Niall’s virtual diary archives – Wednesday 13th May 2015

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Wednesday 13th May 2015: 11.10pm. Link shared: https://github.com/ned14/Boost.APIBind/blob/master/include/cpp_feature.h

As part of publicising my C++ Now 2015 talk this week, here is part 11 of 20 from its accompanying Handbook of Examples of Best Practice for C++ 11/14 (Boost) libraries:

11. PORTABILITY: Consider not doing compiler feature detection yourself

Something extremely noticeable about nearly all the reviewed C++ 11/14 libraries is that they manually do compiler feature detection in their config.hpp, usually via old fashioned compiler version checking. This tendency is not unsurprising as the number of potential C++ compilers your code usually needs to handle has essentially shrunk to three unlike the dozen common compilers implementing the 1998 C++ standard, and the chances are very high that three compilers will be upper bound going into the long term future. This makes compiler version checking a lot more tractable than say fifteen years ago.

However, C++ 1z is expected to provide a number of feature detection macros via the work of SG-10, and GCC and clang already partially support these, especially in very recent compiler releases. To fill in the gaps in older editions of GCC and clang, and indeed MSVC at all, you might consider making use of the header file at  https://github.com/ned14/Boost.APIBind/blob/master/include/cpp_feature.h which provides the following SG-10 feature detection macros on all versions of GCC, clang and MSVC:

cpp_exceptions - Whether C++ exceptions are available
cpp_rtti - Whether C++ RTTI is available
cpp_alias_templates
cpp_alignas
cpp_decltype
cpp_default_function_template_args
cpp_defaulted_functions
cpp_delegated_constructors
cpp_deleted_functions
cpp_explicit_conversions
cpp_generalized_initializers
cpp_implicit_moves
cpp_inheriting_constructors
cpp_inline_namespaces
cpp_lambdas
cpp_local_type_template_args
cpp_noexcept
cpp_nonstatic_member_init
cpp_nullptr
cpp_override_control
cpp_reference_qualified_functions
cpp_range_for
cpp_raw_strings
cpp_rvalue_references
cpp_static_assert
cpp_thread_local
cpp_auto_type
cpp_strong_enums
cpp_trailing_return
cpp_unicode_literals
cpp_unrestricted_unions
cpp_user_defined_literals
cpp_variadic_templates
cpp_contextual_conversions
cpp_decltype_auto
cpp_aggregate_nsdmi
cpp_digit_separators
cpp_init_captures
cpp_generic_lambdas
cpp_relaxed_constexpr
cpp_return_type_deduction
cpp_runtime_arrays
cpp_variable_templates

The advantage of using these SG-10 macros in C++ 11/14 code is threefold:

1. It should be future proof.
2. It's a lot nicer than testing compiler versions.
3. It expands better if a fourth C++ compiler suddenly turned up.

Why use the  https://github.com/ned14/Boost.APIBind/blob/master/include/cpp_feature.h header file instead of doing it by hand?

1. Complete compiler support for GCC, clang and MSVC all versions.
2. Updates in compiler support will get reflected into cpp_feature.h for you.
3. You benefit from any extra compilers added automatically.
4. If you're using Boost.APIBind you automatically get cpp_feature.h included for you as soon as you include any APIBind header file.

Problems with cpp_feature.h:

1. No support for detecting STL library feature availability. One can do this somewhat with GCC as it always pairs to a libstdc++ version, and of course one can do this for MSVC. However clang pairs to whatever is the latest STL on the system, plus GCC combined with libc++ is becoming increasingly common on Linux. In short you are on your own for STL library feature detection as I am unaware of any easy way to abstract this without the SG-10 library feature detection facilities built into the compiler.

Incidentally Boost.APIBind wraps these SG-10 feature macros into Boost.Config compatible macros in  https://github.com/ned14/Boost.APIBind/blob/master/include/boost/config.hpp which would be included, as with Boost, using "boost/config.hpp". You can therefore if you really want use the Boost feature detection macros instead, even without Boost being present.

http://cppnow2015.sched.org/event/37beb4ec955c082f70729e4f6d1a1a05#.VUuMqvkUUuU

#cpp  #cplusplus #cppnow   #cppnow2015   #c++ #boostcpp   #c++11 #c++14

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