C++ In a Nutshell

Declarations and Definitions

A declaration is the all-encompassing term for anything that tells the compiler about an identifier. In order to use an identifier, the compiler must know what it means: is it a type name, a variable name, a function name, or something else? Therefore, a source file must contain a declaration (directly or in an #include file) for every name it uses.

Definitions

A definition defines the storage, value, body, or contents of a declaration. The difference between a declaration and a definition is that a declaration tells you an entity’s name and the external view of the entity, such as an object’s type or a function’s parameters, and a definition provides the internal workings of the entity: the storage and initial value of an object, a function body, and so on.

In a single source file, there can be at most one definition of an entity. In an entire program, there must be exactly one definition of each function or object used in the program, except for inline functions; an inline function must be defined in every source file that uses the function, and the definitions must all be identical.

A program can have more than one definition of a given class, enumeration, inline function, or template, provided the definitions are in separate source files, and each source file has the same definition.

These rules are known as the One Definition Rules, or ODR.

Before you can use an entity (e.g., calling a function or referring to an object), the compiler needs the entity’s ...