列表初始化的复制省略，它在标准中的何处规定？

Question

在[dcl.init]/17.6中明确写到，对于括号初始化的情况，会出现copy elision：

If the initializer expression is a prvalue and the cv-unqualified version of the source type is the same class as the class of the destination, the initializer expression is used to initialize the destination object. [ Example: T x = T(T(T())); calls the T default constructor to initialize x. — end example ]

但是在列表初始化的情况下，上面的段落不适用，我没有发现任何类似的东西。参见 [dcl.init.list]。

那么为什么在这种情况下存在复制省略：T x{T(T())}; 根据 C++17 标准。

Answer 1

根据目前的草案，这种情况下没有复制省略。

考虑以下示例：

#include <iostream>
#include <initializer_list>

struct S {
    S() {std::cout << "default\n";}
    S(const S&) {std::cout << "copy\n";}
    S(std::initializer_list<S>) {std::cout << "initializer list\n";}
};

int main()
{
    S s = S{S()};
}

根据Core Language Issue 2137, the constructor taking std::initializer_list as parameter should be chosen (Clang may choose copy constructor or perform copy elision here, which ）。因此，此类列表初始化应考虑构造函数。

The problem is that when copy/move constructor is selected, it is reasonable to elide this copy/move.事实上，Core Language Issue 2327已经解决了这个缺陷。

列表初始化的复制省略，它在标准中的何处规定？

Copy elision for list-initialization, where is it stated in the standard?

c++

language-lawyer

copy-elision

list-initialization

c++17