Where is getline() function coming from?

Question

I am starting learning C++ a little while ago. Came to know about header files and preprocessor statements.

I know that std::cin and std::cout are the objects/Function is declared in standard library IOSTREAM. But when taking input as a string and to read the whole line. We have to getline() function or at least the instructor is using it in the video.

Now I have checked on the internet and most of the sites are showing that getline() is defined under STRING file/Library. But the thing is my program is working perfectly fine even without including the string file. So what I am missing.? or doing something wrong. and if you can please also explain how getline function is working. and also please what's the actual difference between using namespace std, and using std:: Thank you

#include <iostream> 

int main() 
{ 
    std::string str; 
  
    std::cout << "Please enter your name: \n"; 
    getline(std::cin, str); 
    std::cout << "Hello, " << str 
         << " welcome to GfG !\n"; 
  
    return 0; 
} 

Answer 1

You have to include the header <string> to use the function std::getline. It is implementation defined whether the header <iostream> includes the header <string>.

In this call using unqualified name getline

getline(std::cin, str);

the compiler applies the argument dependent lookup ADL (the first argument std::cin is defined in the namespace std::) and finds the corresponding name std::getline in the namespace std::.