How to execute a piece of code only once?

Question

I have an application which has several functions in it. Each function can be called many times based on user input. However I need to execute a small segment of the code wi

Answer 1

As of C++11, static local variables are thread-safe and usually sufficient for most cases, so std::call_once() et al. very well may be overkill.

This looks especially elegant when using C++17's initialisation-within-if and std::exchange():

#include 

void
do_something_expensive_once()
{
    if ( static auto called = false; !std::exchange(called, true) ) {
        do_something_expensive();
    }
}

If this is a pattern you use a lot, then we can encapsulate it via a tag type:

#include 
#include 

template 
auto
call_once()
{
    static auto called = false;
    return !std::exchange(called, true);
}

void
do_something_expensive()
{
    std::cout << "something expensive\n";
}

void
do_something_expensive_once()
{
    if ( call_once() ) {
        do_something_expensive();
    }
}

auto
main() -> int
{
    for (auto i = 0; i < 5; ++i) {
        do_something_expensive_once();
    }

    return 0;
}

This will only print something expensive a single time. Result! It also uses the ability to declare a tag struct in a template argument list, for maximal brevity.

Alternatively, you could template on a function's address, a unique integer, etc.

You can then also pass a callable to call_once(), and so on, and so forth. As usual for C++: the possibilities are endless!