Rewriting C++ methods in C

Question

How can I make equivalents to these methods in C? I read somewhere that they could be "replaced with functions that take a structure pointer as the first parameter," but I'm not sure how to do this, if that is the right thing to do.

struct SCustomKeys
{
    struct SCustomKey Save[10];
    struct SCustomKey Load[10];
    struct SCustomKey Slot[10];

    struct SCustomKey PrintScreen;
    struct SCustomKey LastItem; // dummy, must be last

    //--methods--
    struct SCustomKey &key(int i) { return ((SCustomKey*)this)[i]; }
    struct SCustomKey const &key(int i) const { return ((SCustomKey*)this)[i]; }
};

Here's an example of how they are used:

void ZeroCustomKeys (SCustomKeys *keys)
{
    int i = 0;

    SetLastCustomKey(&keys->LastItem);
    while (!IsLastCustomKey(&keys->key(i))) {
     keys->key(i).key = 0;
     keys->key(i).modifiers = 0;
     i++;
    };
}

More context: http://pastebin.com/m649210e8

Thanks for the help. I haven't been able to the suggested replacement for the C++ method working with this function yet though. Any ideas on how to approach this?

void InitCustomKeys (struct SCustomKeys *keys)
{
    UINT i = 0;

    SetLastCustomKey(&keys->LastItem);
    while (!IsLastCustomKey(&keys->key(i))) {
     SCustomKey &key = keys->key(i);
     key.key = 0;
     key.modifiers = 0;
     key.handleKeyDown = NULL;
     key.handleKeyUp = NULL;
     key.page = NUM_HOTKEY_PAGE;
     key.param = 0;
     i++;
    };

    //an example key
    keys->PrintScreen.handleKeyDown = HK_PrintScreen;
    keys->PrintScreen.code = "PrintScreen";
    keys->PrintScreen.name = L"Print Screen";
    keys->PrintScreen.page = HOTKEY_PAGE_MAIN;
    keys->PrintScreen.key = VK_PAUSE;
}

And the new function I'm trying now is:

 struct SCustomKey* key(struct SCustomKeys *scs, int i) { 
    return &(((SCustomKey*)scs)[i]); 
 }

Answer 1

 struct SCustomKey &key(int i) { return ((SCustomKey*)this)[i]; }
 struct SCustomKey const &key(int i) const {
     return ((SCustomKey*)this)[i]; 
 }

will become

 struct SCustomKey* key(struct SCustomKeys *scs, int i) { 
    /* Shouldn't you be using one of Load/Save/... arrays? */
    return &(((SCustomKey*)scs)[i]); 
 }

C does not have const functions. So, only one version.

Answer 2

The idea is that you would pass in a pointer to an SCustomKeys as the first (well, it doesn't really matter; it could be any) parameter, and use it in place of "this".

Answer 3

Basically, instead of having a member function like:

struct SCustomKey &key(int i) { return ((SCustomKey*)this)[i]; }

You will need to rewrite this as a function that takes a pointer to a SCustomKeys as it's first argument.

The function will look like:

SCustomKey* key(SCustomKeys* customKeys, int i) 
{ return ((SCustomKey*)(customKeys)+i); }

This should provide you with a pointer to the element you are trying to access.