c++ - 来自 cplusplus 网站的 allocator_traits 示例

Question

我不了解 C++ 11 中的 allocator_traits，所以我尝试了 allocator_traits exmple来自 cplusplus 网站。但是，此代码无法在 gcc 4.9、VS 2015 中编译，甚至无法在其自己的网站中编译。

此外，我也不明白为什么我在这个例子中看不到任何 allocator_traits 语法。我只能看到这个例子构建了一个自定义分配器，然后在 vector 中使用分配器。与 allocator_traits 无关。

谁能帮帮我？

// custom allocator example
#include <cstddef>
#include <iostream>
#include <memory>
#include <vector>

template <class T>
struct custom_allocator {
  typedef T value_type;
  custom_allocator() noexcept {}
  template <class U> custom_allocator (const custom_allocator<U>&) noexcept {}
  T* allocate (std::size_t n) { return static_cast<T*>(::new(n*sizeof(T))); }
  void deallocate (T* p, std::size_t n) { ::delete(p); }
};

template <class T, class U>
constexpr bool operator== (const custom_allocator<T>&, const custom_allocator<U>&) noexcept
{return true;}

template <class T, class U>
constexpr bool operator!= (const custom_allocator<T>&, const custom_allocator<U>&) noexcept
{return false;}

int main () {
  std::vector<int,custom_allocator<int>> foo = {10,20,30};
  for (auto x: foo) std::cout << x << " ";
  std::cout << '\n';
  return 0;
}

cplusplus网站编译错误为:

 In member function 'T* custom_allocator<T>::allocate(std::size_t)':
12:74: error: expected type-specifier before ')' token
 In instantiation of 'T* custom_allocator<T>::allocate(std::size_t) [with T = int; std::size_t = long unsigned int]':
/usr/include/c++/4.9/bits/alloc_traits.h:357:32:   required from 'static std::allocator_traits<_Alloc>::pointer std::allocator_traits<_Alloc>::allocate(_Alloc&, std::allocator_traits<_Alloc>::size_type) [with _Alloc = custom_allocator<int>; std::allocator_traits<_Alloc>::pointer = int*; std::allocator_traits<_Alloc>::size_type = long unsigned int]'
/usr/include/c++/4.9/bits/stl_vector.h:170:46:   required from 'std::_Vector_base<_Tp, _Alloc>::pointer std::_Vector_base<_Tp, _Alloc>::_M_allocate(std::size_t) [with _Tp = int; _Alloc = custom_allocator<int>; std::_Vector_base<_Tp, _Alloc>::pointer = int*; std::size_t = long unsigned int]'
/usr/include/c++/4.9/bits/stl_vector.h:1287:27:   required from 'void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const int*; _Tp = int; _Alloc = custom_allocator<int>]'
/usr/include/c++/4.9/bits/stl_vector.h:378:36:   required from 'std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = int; _Alloc = custom_allocator<int>; std::vector<_Tp, _Alloc>::allocator_type = custom_allocator<int>]'
25:57:   required from here
12:77: warning: no return statement in function returning non-void [-Wreturn-type]

gcc 4.9 报告类似的错误。

VS 2015 也在第 12 行报告错误。消息如下:

Severity    Code    Description Project File    Line    Suppression State
Error   C2059   syntax error: ')'   allocator_traits    c:\users\jiang\documents\visual studio 2015\projects\allocator_traits\allocator_traits.cpp  12  

Answer 1

将 ::new 替换为 ::operator new 即可。可能是网站上的错字。您在这里调用 operator new，而不是调用 new 表达式。