98 lines
3.2 KiB
ReStructuredText
98 lines
3.2 KiB
ReStructuredText
.. _lessons:
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.. toctree
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:maxdepth: 2
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Lessons
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=======
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This is a rogue's gallery of experiments, typically unsuccessful.
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One hurdle we've created for ourselves, is we need both gcc and clang to agree
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that an expression can be computed at compile-time;
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otherwise will get false alarms in our IDE (raised by LSP running in the background, which relies on clang).
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Must fully initialize memory
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----------------------------
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Struggled for a while with the implementation of :ref:xo::flatstring_concat
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.. code-block:: cpp
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template <std::size_t N>
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flatstring::flatstring<N>() {
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if (N > 0)
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value_[0] = '\0';
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}
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This implementation satisfies gcc, but not clang: in the following snippet, clang doesn't recognize ``tmp`` as constexpr:
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.. code-block:: cpp
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constexpr n = ...;
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flatstring<n> tmp;
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static_assert(tmp.size() == ...); // tmp not constexpr!
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Correction is to prove to clang that every memory address owned by an empty ``flatstring`` is initialized:
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.. code-block:: cpp
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template <std::size_t N>
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flatstring::flatstring<N>() {
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std::fill_n(value_, N, '\0');
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}
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Still need equality comparison alongside spaceship operator
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-----------------------------------------------------------
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Had the impression that spaceship operator for :ref:xo::flatstring would be sufficient
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to get all six comparison operators:
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.. code-block:: cpp
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template <std::size_t N1,
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std::size_t N2>
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constexpr auto
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operator<=>(const flatstring<N1> & s1,
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const flatstring<N2> & s2) noexcept
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{
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return (std::string_view(s1) <=> std::string_view(s2));
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}
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We observe this is not the case, at least with gcc 13.1; need to separately define :ref:xo::operator==
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.. code-block:: cpp
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template <std::size_t N1,
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std::size_t N2>
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constexpr bool
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operator==(const flatstring<N1> & s1,
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const flatstring<N2> & s2) noexcept
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{
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return ((s1 <=> s2) == std::strong_ordering::equal);
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}
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Constexpr strict about pointer arithmetic
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-----------------------------------------
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Initially attempted to implement :ref:xo::flatstring reverse iterators using char pointers.
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Notice there's an assymetry between reverse iterators and forward iterators.
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We can (and do) implement forward iterators using char pointers.
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The natural value of ``flatstring::end()`` is a char pointer referring to just past the end of
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the string, i.e. to its null terminator. From the compiler's perspective, this is an ordinary
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char pointer, just like other iterator values.
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For reverse iterators this isn't the case. The natural value for ``flatstring::rend()`` might
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seem to be a char pointer referring to just before the first character in the string.
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However this is no longer a valid pointer address -- dereferencing would be undefined behavior.
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In particular, with this implementation, gcc demotes ``flatstring::rend()`` to non-constexpr
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Workaround is to implement a shim iterator class, where representation is pointer to the
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character just after the one the iterator position; iterator's ``operator*`` adjusts pointer before
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dereferencing.
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This works because gcc can observe that we never dereference a reverse iterator with pointer value
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at the beginning of a flatstring.
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