没有用于初始化“字符串"(又称为"basic_string< char>")的匹配构造函数 [英] no matching constructor for initialization of 'string' (aka 'basic_string<char>')

问题描述

这是代码:

  #include< iostream>#include< string>使用命名空间std;Foo类{上市:运算符string()const {return n;}字符串n {"foo"};};int main(int argc，char ** argv){字符串s {Foo {}};cout<<s<<恩德尔返回0;} 

此代码使用gcc 4.8.3进行编译，但不使用clang 3.5进行编译，有人可以告诉我这是怎么回事吗?

我遇到了这样的错误:

  main.cpp:45:12:错误:没有匹配的构造函数来初始化'string'(aka'basic_string< char>')字符串s {Foo {}};^ ~~~~~~~~ 

clang --version:

  clang版本3.5.0(标签/RELEASE_350/final 216961)目标:x86_64-suse-linux螺纹型号:posix 

谢谢

解决方案

我相信这是一个clang错误.根据[dcl.init.list]中列表初始化的规则:

对象或类型为 T 的引用的列表初始化定义如下:

• 如果 T 是类类型，并且初始化列表具有单个类型为 cv U 的元素，其中 U 是 T 或从 T ，[...]
派生的类
• 否则，如果 T 是字符数组，并且[...]
• 否则，如果 T 是一个聚合，则[...]
• 否则，如果初始化列表中没有元素[...]
• 否则，如果 T 是 std :: initializer_list< E> 的特殊化，则[...]
• 否则，如果 T 是类类型，则考虑构造函数.列举适用的构造函数并通过过载分辨率(13.3、13.3.1.7)来选择最佳的.如果转换范围缩小(请参见以下)转换为任何参数，则程序格式不正确.
• [...]

T 是类类型，因此我们考虑 basic_string 构造函数.该列表中的#7(复制构造函数)是适用的可行构造函数，因此应选择它.届时，这些表达式应等效:

  struct Foo {运算符std :: string()const {返回"hello"；}};std :: string s {Foo {}};//错误std :: string s(Foo {});//好的std :: string s = Foo {};//好的 

但是由于某种原因，在列表初始化的情况下，clang抱怨有:

从 Foo 到 const std :: __ cxx11 :: basic_string< char>的未知转换& 作为第一个参数

尽管有，所以我将其归档为 LLVM错误23658 ./p>

Here is the code:

#include <iostream>
#include <string>

using namespace std; 

class Foo { 
public:
    operator string() const { return n; }
    string n {"foo"};
};

int main (int argc, char** argv) {

    string s {Foo{}};
    cout << s << endl;

    return 0;
}

This code compiles using gcc 4.8.3, but it does not compile using clang 3.5, can someone tell me what's wrong with it?

I got an error like this:

main.cpp:45:12: error: no matching constructor for initialization of 'string' (aka 'basic_string<char>')
    string s {Foo{}};
           ^ ~~~~~~~

clang --version:

clang version 3.5.0 (tags/RELEASE_350/final 216961)
Target: x86_64-suse-linux
Thread model: posix

Thanks

解决方案

I believe this is a clang bug. According to the rules for list-initialization in [dcl.init.list]:

List-initialization of an object or reference of type T is defined as follows:

• If T is a class type and the initializer list has a single element of type cv U, where U is T or a class derived from T, [...]
• Otherwise, if T is a character array and [...]
• Otherwise, if T is an aggregate, [...]
• Otherwise, if the initializer list has no elements [...]
• Otherwise, if T is a specialization of std::initializer_list<E>, [...]
• Otherwise, if T is a class type, constructors are considered. The applicable constructors are enumerated and the best one is chosen through overload resolution (13.3, 13.3.1.7). If a narrowing conversion (see below) is required to convert any of the arguments, the program is ill-formed.
• [...]

T is a class type, so we consider the basic_string constructors. #7 in that list (the copy constructor) is an applicable, viable constructor and so it should be chosen. At that point, these expressions should be equivalent:

struct Foo {
    operator std::string() const { return "hello"; }
};

std::string s{Foo{}};  // error
std::string s(Foo{});  // OK
std::string s = Foo{}; // OK

For some reason though, in the list-initialization case, clang complains that there is:

no known conversion from Foo to const std::__cxx11::basic_string<char> & for 1st argument

There is though, so I filed this as LLVM Bug 23658.

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