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Generate feature test, in two stages.
This is yet another attempt at #732: go back to writing all feature test macros to a config header, so that we can build libpqxx itself and the client application against the same version of the language — even if the user actually builds them with different options that may affect the feature set. Things I wanted for this work: 1. Less of that repetitive configuration. It's tedious and error-prone. 2. Reduced risk of inconsistency between autoconf and CMake. 3. Portability. 4. Simple way to add a feature check based on a C++ feature test macro. 5. Reasonable simplifity. 6. Fast build configuration. I think this solution satisfies most of these. A new configuration file `cxx_features.txt` lets me define features that we detect simply by checking a C++ feature test macro. (We'll still need to enter each libpqxx feature macro that we want to expose into `configitems`.) A first Python script, `generate_cxx_checks.py` generates the C++ code snippets that check for each of the C++ feature check macros. It just adds those to `config-tests/`. The C++ files inside `config-tests/` now have names that correspond _directly_ to the libpqxx feature macros. It eliminates the pointless separate file name, but also, it lets me generate autoconf and CMake config based on just the file names! Then, a second script `generate_check_config.py` produces pieces of autoconf and CMake configuration to actually run these checks. The respective main configurations use include directives to incorporate these configs. These pieces of configuration will reference both the pre-existing, hand-written feature test snippets, and the ones we just generated from C++ feature test macros. At this stage there's no difference between the two kinds. I believe this is as portable as it ever was, because nothing really changes structurally. Yes, the scripts are tailored for recent Python language versions, but I think that's about it. Of course the C++ feature test macros were introduced in C++20 so they're not going to do much good for C++17. But that should stop mattering soon, as we transition to C++20 as the minimum language version. I believe the new system is not the simplest thing, but is easier to manage than all those manual compilation checks across two build systems. That was just painful. And who knows, some day this new approach may support adding yet another build system. The only thing that hasn't improved is build configuration speed. The only positive thing I can say there is that the new mechanism generates all the extra files during `autogen.sh`, so that part shouldn't affect build configuration speeds. But it's not going to be as fast as my last plan for this work. (That experiment checked all the C++ feature test macros by running a single piece of generated C++ code through the preprocessor, and generating a new config header from it.)
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| // Test for C++20 std::cmp_greater etc. support. | ||
| // C++20: Assume support. | ||
| #include <utility> | ||
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config-tests/concepts.cxx → config-tests/PQXX_HAVE_CONCEPTS.cxx
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| Original file line number | Diff line number | Diff line change |
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| // Test for std::span. | ||
| // C++20: Assume support. | ||
| #include <span> | ||
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| int main(int argc, char **argv) | ||
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config-tests/thread_local.cxx → config-tests/PQXX_HAVE_THREAD_LOCAL.cxx
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| Configuration tests | ||
| =================== | ||
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| Libpqxx comes with support for different build systems: the GNU autotools, | ||
| CMake, Visual Studio's "nmake", and raw GNU "make" on Windows. | ||
| Libpqxx comes with support for two different build systems: the GNU autotools, | ||
| and CMake. | ||
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| For several of these build systems, we need to test things like "does this | ||
| compiler environment support `std::to_chars` for floating-point types?" | ||
| We need to teach both of these to test things like "does this compiler | ||
| environment support `std::to_chars` for floating-point types?" | ||
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| We test these things by trying to compile a particular snippet of code, and | ||
| seeing whether that succeeds. | ||
| In both build systems we test these things by trying to compile a particular | ||
| snippet of code, found in this directry, and seeing whether that succeeds. | ||
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| To avoid duplicating those snippets for multiple build systems, we put them | ||
| here. Both the autotools configuration and the CMake configuration can refer to | ||
| them that way. | ||
| them that way. We generate autoconf and cmake configuration automatically to | ||
| inject those checks, avoiding tedious repetition. | ||
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| It took a bit of nasty magic to read a C++ source file into m4 and treat it as | ||
| a string literal, without macro expansion. There is every chance that I missed | ||
| something, so be prepared for tests failing for unexpected reasons! Some C++ | ||
| syntax may end up having an unforeseen meaning in m4, and screw up the handling | ||
| of the code snippet. Re-configure, and read your logs carefully after editing | ||
| these snippets. | ||
| Some of the checks are based on C++20 feature test macros. We generate those | ||
| automatically using `tools/generate_cxx_checks.py`. | ||
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| It took a bit of nasty magic to read a C++ source file into m4 for the autoconf | ||
| side and treat it as a string literal, without macro expansion. There is every | ||
| chance that I missed something, so be prepared for tests failing for unexpected | ||
| reasons! Some C++ syntax may end up having an unforeseen meaning in m4, and | ||
| screw up the handling of the code snippet. Re-configure, and read your logs | ||
| carefully after editing these snippets. |
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