But a compiler is free to decide, if a function qualifies to be an inline function. If the inline function is found to have larger chunk of code, it will not be treated as an inline function, but as like other normal functions.

   Inline functions are treated like macro definitions by the C++ compiler. They are declared with the keyword inline as follows. 

   //Declaration for C++ Tutorial inline sample:
    int add(int x,int y);

   //Definition for C++ Tutorial inline sample:
    inline int add(int x,int y)
    {
        return x+y;
    }
    In fact, the keyword inline is not necessary. If the function is defined with its body directly and the function has a smaller block of code, it will be automatically treated as inline by the compiler. 

   As implied, inline functions are meant to be used if there is a need to repetitively execute a small block of code, which is smaller. When such functions are treated inline, it might result in a significant performance difference.