How to solve "Access violating reading location error" in c++? - c++

Question

Goodday all,
I'm getting the following error:
First-chance exception at 0x67887AB7 (SDL2_mixer.dll) in Racing.exe: 0xC0000005: Access violation reading location 0xCCCCCCD4.
I think the problem could be with the pointer, but I'm not experienced enough in c++ to find it.
I'm hoping someone can tell me what I did wrong, and hopefully I can learn from it :)
Trying to call it from my gamebus.
Music Mus;
Mus.SpeelGeluid("crash");
Music class
bool Music::LoadMusic(){
//Load music
Mix_Music *gMusic = NULL;
gMusic = Mix_LoadMUS("MusicTest.wav");
Mix_PlayMusic(gMusic, -1);
bool success = true;
if (gMusic == NULL)
{
printf("Failed to load Music background song! SDL_mixer Error: %s\n", Mix_GetError());
success = false;
}
// Load sound effects
Mix_Chunk *gCrash = NULL;
gCrash = Mix_LoadWAV("Crash.wav");
if (gCrash == NULL)
{
printf("Failed to load scratch sound effect! SDL_mixer Error: %s\n", Mix_GetError());
success = false;
}
return success;
}
void Music::SpeelGeluid(string soundname){
cout << soundname << endl;
if (soundname == "crash")
{
try
{
Mix_PlayChannel(-1, gCrash, 0);
}
catch (int e)
{
cout << "An exception occurred. Exception Nr. " << e << '\n';
}
}
else
{
} }
Thank you for your time

Answer 1

You have two major problems:
First, you're using gMusic before checking whether it's NULL.
This may crash Mix_PlayMusic, and actually means that compilers may optimise out the later NULL check.
Second, and most likely the cause of your problem, you're declaring gCrash and gMusic as local variables in LoadMusic.
From the names, and your use of gCrash in SpeelGeluid, my educated guess is that you also have two global variables, or possibly member variables, with the same names where you intend to store the results of loading the files.
Your local variables are hiding these globals, and you're only modifying the local variables.
Remove the lines
Mix_Music *gMusic = NULL;
and
Mix_Chunk *gCrash = NULL;
to get rid of these local variables.
And do initialise the globals to NULL when you define them.

Answer 2

Check your pointers as soon as you've allocated them and handle them appropriately. You nearly do:
gMusic = Mix_LoadMUS("MusicTest.wav");
Mix_PlayMusic(gMusic, -1); // <- You're using the pointer here before you check it
bool success = true;
if (gMusic == NULL) // <- this needs to be immediately after assignment
That's just from the code you've pointed. It's also possible you've missed a generic initialisation call from the SDL2_mixer.dll.
Do debug it and step through line by line so you know which bit is causing the error.