c++ - 为什么 std::string {"const char ptr"} 有效？

Question

我可以看到 std::string 只有一个 CTOR 为 initializer_list : string (initializer_list<char> il);所以初始化列表应该使用字符，对吧？为什么 std::string{"some_str"}有效，它得到 const char* ，对吧？

Answer 1

n3337 13.3.1.7/1

When objects of non-aggregate class type T are list-initialized(8.5.4), overload resolution selects the constructor in two phases:

— Initially, the candidate functions are the initializer-listconstructors (8.5.4) of the class T and the argument list consists ofthe initializer list as a single argument.

— If no viableinitializer-list constructor is found, overload resolution isperformed again, where the candidate functions are all theconstructors of the class T and the argument list consists of theelements of the initializer list.

std::string 有很多 constructors .其中之一，接收 const char*。

因此，首先编译器将在重载解析中采用 initializer_list c-tor，但当 string 由 const char*< 构造时，它不是可行的候选者，然后编译器会查看其他构造函数并选择最好的一个，即

basic_string( const CharT* s,
              const Allocator& alloc = Allocator() );

你可以用简单的例子来检查它:

#include <initializer_list>
#include <iostream>

class String
{
public:
   String(const std::initializer_list<char>&) { std::cout << "init-list c-tor called" << std::endl; }
   String(const char*) { std::cout << "const char* c-tor called" << std::endl; }
};

int main()
{
   String s{"hello"};
}

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