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. The following code fragment shows an inline function definition.
inline int add(int i, int j)
return i + j;
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.

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A symbol, name, or key that represents a list of commands, actions, or keystrokes. Many programs allow you to create macros so that you can enter a single character or word to perform a whole series of actions. As in the following example we are defining a macro with named PI and it will be replaced by the constant value 3.1415926 wherever PI is used in the program.
#define PI 3.1415926
Here the symbol PI will be replaced with the actual value (i.e. 3.1415926) in the program. These are simple macros like symbolic names mapped to constants.