Why can I call getline without using std::getline?

Question

I am following the C++ Primer book and trying out all the code examples. I am intrigued by this one:

#include <iostream>
#include <string>

using std::string;
using std::cin;
using std::cout;
using std::endl;

int main()
{
    string line;
    while (getline(cin,line))
        cout << line << endl;
    return 0;
}

Before compiling this code I was guessing that the compilation would fail, since I am not using

while (std::getline(cin,line))

Why is getline in the global namespace? As I understand, this should only happen if I used

namespace std;

or

using std::getline;

I am using g++ version 4.8.2 on Linux Mint Debian Edition.

Answer 1

This is argument dependent lookup.

Unqualified lookup (what you are doing when you just call getline() instead of std::getline()) will start by trying to do normal name lookup for getline. It will find nothing - you have no variables, functions, classes, etc. in scope with that name.

We will then look in the "associated namespaces" of each of the arguments. In this case, the arguments are cin and line, which have types std::istream and std::string respectively, so their associated namespaces are both std. We then redo lookup within namespace std for getline and find std::getline.

There are many more details, I encourage you to read the reference I cited. This process is additionally known as Koenig lookup.

Answer 2

Since std::getline() for std::string is defined in the header I would have to say that Argument-dependent lookup is coming into play.