The inline function specifier

An inline function is one for which the compiler copies the code from the function definition directly into the code of the calling function rather than creating a separate set of instructions in memory. Instead of transferring control to and from the function code segment, a modified copy of the function body may be substituted directly for the function call. In this way, the performance overhead of a function call is avoided. Using the inline specifier is only a suggestion to the compiler that an inline expansion can be performed; the compiler is free to ignore the suggestion.

Any function, with the exception of main, can be declared or defined as inline with the inline function specifier. Static local variables are not allowed to be defined within the body of an inline function.

C++ functions implemented inside of a class declaration are automatically defined inline. Regular C++ functions and member functions declared outside of a class declaration, with the exception of main, can be declared or defined as inline with the inline function specifier. Static locals and string literals defined within the body of an inline function are treated as the same object across translation units; see Linkage of inline functions for details.

The following code fragment shows an inline function definition:

inline int add(int i, int j) { return i + j; }

The use of the inline specifier does not change the meaning of the function. However, the inline expansion of a function may not preserve the order of evaluation of the actual arguments.

The most efficient way to code an inline function is to place the inline function definition in a header file, and then include the header in any file containing a call to the function which you would like to inline.

Related information

• The noinline function attribute
• LANGLVL in the ILE C/C++ Compiler Reference