add_custom_target(
+          ccov-all-report
+          COMMAND
+            powershell -Command $$FILELIST = Get-Content
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list\; llvm-cov.exe
+            report $$FILELIST
+            -instr-profile=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata
+            ${EXCLUDE_REGEX}
+          DEPENDS ccov-all-processing)
+      else()
+        add_custom_target(
+          ccov-all-report
+          COMMAND
+            ${LLVM_COV_PATH} report `cat
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list`
+            -instr-profile=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata
+            ${EXCLUDE_REGEX}
+          DEPENDS ccov-all-processing)
+      endif()
+
+      # Export coverage information so continuous integration tools (e.g.
+      # Jenkins) can consume it
+      add_custom_target(
+        ccov-all-export
+        COMMAND
+          ${LLVM_COV_PATH} export `cat
+          ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list`
+          -instr-profile=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata
+          -format="text" ${EXCLUDE_REGEX} >
+          ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/coverage.json
+        DEPENDS ccov-all-processing)
+
+      # Generate HTML output of all added targets for perusal
+      if(WIN32)
+        add_custom_target(
+          ccov-all
+          COMMAND
+            powershell -Command $$FILELIST = Get-Content
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list\; llvm-cov.exe show
+            $$FILELIST
+            -instr-profile=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata
+            -show-line-counts-or-regions
+            -output-dir=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged
+            -format="html" ${EXCLUDE_REGEX}
+          DEPENDS ccov-all-processing)
+      else()
+        add_custom_target(
+          ccov-all
+          COMMAND
+            ${LLVM_COV_PATH} show `cat
+            ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/binaries.list`
+            -instr-profile=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.profdata
+            -show-line-counts-or-regions
+            -output-dir=${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged
+            -format="html" ${EXCLUDE_REGEX}
+          DEPENDS ccov-all-processing)
+      endif()
+
+    elseif(CMAKE_C_COMPILER_ID MATCHES "GNU" OR CMAKE_CXX_COMPILER_ID MATCHES
+                                                "GNU")
+      set(COVERAGE_INFO "${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged.info")
+
+      # Nothing required for gcov
+      add_custom_target(ccov-all-processing COMMAND ;)
+
+      # Exclusion regex string creation
+      set(EXCLUDE_REGEX)
+      foreach(EXCLUDE_ITEM ${add_code_coverage_all_targets_EXCLUDE})
+        set(EXCLUDE_REGEX ${EXCLUDE_REGEX} --remove ${COVERAGE_INFO}
+                          '${EXCLUDE_ITEM}')
+      endforeach()
+
+      if(EXCLUDE_REGEX)
+        set(EXCLUDE_COMMAND ${LCOV_PATH} ${EXCLUDE_REGEX} --output-file
+                            ${COVERAGE_INFO})
+      else()
+        set(EXCLUDE_COMMAND ;)
+      endif()
+
+      # Capture coverage data
+      if(${CMAKE_VERSION} VERSION_LESS "3.17.0")
+        add_custom_target(
+          ccov-all-capture
+          COMMAND ${CMAKE_COMMAND} -E remove -f ${COVERAGE_INFO}
+          COMMAND ${LCOV_PATH} --directory ${CMAKE_BINARY_DIR} --capture
+                  --output-file ${COVERAGE_INFO}
+          COMMAND ${EXCLUDE_COMMAND}
+          DEPENDS ccov-preprocessing ccov-all-processing)
+      else()
+        add_custom_target(
+          ccov-all-capture
+          COMMAND ${CMAKE_COMMAND} -E rm -f ${COVERAGE_INFO}
+          COMMAND ${LCOV_PATH} --directory ${CMAKE_BINARY_DIR} --capture
+                  --output-file ${COVERAGE_INFO}
+          COMMAND ${EXCLUDE_COMMAND}
+          DEPENDS ccov-preprocessing ccov-all-processing)
+      endif()
+
+      # Generates HTML output of all targets for perusal
+      add_custom_target(
+        ccov-all
+        COMMAND ${GENHTML_PATH} -o ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged
+                ${COVERAGE_INFO} -p ${CMAKE_SOURCE_DIR}
+        DEPENDS ccov-all-capture)
+
+    endif()
+
+    add_custom_command(
+      TARGET ccov-all
+      POST_BUILD
+      COMMAND ;
+      COMMENT
+        "Open ${CMAKE_COVERAGE_OUTPUT_DIRECTORY}/all-merged/index.html in your browser to view the coverage report."
