allocator_traits example from cplusplus website - c++

Question

I don't understant allocator_traits in C++ 11, so I tried the allocator_traits exmple from cpluspluswebsite. However, this code can't compile in gcc 4.9, VS 2015, even not in its own website.
What's more, I also don't understand why I can't see any allocator_traits syntax in this example. I can just see this example build a custom allocator, then use the allocator in vector. Nothing about allocator_traits.
Can anyone help me?
// 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;
}
The compilation error in cplusplus website is:
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 reports similar error.
VS 2015 reports error also in line 12. The message is below:
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

Replace ::new by ::operator new and it will work. Probably it was a typo on the website. You are calling here operator new, not invoking a new expression.