# Basic concepts

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This section provides definitions for the specific terminology and the concepts used when describing the C++ programming language.

A C++ program is a sequence of text files (typically header and source files) that contain declarations. They undergo translation to become an executable program, which is executed when the OS calls its main function.

Certain words in a C++ program have special meaning, and these are known as keywords. Others can be used as identifiers. Comments are ignored during translation. Certain characters in the program have to be represented with escape sequences.

The entities of a C++ program are values, objects, references, functions, enumerators, types, class members, templates, template specializations, namespaces, parameter packs, and the this pointer. Preprocessor macros are not C++ entities.

Entities are introduced by declarations, which associate them with names and define their properties. The declarations that define all properties required to use an entity are definitions. A program must contain only one definition of any non-inline function or variable that is odr-used.

Definitions of functions include sequences of statements, some of which include expressions, which specify the computations to be performed by the program.

Names encountered in a program are associated with the declarations that introduced them using name lookup. Each name is only valid within a part of the program called its scope. Some names have linkage which makes them refer to the same entities when they appear in different scopes or translation units.

Each object, reference, function, expression in C++ is associated with a type, which may be fundamental, compound, or user-defined, complete or incomplete, etc.

Named objects and named references to objects are known as variables.

### See also

C documentation for Basic concepts

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