When compiling the following piece of code (gcc-4.8, --std=c++11):
#include <atomic>
#include <utility>
#include <cstdint>
struct X {
std::atomic<std::pair<uint32_t, uint32_t>> A;
};
I get the following compilation error:
/usr/local/include/c++/4.8.2/atomic:167:7: error: function 'std::atomic<_Tp>::atomic() [with _Tp = std::pair<unsigned int, unsigned int>]' defaulted on its first declaration with an exception-specification that differs from the implicit declaration 'constexpr std::atomic<std::pair<unsigned int, unsigned int> >::atomic()'
With a more recent compiler (gcc-9 with --std=c++17), I get:
In instantiation of 'struct std::atomic<std::pair<int, int> >': error: static assertion failed: std::atomic requires a trivially copyable type static_assert(__is_trivially_copyable(_Tp),
demos:
I can't figure out the reason why; can anyone help me please?
std::atomic<T> requires T to be TriviallyCopyable.
You cannot define an std::atomic<std::pair<...>> because std::pair is not trivially copiable. For more information about that, read Why can't std::tuple be trivially copyable?.
As a workaround, you can define your own simplified trivially copiable pair:
#include <atomic>
#include <cstdint>
struct X
{
using pair = struct { std::uint32_t first; std::uint32_t second; };
std::atomic<pair> A;
};