In-line Function in C++

What is inline functions?:

  • The inline functions are a C++ enhancement feature to increase the execution time of a program.

  • It reduces the function call overhead.

  • Inline function is a function that is expanded in line when it is called.

  • When the inline function is called whole code of the inline function gets inserted or substituted at the point on inline function call.

  • This substitution is performed by C++ compiler at compile time.

  • Inline function may increase efficiency if it is small.

  • To declare a function as inline use the keyword inline before the function name.


inline return-type function-name( parameter-list)
   //function definition

Note:A function definition in a class definition is an inline function definition, even without the use of the inline specifier.


Below is an inline function that finds the cube of a number.

using namespace std;
inline int cube(int x)
   return x*x*x;
int main() {
  int n, res;
  cout<<"Enter Number :";
  res = cube(n);
  cout<<" CUBE of "<< n;
  cout<<" is :"<< res;
  cout<< endl;
  return 0; 

Inline is only a request to the compiler, not a command.

Compiler may not perform inlining in such cases:--
  1. If a function contains a loop.
  2. If a function contains static variable.
  3. If a function is recursive.
  4. If a function contains switch or goto statement.

Inline Function Advantages:

  1. Function call overhead does not occur.
  2. It also saves the overhead of push/pop variable on the stack when function is called.
  3. It also saves overhead of return call from function.

Inline Function Disadvantages:

  1. If you use too many inline functions the the size of the binary executable file will be large, because of duplication of the same code.
  2. Inline function may increase compile time overhead.
  3. Too much inlining can reduce your instruction cache hit rate.

Next topic is Function Overloading


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