Argument-dependent Name Lookup - Example

Example

An example of ADL looks like this:

namespace NS { class A {}; void f( A *&, int ) {} } int main { NS::A *a; f( a, 0 ); //calls NS::f }

A common pattern in the C++ Standard Library is to declare overloaded operators that will be found in this manner. For example, this simple Hello World program would not compile if it weren't for ADL:

#include int main { std::cout << "Hello World, where did operator<< come from?" << std::endl; return 0; }

Using << is equivalent to calling operator<<, which however lacks the std qualifier. In this case, function std::ostream& std::operator<<(std::ostream&, const char*) is found through ADL.

Note that std::endl is a function but it needs full qualification, since it is used as an argument to operator<< (std::endl is a function pointer, not a function call).

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