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

Additionally to @Basile's answer, you can use a lambda to encapsulate the static variable as follows:

if ([] {
    static bool is_first_time = true;
    auto was_first_time = is_first_time;
    is_first_time = false;
    return was_first_time; } ()) 
{ 
    // do the initialization part
}

This makes it easy to convert into a general-purpose macro:

#define FIRST_TIME_HERE ([] { \
    static bool is_first_time = true; \
    auto was_first_time = is_first_time; \
    is_first_time = false; \
    return was_first_time; } ())

Which can be placed anywhere you want call-by-need:

if (FIRST_TIME_HERE) {
    // do the initialization part
}

And for good measure, atomics shorten the expression and make it thread-safe:

#include <atomic>
#define FIRST_TIME_HERE ([] { \
    static std::atomic<bool> first_time(true); \
    return first_time.exchange(false); } ())

Answer 2

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 <utility>

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 <iostream>
#include <utility>

template <typename T>
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<struct TagForSomethingExpensive>() ) {
        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!

Answer 3

Use global static objects with constructors (which are called before main)? Or just inside a routine

static bool initialized;
if (!initialized) {
   initialized = true;
   // do the initialization part
}

There are very few cases when this is not fast enough!

---

addenda

In multithreaded context this might not be enough:

You may also be interested in pthread_once or constructor function __attribute__ of GCC.

With C++11, you may want std::call_once.

You may want to use <atomic> and perhaps declare static volatile std::atomic_bool initialized; (but you need to be careful) if your function can be called from several threads.

^{But these might not be available on your system; they are available on Linux!}

Answer 4

Compact version using lambda function:

void foo()
{
    static bool once = [](){
        cout << "once" << endl;
        return true;
    } ();
    cout << "foo" << endl;
}

Code within lambda function is executed only once, when the static variable is initialized to the return value of lambda function. It should be thread-safe as long as your compiler support thread-safe static initialization.

Answer 5

Using C++11 -- use the std::call_once

#include <mutex>

std::once_flag onceFlag;

{
    ....
    std::call_once ( onceFlag, [ ]{ /* my code body here runs only once */ } );
    ....
}

Answer 6

You can use local static variable:

void foo()
{
     static bool wasExecuted = false;
     if (wasExecuted)
         return;
     wasExecuted = true;

     ...
}