Why does a const char* cast to std::string work?

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";

(T)x = T(x). The two are identical except in name (C-style cast vs function-style cast). — Konrad Rudolph
– Konrad Rudolph, Commented Jan 12, 2016 at 17:58

Jan Hudec · Accepted Answer · 2016-01-12 18:01:52Z

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.

makes complete sense...for some reason I always forget about the "valid user defined conversions" with respect to static casting.

anonymous coward · Accepted Answer · 2016-01-12 17:55:13Z

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

NathanOliver · Accepted Answer · 2016-01-12 18:10:54Z

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

Collectives™ on Stack Overflow

Why does a const char* cast to std::string work?

3 Answers 3

1 Comment

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Linked

Related