int main() { return 0; }

GLIBCXX_CONCEPT_CHECKS_

Nov 4, 2017 by htfy96

最近发现了libstdc++中的_GLIBCXX_CONCEPT_CHECKS这个宏，感觉很有意思……在此记录一下。

Introduction

_GLIBCXX_CONCEPT_CHECKS, according to online doc of gcc, is a macro that performs compile-time checking on certain template instantiations to detect violations of the requirements of the standard. This macro has no effect for freestanding implementations.

Example

#include <set>

struct A
{
    int a, b;
};

int main()
{
    std::set<A> h_a;
    // h_a.insert(A { 1, 2 });
};

std::set requires operator < defined on A, which is missing in our code. However, libstdc++ does not perform the check in the constructor of std::set<A>, thus the error is only raised when we call member functions(e.g. insert) that requires it. Such behavior prohibits early detection of unsatisfied constraints(e.g. Compare).

Without _GLIBCXX_CONCEPT_CHECKS

Without .insert

compiles without error

With .insert

|| In file included from /usr/include/c++/7.2.0/bits/stl_tree.h:65:0,
||                  from /usr/include/c++/7.2.0/set:60,
||                  from /tmp/a.cpp:2:
|| /usr/include/c++/7.2.0/bits/stl_function.h: In instantiation of ‘constexpr bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = A]’:
/usr/include/c++/7.2.0/bits/stl_tree.h|2038 col 11| required from ‘std::pair<std::_Rb_tree_node_base*, std::_Rb_tree_node_base*> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_get_insert_unique_pos(const key_type&) [with _Key = A; _Val = A; _KeyOfValue = std::_Identity<A>; _Compare = std::less<A>; _Alloc = std::allocator<A>; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::key_type = A]’
/usr/include/c++/7.2.0/bits/stl_tree.h|2091 col 28| required from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_Arg&&) [with _Arg = A; _Key = A; _Val = A; _KeyOfValue = std::_Identity<A>; _Compare = std::less<A>; _Alloc = std::allocator<A>]’
/usr/include/c++/7.2.0/bits/stl_set.h|510 col 48| required from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(std::set<_Key, _Compare, _Alloc>::value_type&&) [with _Key = A; _Compare = std::less<A>; _Alloc = std::allocator<A>; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree_const_iterator<A>; std::set<_Key, _Compare, _Alloc>::value_type = A]’
/tmp/a.cpp|14 col 26| required from here
/usr/include/c++/7.2.0/bits/stl_function.h|386 col 20 error| no match for ‘operator<’ (operand types are ‘const A’ and ‘const A’)
||        { return __x < __y; }
||                 ~~~~^~~~~
// ...

With _GLIBCXX_CONCEPT_CHECKS

Without .insert

|| In file included from /usr/include/c++/7.2.0/bits/stl_tree.h:65:0,
||                  from /usr/include/c++/7.2.0/set:60,
||                  from /tmp/a.cpp:2:
|| /usr/include/c++/7.2.0/bits/stl_function.h: In instantiation of ‘constexpr bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = A]’:
/usr/include/c++/7.2.0/bits/boost_concept_check.h|355 col 11| required from ‘void __gnu_cxx::_BinaryFunctionConcept<_Func, _Return, _First, _Second>::__constraints() [with _Func = std::less<A>; _Return = bool; _First = A; _Second = A]’
/usr/include/c++/7.2.0/bits/stl_set.h|101 col 7| required from ‘class std::set<A>’
/tmp/a.cpp|13 col 17| required from here
/usr/include/c++/7.2.0/bits/stl_function.h|386 col 20 error| no match for ‘operator<’ (operand types are ‘const A’ and ‘const A’)
||        { return __x < __y; }
||                 ~~~~^~~~~
...

Limitations

According to Compile Time Checks in libstdc++ manual:

Please note that the concept checks only validate the requirements of the old C++03 standard. C++11 was expected to have first-class support for template parameter constraints based on concepts in the core language. This would have obviated the need for the library-simulated concept checking described above, but was not part of C++11.

Tags:  c++