make_pair namespace pollution

Question

In the code I've been writing recently, I've noticed a strange behaviour.

When I use make_pair with the first argument being an std::pair, make_pair becomes "magically" available in the namespace (I don't have to use an std:: qualifier)

#include <iostream>

int main()
{   
    int i1 = 2; int i2 = 10; int i3 = 0;

    // constructing a pair using std::make_pair, everything's okay
    std::pair<int,int> key = std::make_pair(i1, i2);

    // here, why is make_pair suddenly magically available without the
    // std:: namespace qualifier?
    // At the same time, using ::make_pair yields and error
    // (make_pair has not declared...)
    std::pair<std::pair<int,int>, int> mypair = make_pair(key, i3);

    std::cout << mypair.first.first << "\n";
    std::cout << mypair.first.second << "\n";
    std::cout << mypair.second << "\n";

    return 0;
}

The compiles just fine (with -Wall and -pedantic-errors) and outputs:

2
10
0

Why is this happening? I've looked into cppreference and didn't find any hint of this behaviour being correct. Am I missing anything?

FYI, I'm using gcc 4.6.3

Answer 1

That's a less-known feature of C++, as @jrok pointed out blazingly fast, Koenig Lookup, or in modern C++ ¹⁾, ADL (Argument-Dependent Lookup). What it does is basically searches in the namespaces of the arguments for the function that you want to call (make_pair in this example). The argument triggering the ADL is obviously std::pair.

¹⁾_{the naming has been changed, though a lot of people know the first term}

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Perhaps it's worth mentioning that ADL is quite important for one particular type of function: operators. If not for ADL, it would be impossible for even the trivial C++ "hello, world!" to work, because this:

std::cout << "Hello, world!";

Would have to be written as this:

std::operator<< (std::cout, "Hello, world!");

Thanks to ADL, << is properly resolved to be in std namespace.

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References:

• Wikipedia Article
• Great video by Stephan T. Lavavej, Core C++ Part 1
• A broader look on ADL and its uses by Herb Sutter
• It can be found in The C++ Standard (I was using n3485 version) in § 3.4.2