Let's revisit a core concept in C++: how the compiler finds the names you use in your code. From qualified and unqualified name lookups to the special case of Argument-Dependent Lookup (ADL), understanding these mechanisms is essential for writing clear and correct C++ programs.

Three types of name lookups in C++

by Sandor Dargo

From the article:

Let’s get back to some basics this week and talk about name lookups in C++. In other words: when you refer to a symbol in your code, how does the compiler find it?

Essentially, we can differentiate between three kinds of lookups:

• Qualified name lookup
• Unqualified name lookup
• Argument-Dependent Lookup (ADL)

Let’s explore them in that order.

Qualified Name Lookup

The term qualified refers to symbols that are explicitly scoped using the :: operator. In other words, these are names that appear to the right of a ::, such as x in a::b::x.

Before the compiler can perform a qualified name lookup, it must first resolve the left-hand side of the :: operator. This identifies the namespace or class being referenced.

Qualified name lookup is relatively simple: it only searches the explicitly named scope. It does not search enclosing or outer scopes.