Spawn 2 enemies per second in GLUT - c++

Question

I'm making a game in openGL and I want to spawn 2 enemies per second on the screen. The problem is, I'm calling the spawn function inside idle:
idle
{
// ...
world.spawnEnemy();
}
And then, in the spawnEnemy function:
void World::spawnEnemy()
{
Enemy newEnemy, newEnemy2;
float start;
start = glutGet(GLUT_ELAPSED_TIME);
// 1/2 a second passes
while ( (glutGet(GLUT_ELAPSED_TIME) - start) <= 500 )
{
;
}
// create enemy
newEnemy();
pushToEnemyList(newEnemy);
// another 1/2 second passes
while ( (glutGet(GLUT_ELAPSED_TIME) - start) <= 1000 )
{
;
}
// create enemy
newEnemy2();
pushToEnemyList(newEnemy2);
}
And this is not working, of course. Probably because idle is called everytime and I'm calling a function inside idle that waits for a certain amount of time to pass and then everything starts going wrong. The solution is to do the spawning in the idle function (without calling any function, doing all the logic inside idle), but I don't like this idea. I need to do more things in a certain number of time, for example, my enemies have guns and they will have to shoot n times every second. And if I do all of these directly inside of idle it will become a mess. So, basically, how do I make this work:
idle
{
// ...
world.spawnEnemy();
world.enemyShoot();
// another functionalities that depend on time
}
instead of this:
idle
{
// ...
// logic of spawnEnemy directly here in the idle function
// logic of enemyShoot directly here in the idle function
// logic of another functionalities that depend on time directly here in the idle function
}

Answer 1

There is quite a few ways to do this here the most common:
Measure elapsed time
Simply remember time of last spawn t0 and on each (idle) iteration get current time t. If t0+T<=t then spawn new enemy ans set t0=t. The T is the period of spawning in the same units as the measured time. In your case 500ms.
As mentioned OnIdle Event will take 100% of one CPU core for itself. To remedy this use Sleep(x); where x<<T that will ease up on the CPU load (even Sleep(1); will be a big difference).
To measure time use any OS routine you got available (with high enough accuracy) I am using PerformanceCounter on Windows but there are other alternatives like RDTSC etc ...
Timers
Modern messaging based OS provides timer function which will fire your event periodically (usually with ~1ms accuracy). Just study your API to implement it. Then you just spawn your enemy inside timer event without any other logic or timing code ...
In case you do not have any timers you can fake this by creating thread with code like this inside:
for (;!spawn_exit;)
{
if (spawn_enabled) spawn();
Sleep(T);
}
where volatile bool spawn_enabled=true,spawn_exit=false; are used to control the spawning and stop the thread (before App shutdown) But in this case be careful with multi threaded access in the spawn() function ...