How does std::string construct an object with the name as its constructor argument? [duplicate]

Question

#include <string>          
#include <iostream>        

std::string(foo);          

int main()                 
{                          
    std::cout << foo.size() << "\n";                                                
    return 0;              
}                          

Results in 0, instead of an expected compile error for foo being undefined.

How is it able to do this? What is this called?

Answer 1

std::string(foo);  //#1        

is the same as

std::string (foo); //#2         

is the same as

std::string foo; //#3          

The parentheses in #2 are redundant. They are needed in #1 as there is no whitespace separating std::string and foo.