C ++ getline（）不需要命名空间声明

Question

Why is getline() from header string in local scope and can be used:

#include <iostream>
#include <string>

int main() {
    std::string str;
    getline(std::cin, str);
    std::cout << str << "\n";
    return 0;
}

That works with gcc. But why? It is defined in header string , which should required me to use std::getline() instead of getline() .

Answer 1

You're experiencing Argument Dependent Lookup (ADL, and also referred to as Koenig Lookup). Since one or more of the arguments is a type defined in the std namespace, it searches for the function in the std namespace in addition to wherever else it would search. I point you to Stephan T. Lavavej's video to learn more about it and name lookup in general.