C++成员初始化、复制初始化和默认初始化

Question

C++成员初始化、复制初始化和默认初始化

c++compilationcompiler-errorsinitialization

4

来自《C++编程语言》第四版的17.3.1节“不需要构造函数的初始化”，第489页。

书中示例中的标记行无法编译，会出现以下错误 -

$ g++ -std=c++11 ch17_pg489.cpp
ch17_pg489.cpp: In function 'int main()':
ch17_pg489.cpp:32:34: error: could not convert 's9' from 'Work' to 'std::string {aka std::basic_string<char>}'
      Work currently_playing { s9 }; // copy initialization

我有Cygwin

$ g++ --version
g++.exe (tdm64-2) 4.8.1

引用上述部分的文本，如下所示：

we can initialize objects of a class for which we have not defined a constructor using
• memberwise initialization,
• copy initialization, or
• default initialization (without an initializer or with an empty initializer list).


#include <iostream>

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

/* 
    // This correctly prints the respective fields
    std::cout << s9.author << " | " 
                        << s9.name << " | "
                        << s9.year << std::endl;
*/

  // Fails to compile
    Work currently_playing { s9 }; // copy initialization

    Work none {}; // default initialization

    return 0;
}

据我的理解，编译器生成的默认复制构造函数提供了复制初始化方法，或者仅仅是成员逐一复制（像C语言中一样将一个结构体赋值给另一个结构体）。因此，程序应该能够编译通过。这是编译器的问题吗？有任何解释吗？

- nightlytrails

1个回答

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- user657267 · Accepted Answer

正如您在第四版勘误表中所看到的。

http://www.stroustrup.com/4th.html

People have pointed out that the {} doesn't work for copy construction:
X x1 {2};     // construct from integer (assume suitable constructor)
X x2 {x1};        // copy construction: fails on GCC 4.8 and Clang 3.2
I know that. It's a bug in the standard. Fixed for C++14. For now use one of the traditional notations:
X x3(x1);     // copy construction
X x4 = x1;        // copy construction

这是针对GCC 4.10的修正。

以下是缺陷报告本身。