What is “Argument-Dependent Lookup” (aka ADL, or “Koenig Lookup”)?

Question

What are some good explanations on what argument dependent lookup is? Many people also call it Koenig Lookup as well.

Preferably I\'d like to know:

• Why

Answer 1

In Koenig Lookup, if a function is called without specifying its namespace, then the name of a function is also searched in namespace(s) in which the type of the argument(s) is defined. That is why it is also known as Argument-Dependent name Lookup, in short simply ADL.

It is because of Koenig Lookup, we can write this:

std::cout << "Hello World!" << "\n";

Otherwise, we would have to write:

std::operator<<(std::operator<<(std::cout, "Hello World!"), "\n");

which really is too much typing and the code looks really ugly!

In other words, in the absence of Koenig Lookup, even a Hello World program looks complicated.