SDL_Keycodes are too big for storage

Question

While searching up methods of detecting multiple keys at once in SDL 2, I came across this piece of code for SDL 1.x:

//author: Rob Loach
// Global key buffer
bool keys[256];

while(SDL_PollEvent(&mainEvent))
{
   if(mainEvent.type == SDL_KEYDOWN)
   {
       keys[mainEvent.key.keysym.sym] = true;
   }
   else if(mainEvent.type == SDL_KEYUP)
   {
       keys[mainEvent.key.keysym.sym] = false;
   }
}

I tried implementing it in SDL2 and a std::array<bool, 256>, but I had Segmentation fault: 11 with the up button.

That's when I looked at this: https://wiki.libsdl.org/SDLKeycodeLookup.

Most of the 'special' keys including arrow, function, symbols, and so on have decimal representations in the billions.

Even with the simple code printf("%d\n", e.key.keysym.sym); on, say the up button gives:

1073741906
Segmentation fault: 11

I am on a Mac, if it makes any difference with the error codes.

So, what solutions are there to this in SDL 2?

You could use a `std::map`, that way you won't run into that problem. I don't think there is much to help you in `SDL2` specifically though. — olevegard, Jun 13 '15 at 06:54
@olevegard Perfect! Would you mind writing that as an answer? — hpm, Jun 13 '15 at 07:06

olevegard · Accepted Answer · 2016-04-08T06:43:29.107

First of all, bools don't default to anything in C++, you need to initialize them. The fact that they appear to always be true is that they're byte in size. Which means they have a size between 0 and 255 inclusive. Only 0 would mean false so its a 255 / 256 chance of true.

As for your solution, you would simply define your std::map as this :

std::map<SDLKey, bool> keyMap;

An std::map is initially empty, so you need to check that the items actually exists when you try to look it up.

bool IsKeyDown(SDLKey key)
{
    // Look for element
    auto it = keyMap.find(key);

    if (it == keyMap.end())
        // No element found, which means this key hasn't been pressed
        return false;

    // 'it' is an iterator, so we use * to return its value
    return it->second; 
}

When you try to set an item, it will automtically get created if it doesn't exists :

bool SetIsKeyDown(SDLKey key, bool isDown)
{
    keyMap[key] = isDown
}

So the fact the std::map is initially empty means you don't need to fill it. You can if you want to, but it's not required like it is with an array.

Changed some things in a edit. `it` is a std::pair, with the first object containing the key and the second object containing the value, therefore, `return (*it);` will not work, `return it->second;` will work. Also, `it` will be equal to `keyMap.end();` if it is *NOT* found, therefore, the correct code is `if( it == keyMap.end();`. Thanks for a great answer though :) — ThatPixelCherry, Apr 08 '16 at 04:59
@ThatPixelCherry thanks, I should learn to run my code through a compiler before I submit... — olevegard, Apr 08 '16 at 06:44

SDL_Keycodes are too big for storage

1 Answers1