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Feature-test macros + config header!
We're getting bug reports (e.g. #732) for situations where people have a libpqxx compiled as C++17 but an application compiled as C++20, or _vice versa._ Generally it's probably not wise to link C++17-compiled code to code compiled as C++20. But two things I did exacerbate the problem: 1. I moved a bunch of C++ feature checks to compile time, using C++20's feature test macros. It looked like a much cleaner, easier way forward with reduced maintenance overhead. 2. Usage of C++20's `std::source_location` _when present_ affects the ABI. So effectively there's one ABI with, and one without. I see that mostly as the price of doing libraries in C++ — it's generally dangerous to package library binaries, unless they've been designed to be compatible, or they come as a range of binaries for various compilers, versions, and configurations. And the real problem is that _these two changes interacted._ The detection of support for `std::source_location` happened at compile time. And so if you compile libpqxx in C++17 (without this feature) and the application in C++20 (with this feature), the two will not be link-compatible! In this first commit I'm prototyping a new approach that I hope will combine the ease of maintenance of feature test macros with the ABI stability of a configuration header. Configuration speed should lie somewhere inbetween: no more compiling separate little checks for every individual C++ feature. But it's not easy. There are 2 orthogonal binary choices, leaving me with 4 scenarios to support: * Autoconf vs. CMake * Cross-compiled vs. native. How does cross compilation factor into it? It works like this: I need to check C++ feature test macros. I check them in C++. But I don't want to keep editing that C++ code for every feature — there's a repetitive preprocessor dance that I don't think I could simplify to one simple line of code, because I'd need to pass macro names as parameters to macros. So, I write a minimal config file and run it through a Python script that generates the C++ code for me. Then I have the build configuration machinery compile _and run_ that C++ code, and generate the configuration header. Yes, alright, but how does cross compilation factor into it? First, if you're cross-compiling, it's not a given that you can _run_ the binary you produce on the same system! The whole point is that the two systems are different. And two, you'll have a native compilation environment but there's no guarantee that it will resemble the cross compilation environment at all. So if you compile a binary to run locally, you may get very different results. So for cross-compilation, the Python script just generates a minimal configuration header that just has all features disabled. And in this first commit I've got that working for autoconf. But I'm still struggling with CMake (thanks @KayEss for helping with this). If it gets too difficult, I may go a different route: generate C++ code from Python, but only run it through the preprocessor. (I believe autoconf has a standard, portable way of running the preprocessor; let's hope CMake has one as well.) The output will still C++ code, but now it's been preprocessed, so hopefully it'll be possible to tell portably which features are present. And ironically, I think I'd then have to have another Python script to _postprocess_ the preprocessed code and turn it into a ready-to-use config header.
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@@ -20,6 +20,65 @@ You have another version of autoconf. It may work, but is not guaranteed to. | |
| If you have problems, you may need to regenerate the build system entirely. | ||
| To do so, use the procedure documented by the package, typically 'autoreconf'.])]) | ||
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| # =========================================================================== | ||
| # https://www.gnu.org/software/autoconf-archive/ax_file_escapes.html | ||
| # =========================================================================== | ||
| # | ||
| # SYNOPSIS | ||
| # | ||
| # AX_FILE_ESCAPES | ||
| # | ||
| # DESCRIPTION | ||
| # | ||
| # Writes the specified data to the specified file. | ||
| # | ||
| # LICENSE | ||
| # | ||
| # Copyright (c) 2008 Tom Howard <[email protected]> | ||
| # | ||
| # Copying and distribution of this file, with or without modification, are | ||
| # permitted in any medium without royalty provided the copyright notice | ||
| # and this notice are preserved. This file is offered as-is, without any | ||
| # warranty. | ||
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| #serial 8 | ||
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| AC_DEFUN([AX_FILE_ESCAPES],[ | ||
| AX_DOLLAR="\$" | ||
| AX_SRB="\\135" | ||
| AX_SLB="\\133" | ||
| AX_BS="\\\\" | ||
| AX_DQ="\"" | ||
| ]) | ||
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| # =========================================================================== | ||
| # https://www.gnu.org/software/autoconf-archive/ax_print_to_file.html | ||
| # =========================================================================== | ||
| # | ||
| # SYNOPSIS | ||
| # | ||
| # AX_PRINT_TO_FILE([FILE],[DATA]) | ||
| # | ||
| # DESCRIPTION | ||
| # | ||
| # Writes the specified data to the specified file. | ||
| # | ||
| # LICENSE | ||
| # | ||
| # Copyright (c) 2008 Tom Howard <[email protected]> | ||
| # | ||
| # Copying and distribution of this file, with or without modification, are | ||
| # permitted in any medium without royalty provided the copyright notice | ||
| # and this notice are preserved. This file is offered as-is, without any | ||
| # warranty. | ||
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| #serial 8 | ||
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| AC_DEFUN([AX_PRINT_TO_FILE],[ | ||
| AC_REQUIRE([AX_FILE_ESCAPES]) | ||
| printf "$2" > "$1" | ||
| ]) | ||
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| # Copyright (C) 2002-2021 Free Software Foundation, Inc. | ||
| # | ||
| # This file is free software; the Free Software Foundation | ||
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,6 @@ | ||
| __cpp_concepts pqxx_have_concepts | ||
| __cpp_multidimensional_subscript pqxx_have_multidim | ||
| __cpp_lib_source_location pqxx_have_source_location | ||
| __cpp_lib_span pqxx_have_span | ||
| __cpp_lib_ssize pqxx_have_ssize | ||
| __cpp_lib_unreachable pqxx_have_unreachable |
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