xo-indentlog: + cond feature

xo-indentlog/include/xo/indentlog/print/cond.hpp

@@ -0,0 +1,92 @@
+/** @file cond.hpp
+ *
+ *  author: Roland Conybeare
+ **/
+
+#pragma once
+
+#include "ppdetail_atomic.hpp"
+#include <iostream>
+#include <type_traits>
+
+namespace xo {
+
+    /** @class cond_impl
+     *  @brief conditional stream inserter implementation
+     *
+     *  @tparam T  type printed when condition is true
+     *  @tparam U  type printed when condition is false
+     **/
+    template <typename T, typename U>
+    struct cond_impl {
+    public:
+        cond_impl(bool condition, T const & then_value, U const & else_value)
+            : condition_{condition},
+              then_value_{then_value},
+              else_value_{else_value} {}
+
+        bool condition() const { return condition_; }
+        T const & then_value() const { return then_value_; }
+        U const & else_value() const { return else_value_; }
+
+    private:
+        bool condition_;
+        T then_value_;
+        U else_value_;
+    };
+
+    /** Create conditional stream inserter.
+     *
+     *  @param condition   when true, print @p then_value; otherwise print @p else_value
+     *  @param then_value  value to print when condition is true
+     *  @param else_value  value to print when condition is false
+     *
+     *  Example:
+     *  @code
+     *    std::cout << cond(ptr != nullptr, xtag("ptr", *ptr), "nullptr");
+     *  @endcode
+     **/
+    template <typename T, typename U>
+    auto
+    cond(bool condition, T && then_value, U && else_value)
+    {
+        return cond_impl<std::decay_t<T>, std::decay_t<U>>
+            (condition,
+             std::forward<T>(then_value),
+             std::forward<U>(else_value));
+    }
+
+    /** Stream insertion operator for cond_impl **/
+    template <typename T, typename U>
+    inline std::ostream &
+    operator<<(std::ostream & os, cond_impl<T, U> const & c)
+    {
+        if (c.condition())
+            os << c.then_value();
+        else
+            os << c.else_value();
+        return os;
+    }
+
+#ifndef ppdetail_atomic
+    namespace print {
+        /** Pretty-printing support for cond_impl **/
+        template <typename T, typename U>
+        struct ppdetail<cond_impl<T, U>> {
+            using target_type = cond_impl<T, U>;
+
+            static bool print_pretty(const ppindentinfo & ppii,
+                                     const target_type & c)
+            {
+                if (c.condition())
+                    return ppdetail<T>::print_pretty(ppii, c.then_value());
+                else
+                    return ppdetail<U>::print_pretty(ppii, c.else_value());
+            }
+        };
+    }
+#endif
+
+}
+
+/* end cond.hpp */