+    )
+  endif()
+endfunction()

cmake/cxx.cmake

@@ -0,0 +1,96 @@
+# ----------------------------------------------------------------
+# use this in subdirs that compile c++ code
+#
+macro(xo_include_options target)
+    # ----------------------------------------------------------------
+    # PROJECT_SOURCE_DIR:
+    #   so we can for example write
+    #     #include "ordinaltree/foo.hpp"
+    #   from anywhere in the project
+    # PROJECT_BINARY_DIR:
+    #   since generated version file will be in build directory,
+    #   need that build directory to also appear in
+    #   compiler's include path
+    #
+    target_include_directories(
+      ${target} PUBLIC
+      $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>              # e.g. for #include "indentlog/scope.hpp"
+      $<INSTALL_INTERFACE:include>
+      $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include/${target}>    # e.g. for #include "Refcounted.hpp" in refcnt/src
+      $<INSTALL_INTERFACE:include/${target}>
+      $<BUILD_INTERFACE:${PROJECT_BINARY_DIR}>                      # e.g. for generated config.hpp file
+    )
+
+    # ----------------------------------------------------------------
+    # make standard directories for std:: includes explicit
+    # so that
+    # (1) they appear in compile_commands.json.
+    # (2) clangd (run from emacs lsp-mode) can find them
+    #
+    if(CMAKE_EXPORT_COMPILE_COMMANDS)
+      set(CMAKE_CXX_STANDARD_INCLUDE_DIRECTORIES ${CMAKE_CXX_IMPLICIT_INCLUDE_DIRECTORIES})
+    endif()
+endmacro()
+
+# ----------------------------------------------------------------
+# variable
+#   XO_ADDRESS_SANITIZE
+# determines whether to enable address sanitizer for the XO project
+# (see toplevel CMakeLists.txt)
+# ----------------------------------------------------------------
+if(XO_ADDRESS_SANITIZE)
+    add_compile_options(-fsanitize=address)
+    add_link_options(-fsanitize=address)
+endif()
+
+# XO_STANDARD_COMPILE_OPTIONS: use these when XO_ADDRESS_SANITIZE=OFF
+set(XO_STANDARD_COMPILE_OPTIONS -Werror -Wall -Wextra)
+
+# XO_ADDRESS_SANITIZE_COMPILE_OPTIONS: use when XO_ADDRESS_SANITIZE=ON
+#
+# address sanitizer build complains about _FORTIFY_SOURCE redefines
+#    In file included from <built-in>:460:
+#    <command line>:1:9: error: '_FORTIFY_SOURCE' macro redefined [-Werror,-Wmacro-redefined]
+#    #define _FORTIFY_SOURCE 2
+#
+set(XO_ADDRESS_SANITIZE_COMPILE_OPTIONS -Werror -Wall -Wextra -Wno-macro-redefined)
+
+# XO_COMPILE_OPTIONS: use these with xo_compile_options() macro
+if(XO_ADDRESS_SANITIZE)
+    set(XO_COMPILE_OPTIONS ${XO_ADDRESS_SANITIZE_COMPILE_OPTIONS})
+else()
+    set(XO_COMPILE_OPTIONS ${XO_STANDARD_COMPILE_OPTIONS})
+endif()
+
+# ----------------------------------------------------------------
+# generally want all the errors+warnings!
