SDL: Is defining the event struct with every iteration of the event loop inefficient?

Question

This is the example given on the wiki and the way every tutorial I've seen does it...

while (1) {
    SDL_Event event;
    while (SDL_PollEvent(&event)) {
        /* handle your event here */
    }
    /* do some other stuff here -- draw your app, etc. */
    }

If I have this kind of event loop within an update function within my game loop, am I wrong in thinking the program is re-defining and re-allocating a place for a new event struct every iteration of the loop? And am I wrong in thinking that's a inefficient way of doing it? Can't the event struct be defined once elsewhere and filled with different events throughout the life of the program without needing totally redefined every iteration of the update function? And if so where would I want the event struct to be defined? Globally within my game or main source file or what?

What would be the benefit, if any, of having the event allocated on heap?

Attributed to [Donald Knuth](https://st]ackify.com/premature-optimization-evil/): "The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming.” This is a great example :) Yes, the compiler will allocate a new struct each loop iteration (from the stack). No - there is no real "inefficiency" in doing so :) — paulsm4, Oct 22 '20 at 21:05
Thanks a lot, yeah I'm probably worrying too much about making sure I'm doing everything in the absolute most efficient way when I'm still such a beginner. — Nick B, Oct 22 '20 at 21:08

G. Sliepen · Accepted Answer · 2020-10-22T21:08:57.540

4

First, the variable event is declared on the stack, this costs basically nothing. Since it is a POD type, there is no constructor and/or destructor that has to be run. Also, the lifetime of the variable ends at the end of the loop iteration, and will start again at the start of the next iteration. The compiler will see that it can reuse the variable between iterations.

edited Oct 22 '20 at 21:08

answered Oct 22 '20 at 21:06

G. Sliepen

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Ok, now what about an object whose constructor executes the Sieve of Eratosthenes? – Sam Varshavchik Oct 22 '20 at 21:07
2

Yes then that would be different of course. But SDL is a C library, so SDL_Event is a POD. – G. Sliepen Oct 22 '20 at 21:07
Okay, so the same variable is essentially being brought back to life with each iteration? Will it use the same location in memory? – Nick B Oct 22 '20 at 21:10
2

@NickB There is nothing in the C++ standard mandating that it has to be at the same location, but in practice it is. – G. Sliepen Oct 22 '20 at 21:10

SDL: Is defining the event struct with every iteration of the event loop inefficient?

1 Answers1