为什么const char *强制转换为std :: string有效？

Question

This cast puzzles me:

#include <string>
#include <iostream>
#include <memory>

using namespace std;

int main() {
    string str1 =  (string)"I cast this thing" +  " -- then add this";
    cout << str1 << endl;
}

Can someone explain why this c-style cast to string works (or is allowed)? I compared the generated optimized assembly with that from:

string str1 =  string("I construct this thing") +  " -- then add this";

and they appear to be identical, so I feel like I'm forgetting some c++ semantics that actually allow this kind of cast/construction to be interchanged.

 std::string str2 =  std::string("I construct this thing") +  " -- then add this";

Answer 1

Yep, if you have a constructor that takes in a single argument like that it will be used to cast the argument type to the object type. This is why we can pass const char* to functions taking strings.

Converting constructor

Answer 2

A C-style cast will do a const cast and static cast or reinterpret cast whichever is possible.

A static cast will use a user-defined conversion if defined.

std::string has a constructor string(const char *) .

The syntaxes std::string("something") , static_cast<std::string>("something") and (std::string)"something" are equivalent. They will all construct a temporary std::string using the std::string::string(const char *) constructor. The only difference between the syntaxes would be when casting pointers.

Answer 3

std::string has a constructor in the form of

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

This acts as a conversion operator. If you cast a string literal to a std::string it will call this constructor and create a temporary std::string