+# however: address sanitizer generates error on _FORTIFY_SOURCE
+#
+macro(xo_compile_options target)
+    target_compile_options(${target} PRIVATE ${XO_COMPILE_OPTIONS})
+endmacro()
+
+# ----------------------------------------------------------------
+# use this for a subdir that builds a library
+#
+macro(xo_install_library target)
+    install(
+        TARGETS ${target}
+        EXPORT ${target}Targets
+        LIBRARY DESTINATION lib COMPONENT Runtime
+        ARCHIVE DESTINATION lib COMPONENT Development
+        RUNTIME DESTINATION bin COMPONENT Runtime
+        PUBLIC_HEADER DESTINATION include COMPONENT Development
+        BUNDLE DESTINATION bin COMPONENT Runtime
+    )
+endmacro()
+
+# ----------------------------------------------------------------
+# use this when relying on indentlog [[https://github.com/rconybea/indentlog]] headers
+#
+macro(xo_indentlogg_dependency target)
+    find_package(indentlog REQUIRED)
+    #add_dependencies(${target} indentlog)
+    target_link_libraries(${target} PUBLIC indentlog)
+    #target_include_directories(${target} PUBLIC ${indentlog_DIR}/../../../include)
+endmacro()

cmake/subsysConfig.cmake.in

@@ -0,0 +1,4 @@
+@PACKAGE_INIT@
+
+include("${CMAKE_CURRENT_LIST_DIR}/@XO_PROJECT_NAME@Targets.cmake")
+check_required_components("@PROJECT_NAME@")

include/subsys/Subsystem.hpp

@@ -0,0 +1,303 @@
+/* file Subsystem.hpp
+ *
+ * author: Roland Conybeare, Aug 2022
+ */
+
+#pragma once
+
+#include "indentlog/scope.hpp"
+#include <functional>
+#include <list>
+#include <string_view>
+#include <cstddef>
+
+/* e.g. XO_SUBSYSTEM_TAG(simulator) => xo::S_simulator_tag */
+#define XO_SUBSYSTEM_TAG(subsys_name) xo::S_ ## subsys_name ## _tag
+
+/* e.g. XO_SUBSYSTEM_REQUIRE(simulator) =>
+ *        xo::InitSubsys<xo::S_simulator_tag>::require()
+ */
+#define XO_SUBSYSTEM_REQUIRE(subsys_name) xo::InitSubsys<XO_SUBSYSTEM_TAG(subsys_name)>::require();
+
+/* e.g. XO_SUBSYSTEM_PROVIDE(simulator, &init) =>
+ *        xo::Subsystem::provide<xo::S_simulator_tag>("simulator", &init)
+ */
+#define XO_SUBSYSTEM_PROVIDE(subsys_name, init_addr) xo::Subsystem::provide<XO_SUBSYSTEM_TAG(subsys_name)>(STRINGIFY(subsys_name), init_addr)
+
+//#define VERIFY_SUBSYSTEM(tag) Subsystem::verify_present<tag>(STRINGIFY(tag))
+
+namespace xo {
+    using xo::tostr;
+
+    /* evidence that one or more subsystems have been initialized.
+     * Used to prevent static linker stripping must-run initialization code
+     */
+    class InitEvidence {
+    public:
+        InitEvidence() = default;
+        InitEvidence(std::uint64_t x) : evidence_{x} {}
+
+        std::uint64_t evidence() const { return evidence_; }
+
+        InitEvidence operator^=(InitEvidence x) {
+            this->evidence_ ^= x.evidence_;
+
+            return *this;
+        } /*operator^=*/
+
+        InitEvidence operator^(InitEvidence x) {
+            return InitEvidence(this->evidence_ ^ x.evidence_);
+        }
+
+    private:
+        /* we don't care about the specific value computed here,
+         * purpose is to be sufficiently impenentrable to compiler such
+         * that static linker can't optimize it away
+         */
+        std::uint64_t evidence_ = 0;
+    }; /*InitEvidence*/
+
+    /* Goals:
+     * 1. provide for code that must run once (and only once)
+     *    to initialize subsystems
+     * 2. in executable,  want to run such code after main() starts
+     *    instead of relying on static initializers;
+     *    that way init behavior can be parameterized based on
+     *    program arguments
+     *
+     * Use
+     *   // subsystem foo
+     *
+     *   enum Foo_tag {};
+     *
+     *   // guarantees that if anything gets initialized,  then
+     *   // foo_init() is included
+     *   //
+     *   template<>
+     *   struct InitSubsys<Foo_tag> {
+     *     static void foo_init() { ... }
+     *
+     *     static InitEvidence require() {
+     *       return Subsystem::require<Foo_tag>("foo", &foo_init);
+     *     }
+     *   };
+     *
+     *   .. register other subsystems ..
