c++11 - 使用统一初始化调用复制构造函数

Question

我试图通过统一初始化调用默认复制构造函数，但它不起作用。

例如:

#include <string>

struct Work {
        std::string author;
        std::string name;
        int year;
};

int main() {
        Work s9 {"Beethoven", "Symphony No. 9 in D minor, Op. 125; Choral", 1824}; // memberwise initialization
        Work currently_playing {s9};    // copy initialization

        return 0;
}

我将其编译为:g++ -std=c++11 -c Ex1.cpp

编译器报错:

Ex1.cpp: In function ‘int main()’:
Ex1.cpp:11:28: error: could not convert ‘s9’ from ‘Work’ to ‘std::string {aka std::basic_string<char>}’
  Work currently_playing {s9}; // copy initialization
                            ^

统一初始化不能复制初始化对象吗？

Answer 1

这是 C++11 标准中的错误。 C++14 改变了列表初始化的执行方式。在 C++11 中，如果 X 是 aggregate type，X{X{}} 将执行聚合初始化 . C++14 添加了一个额外的子句，以便此代码正确调用相应的构造函数； [dcl.init.list]/3.2:

If T is an aggregate class and the initializer list has a single element of type cv U, where U is T or a class derived from T, the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).

来源:

http://eel.is/c++draft/dcl.init.list#3.2

http://en.cppreference.com/w/cpp/language/list_initialization#Explanation