C ++成员初始化，复制初始化和默认初始化

Question

From the book, The C++ Programming Language, 4th edition, Section "17.3.1 Initialization Without Constructors", Page 489

The marked line in the example from the book fails to compile with this error -

$ 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

I have Cygwin

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

To quote from the text from the aforementioned section,

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;
}

As per my understanding, either copy initialization would be provided by the default copy constructor generated by the compiler Or it would be simply a member wise copy (assigning one struct to another, as in C). So the program should have compiled here.

Or is this the compiler quirk??

Any explanations ?

Answer 1

As you can see under the errata for the 4th edition

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 

It's fixed for GCC 4.10

Here's the defect report itself.