c++ - std::string 为 1 个字符串分配内存 2 次

Question

#include <iostream>
#include <string>

void* operator new(size_t size) {
    std::cout << "Allocated: " << size << " Bytes\n";
    return malloc(size);
}

void operator delete(void* var) {
    std::cout << "Deleted\n";
    free(var);
}

int main() {
    std::string name0 = "Ahmed Zaki Marei";
    //std::string name1 = "Lara Mohammed";

    std::cout << name0 << "\n";
    //std::cout << name1 << "\n";
}

当我尝试运行此代码时，它给了我以下输出:

Allocated: 8 Bytes
Allocated: 32 Bytes
Ahmed Zaki Marei
Deleted
Deleted

为什么它先分配 8 个字节然后分配 32 个字节？
有谁能解释一下吗？谢谢! :)

Answer 1

作为 C.M.评论中已经指出:这是 Debug模式下的 MS STL 行为，与 _ITERATOR_DEBUG_LEVEL 相关。在 Release 中一切都很好。
很好奇所以我自己测试了它，堆栈在 new 的第一个断点上读取了这个:

operator new(unsigned int size)
std::_Default_allocate_traits::_Allocate(const unsigned int _Bytes)
std::_Allocate<8,std::_Default_allocate_traits,0>(const unsigned int _Bytes)
std::allocator<std::_Container_proxy>::allocate(const unsigned int _Count)
std::_Container_proxy_ptr12<std::allocator<std::_Container_proxy>>::_Container_proxy_ptr12<std::allocator<std::_Container_proxy>>(std::allocator<std::_Container_proxy> & _Al_, std::_Container_base12 & _Mycont)
std::string::basic_string<char,std::char_traits<char>,std::allocator<char>>(const char * const _Ptr)
main()

让我们看看 basic_string构造函数:

    basic_string(_In_z_ const _Elem* const _Ptr) : _Mypair(_Zero_then_variadic_args_t{}) {
        auto&& _Alproxy = _GET_PROXY_ALLOCATOR(_Alty, _Getal());
        _Container_proxy_ptr<_Alty> _Proxy(_Alproxy, _Mypair._Myval2);

那是什么 _Container_proxy_ptr真的是:

#if _ITERATOR_DEBUG_LEVEL == 0
#define _GET_PROXY_ALLOCATOR(_Alty, _Al) _Fake_allocator()
template <class _Alloc>
using _Container_proxy_ptr = _Fake_proxy_ptr_impl;
#else // _ITERATOR_DEBUG_LEVEL == 0
#define _GET_PROXY_ALLOCATOR(_Alty, _Al) static_cast<_Rebind_alloc_t<_Alty, _Container_proxy>>(_Al)
template <class _Alloc>
using _Container_proxy_ptr = _Container_proxy_ptr12<_Rebind_alloc_t<_Alloc, _Container_proxy>>;
#endif // _ITERATOR_DEBUG_LEVEL == 0

是 _Container_proxy_ptr12 (最终调用 .allocate(1) )在调试
和 _Fake_proxy_ptr_impl (什么都不做)在 Release 中。