7.5 — Inline functions

The use of functions provides many benefits, including:

  • The code inside the function can be reused.
  • It is much easier to change or update the code in a function (which needs to be done once) than for every in-place instance. Duplicate code is a recipe for disaster.
  • It makes your code easier to read and understand, as you do not have to know how a function is implemented to understand what it does.
  • The function provides type checking.

However, one major downside of functions is that every time a function is called, there is a certain amount of performance overhead that occurs. This is because the CPU must store the address of the current instruction it is executing (so it knows where to return to later) along with other registers, all the function parameters must be created and assigned values, and the program has to branch to a new location. Code written in-place is significantly faster.

For functions that are large and/or perform complex tasks, the overhead of the function call is usually insignificant compared to the amount of time the function takes to run. However, for small, commonly-used functions, the time needed to make the function call is often a lot more than the time needed to actually execute the function’s code. This can result in a substantial performance penalty.

C++ offers a way to combine the advantages of functions with the speed of code written in-place: inline functions. The inline keyword is used to request that the compiler treat your function as an inline function. When the compiler compiles your code, all inline functions are expanded in-place -- that is, the function call is replaced with a copy of the contents of the function itself, which removes the function call overhead! The downside is that because the inline function is expanded in-place for every function call, this can make your compiled code quite a bit larger, especially if the inline function is long and/or there are many calls to the inline function.

Consider the following snippet:

This program calls function min() twice, incurring the function call overhead penalty twice. Because min() is such a short function, it is the perfect candidate for inlining:

Now when the program compiles main(), it will create machine code as if main() had been written like this:

This will execute quite a bit faster, at the cost of the compiled code being slightly larger.

Because of the potential for code bloat, inlining a function is best suited to short functions (eg. no more than a few lines) that are typically called inside loops and do not branch. Also note that the inline key word is only a recommendation -- the compiler is free to ignore your request to inline a function. This is likely to be the result if you try to inline a lengthy function!

7.6 -- Function overloading
7.4a -- Returning values by value, reference, and address

28 comments to 7.5 — Inline functions

  • Ashu

    I have Little doubt regarding Inline function:

    void main()
    int i=2;

    inline void fun()
    int i=4;
    printf("%d", i);

    As compiler make copy of this fun() function and insert inside main function body. why compiler not give error of variable int i re declaration.

  • dice3000

    I used to thing that this was done by default to all functions. Now I don't :)

  • silencedwind

    damn, won't paste correctly, but hopefully you get the idea.

  • silencedwind

    using namespace std;

    int main()
    for (int iii = 4; iii > 0; iii--)
    for (int jjj = 1; jjj <= iii; jjj++)
    cout << jjj * iii << " ";
    cout <> end;
    return 0;

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