从std :: option <int>构造std :: optional <volatile int>

问题描述

此代码被拒绝：

const std::optional<int> opt{5};
std::optional<volatile int> opt2 = opt;

但这是可以接受的：

const std::optional<int> opt{5};
std::optional<const int> opt2 = opt;

这是怎么回事？

In file included from /tmp/a.cpp:1:
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/optional:627:4: error: no matching function for call to 'operator new'
          ::new
          ^~
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/optional:1140:10: note: in instantiation of function template specialization 'std::_Optional_base_impl >::_M_construct' requested here
          this->_M_construct(std::forward(__args)...);
                ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/optional:1002:4: note: in instantiation of function template specialization 'std::optional::emplace' requested here
          emplace(*__t);
          ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/new:134:7: note: candidate function not viable: no known conversion from 'volatile int *' to 'const std::nothrow_t' for 2nd argument
void* operator new(std::size_t,const std::nothrow_t&) _GLIBCXX_USE_NOEXCEPT
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/new:143:7: note: candidate function not viable: no known conversion from 'volatile int *' to 'std::align_val_t' for 2nd argument
void* operator new(std::size_t,std::align_val_t)
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/new:168:14: note: candidate function not viable: no known conversion from 'volatile int *' to 'void *' for 2nd argument
inline void* operator new(std::size_t,void* __p) _GLIBCXX_USE_NOEXCEPT
             ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/new:120:7: note: candidate function not viable: requires 1 argument,but 2 were provided
void* operator new(std::size_t) _GLIBCXX_THROW (std::bad_alloc)
      ^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/new:145:7: note: candidate function not viable: requires 3 arguments,but 2 were provided
void* operator new(std::size_t,std::align_val_t,const std::nothrow_t&)
      ^
1 error generated.

解决方法

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