+     *
+     *   Subsystem::initialize_all();  // foo_init() has been called once
+     *
+     * If subsystem bar depends on supporting subsystem {foo, quux},  then write:
+     *
+     *   enum Bar_tag {};
+     *
+     *   template<>
+     *   struct InitSubsys<Bar_tag> {
+     *     static void bar_init() { ... }
+     *
+     *     static InitEvidence require() {
+     *       InitEvidence retval;
+     *
+     *       retval ^= InitSubsys<Foo_tag>::require();
+     *       retval ^= InitSubsys<Quux_tag>::require();
+     *
+     *       retval ^= Subsystem::require<Bar_tag>("bar", &bar_init);
+     *     }
+     *   };
+     *
+     * If using subsystems from a shared library (so no access to cmdline args etc):
+     * e.g. in pyfoo.cpp:
+     *
+     *   InitEvidence s_pyfoo_init = InitSubsys<Foo_tag>::require();
+     * or
+     *   InitEvidence s_pyfoo_init = (InitSubsys<Foo_tag>::require()
+     *                                 ^ InitSubsys<Bar_tag>::require());
+     *
+     * Note: Tag argument here no relation of BuildTag in SubsystemImpl<BuildTag> below
+     */
+    template<typename Tag>
+    struct InitSubsys {};
+
+    /* BuildTag: placeholder; insisting on header-only library */
+    template <typename BuildTag>
+    class SubsystemImpl {
+    public:
+        SubsystemImpl() = default;
+        SubsystemImpl(bool require_flag,
+                  std::string_view subsys_name,
+                  std::function<void ()> init_fn)
+            : require_flag_{require_flag},
+              subsys_name_{subsys_name},
+              init_fn_{init_fn} {}
+
+        /* establish an empty Subsystem record for subsys_name.
+         * record is _not_ linked into s_subsys_l!
+         * idempotent.
+         */
+        template<typename SubsystemTag>
+        static SubsystemImpl * establish() {
+            static SubsystemImpl s_subsys;
+
+            return &s_subsys;
+        } /*establish*/
+
+        template<typename SubsystemTag>
+        static bool verify_present(std::string subsys_tag) {
+            SubsystemImpl * subsys = establish<SubsystemTag>();
+
+            if (!subsys->require_flag()) {
+                throw std::runtime_error(tostr("subsystem not present."
+                                               "(missing InitSubsys<", subsys_tag, ">::require() ?)"));
+                return false;
+            }
+
+            return true;
+        } /*verify_present*/
+
+        /* provide (once only) initialization code for a subsystem with tag SubsystemTag.
+         * ideally this would be called just once for a particular tag;
+         * if called multiple times,   calls after the first are no-ops.
+         */
+        template<typename SubsystemTag>
+        static InitEvidence provide(std::string_view subsys_name,
+                                    std::function<void ()> init_fn) {
+            SubsystemImpl * subsys = establish<SubsystemTag>();
+
+            provide_aux(subsys_name, init_fn, subsys);
+
+            return InitEvidence(reinterpret_cast<std::uint64_t>(subsys));
+        } /*provide*/
+
+        /* throw exception if there's anything left for .initialize_all() to do,
+         * or subsystems have been added since last call to .initialize_all()
+         * Can use this to remind application author to call SubsystemImpl::initialize_all()
+         */
+        static bool verify_all_initialized();
+
+        /* 1. initialize all subsystems: promise that for every preceding call
+         *    to .require<tag>(),  the corresponding initialization function has been
+         *    run exactly once.
