The inline functions are a C++ enhancement feature to increase the execution time of a program. Compiler replace the definition at compile time instead of referring function defination at runtime.

NOTE - This is a suggestion to compiler to make the function inline. If function is big(in term of instruction etc), compiler can ignore the "inline" request and treat it as normal function.

To make an function as inline, start the defination with the keyword "inline"

Class A{public:    inline int add(int a, int b){        return (a + b);    }};Class B{public:    int add(int a, int b);};inline int B::add(int a, int b){    return (a + b):}

It is just a suggestion, not compulsion. Compiler may or may not inline functions you marked as inline. It may also decide to inline functions not marked as inline at compilation or linking time.

All the member functions declared and defined within class are inline by default. So no need to define explictly.

Reference:

What is C++ inline functions, cplusplus.com

[c++] Inline Function