Using std::sort() without prefix “std” and also without “using namespace std;” compiles successfully

Question

As sort() is defined in namespace std it must always be used as std::sort.But the following code compiles correctly even without

Answer 1

This is argument dependent lookup. According to Stroustroup's The C++ Programming Language: 4th Edition, there are two rules that apply here:

1) If an argument is a member of a namespace, the associated namespaces are the enclosing namespaces.

2) If an argument is a built-in type, there are no associated namespaces.

In your first and second cases, begin() and end() return iterators. However, the C++ standard defines an iterator as any variable of any type, upon which an iteration operation can be performed. (In other words, an iterator is a design concept that is enforced via a template.)

According to the other answer, the iterators in the first case are variables of a data type that belongs to the same namespace as sort(). However, the iterators in the second case have a primitive data type. Per Rule #2, these iterators have no associated namespace.