How to run code inside a loop only once without external flag?

Question

I want to check a condition inside a loop and execute a block of code when it's first met. After that, the loop might repeat but the block should be ignored. Is there a pattern for that? Of course it's easy to declare a flag outside of the loop. But I I'm interested in an approach that completely lives inside the loop.

This example is not what I want. Is there a way to get rid of the definition outside of the loop?

bool flag = true;
for (;;) {
    if (someCondition() && flag) {
        // code that runs only once
        flag = false;
    }        
    // code that runs every time
}

Answer 1

It's fairly hacky, but as you said it's the application main loop, I assume it's in a called-once function, so the following should work:

struct RunOnce {
  template <typename T>
  RunOnce(T &&f) { f(); }
};

:::

while(true)
{
  :::

  static RunOnce a([]() { your_code });

  :::

  static RunOnce b([]() { more_once_only_code });

  :::
}

Answer 2

For a less convoluted version of Mobius's answer:

while(true)
{
  // some code that executes every time
  for(static bool first = true;first;first=false)
  {
    // some code that executes only once
  }
  // some more code that executes every time.
}

You could also write this using ++ on a bool, but that's apparently deprecated .

Answer 3

a possibly cleaner way to write this, albeit still with a variable, would be as follows

while(true){
   static uint64_t c;
   // some code that executes every time
   if(c++ == 0){
      // some code that executes only once
   }
   // some more code that executes every time.
 }

The static allows you to declare the variable inside the loop, which IMHO looks cleaner. If your code that executes every time makes some testable change, you could get rid of the variable and write it like this:

while(true){
   // some code that executes every time
   if(STATE_YOUR_LOOP_CHANGES == INITIAL_STATE){
      // some code that executes only once
   }
   // some more code that executes every time.
 }

Answer 4

If you know you only want to run this loop once, why not use break as the last statement in the loop.

Answer 5

1    while(true)
2    {
3        if(someCondition())
4        {
5            // code that runs only once
6            // ...
7          // Should change the value so that this condition must return false from next execution.
8        }        
9    
10        // code that runs every time
11        // ...
12    }

If you expecting the code without any external flag then you need to change the value of condition in last statement of the condition. (7th line in code snippet)