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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.

A function is declared inline by using the inline function specifier or by defining a member function within a class or structure definition. 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.

Advantages :- 

1) It does not require function calling overhead.

2) It also save overhead of variables push/pop on the stack, while function calling.

3) It also save overhead of return call from a function.

4) It increases locality of reference by utilizing instruction cache.

5) After in-lining compiler can also apply intra-procedural optimization if specified. 

This is the most important one, in this way compiler can now focus on dead code elimination, can give more stress on branch prediction, induction variable elimination