+         * 2. harmless to call this multiple times -- will not call any init_fn more than once
+         * 3. can interleave .initialize_all() with .require<tag>() as desired
+         */
+        static InitEvidence initialize_all();
+
+        bool require_flag() const { return require_flag_; }
+        bool init_flag() const { return init_flag_; }
+        std::string_view subsys_name() const { return subsys_name_; }
+
+        InitEvidence initialize();
+
+    private:
+        /* helper for .provide<SubsystemTag>() */
+        static void provide_aux(std::string_view subsys_name,
+                                std::function<void ()> init_fn,
+                                SubsystemImpl * p_subsys);
+
+    private:
+        /* set to true iff .s_subsys_l has been extended since last call to .initialize_all() */
+        static bool s_dirty_flag;
+        /* one member for each unique call to .require() */
+        static std::list<SubsystemImpl *> s_subsys_l;
+
+    private:
+        /* set to true on 1st call to .require() */
+        bool require_flag_ = false;
+        /* set to true when .init_fn() invoked */
+        bool init_flag_ = false;
+        /* unique subsystem name */
+        std::string_view subsys_name_;
+        /* call this function once (at most) to initialize this subsystem */
+        std::function<void ()> init_fn_;
+    }; /*SubsystemImpl*/
+
+    template <typename BuildTag>
+    bool
+    SubsystemImpl<BuildTag>::s_dirty_flag = false;
+
+    template <typename BuildTag>
+    std::list<SubsystemImpl<BuildTag> *>
+    SubsystemImpl<BuildTag>::s_subsys_l;
+
+    template <typename BuildTag>
+    void
+    SubsystemImpl<BuildTag>::provide_aux(std::string_view subsys_name,
+                                         std::function<void ()> init_fn,
+                                         SubsystemImpl<BuildTag> * p_subsys)
+    {
+        if (!p_subsys->require_flag()) {
+            /* 1st call to .provide() for this SubsystemTag */
+
+            using xo::scope;
+            using xo::xtag;
+
+            scope log(XO_ENTER0(chatty),
+                      xtag("subsys", subsys_name),
+                      xtag("address", p_subsys));
+
+            *p_subsys = SubsystemImpl<BuildTag>(true /*require_flag*/,
+                                                subsys_name,
+                                                init_fn);
+
+            s_dirty_flag = true;
+            s_subsys_l.push_back(p_subsys);
+        }
+    } /*provide_aux*/
+
+    template <typename BuildTag>
+    bool
+    SubsystemImpl<BuildTag>::verify_all_initialized()
+    {
+        if (s_dirty_flag) {
+            scope log(XO_ENTER0(error), "required subsystems NOT initialized!?");
+
+            for (SubsystemImpl<BuildTag> * subsys : s_subsys_l) {
+                if (!subsys->init_flag()) {
+                    log && log("missing InitSubsys<S_", subsys->subsys_name(), "_tag>::require()");
+                }
+            }
+
+            throw std::runtime_error("Subsystem::verify_initialized:"
+                                     " Subsystem::initialize_all() never called, or out-of-date");
+            return false;
+        }
+
+        return true;
+    } /*verify_all_initialized*/
+
+    template <typename BuildTag>
+    InitEvidence
+    SubsystemImpl<BuildTag>::initialize_all() {
+        scope log(XO_ENTER0(chatty));
+
+        InitEvidence retval;
+
+        if (s_dirty_flag) {
+            for (SubsystemImpl<BuildTag> * subsys : s_subsys_l) {
+                log && log("init", xtag("subsys", subsys->subsys_name()));
+
+                retval ^= subsys->initialize();
+            }
+        }
+
+        s_dirty_flag = false;
+
+        return retval;
+    } /*initialize_all*/
+
+    template <typename BuildTag>
+    InitEvidence
+    SubsystemImpl<BuildTag>::initialize()
+    {
+        if (!init_flag_) {
+            init_flag_ = true;
+
+            init_fn_();
+        }
+
+        return InitEvidence(reinterpret_cast<std::uint64_t>(this));
+    } /*initialize*/
+
+    using Subsystem = SubsystemImpl<class Subsystem_tag>;
+} /*namespace xo*/
+
+/* end Subsystem.hpp */