passing pointers - c++11

Question

I have this declared above:
char PandaImage[] = "images/panda.png";
SDL_Texture* PandaTexture = nullptr;
I have a function to create textures:
void LoadMedia( SDL_Texture *ThisTexture, char *Image )
{
SDL_Surface* TempSurface = nullptr;
.......................
ThisTexture = SDL_CreateTextureFromSurface( gRenderer, TempSurface );
I call it as:
LoadMedia( PandaTexture, PandaImage );
It builds, logs the image loaded and texture created, but no image
If I hard change the line ( use Panda directly instead of This ):
PandaTexture = SDL_CreateTextureFromSurface( gRenderer, TempSurface );
My image is there.
I have always had trouble with & * and passing.
Is there a good, simple help for me?
Thanks for your kind help - back to Google for now

Answer 1

In short, I think you could solve your problem by changing the function to:
void LoadMedia( SDL_Texture** thisTexture, char* Image)
{
...
(*thisTexture) = SDL_CreateTextureFromSurface( gRenderer, TempSurface);
}
And by calling the function using:
LoadMedia( &PandaTexture, PandaImage);
An explanation:
Variables and Pointers
A variable is used to store data (a primitive or a class instance). For example:
int a = 10;
stores an integer in memory. This means, that symbol 'a' now represents number 10, which is stored somewhere in your computer's memory as 4 bytes.
A pointer is used to store an address (this address points towards a variable). For example:
int* a_address = 1234;
says that there is an integer stored at address 1234 in your computer's memory. A pointer always takes up the same amount of space (4 bytes on a 32 bit machine and 8 bytes on a 64 bit machine), as it simply stores an address.
Getting the Address of a Variable [&]
You will rarely ever set the address of a pointer yourself. Often, pointers are the result of a "new" call. Using "new" reserves memory to store an instance of the class you want to create, and returns the address of the object. In essence, it says: "I created an object for you, and you can find it at this location in your memory".
Alternatively, when you have a normal variable (primitive of class instance), you can find its address by using the & character. For example:
int a = 10;
int* a_address = &a;
says: "store the location of variable a in pointer a_address. Why would you do this? Say you have a very large instance (for example an SDL_Texture consisting of many, many pixels) and you want to pass it to a function (or pass it back outside of the function). If you were to pass it to the function as SDL_Texture thisTexture, you are copying the entire object (a so-called pass by value). This is time consuming. Alternatively, you could simply pass the address to the function, as an SDL_Texture * thisTexture. This is a so called pass by reference, and it is much faster as you can imagine.
Getting the Variable at an Address [*]
Obviously, if you have an address, you also need a way to get the actual variable at that address. This is done using the * character. It is called "dereferencing". For example:
int a = 10;
int* a_address = &a;
int b = (*a_address);
This last line says: "Give me the variable, stored at address a_address, and put it in b".
Function Parameters Going Out-of-scope
When a function ends, its local variables (including parameters) go out-of-scope. This means that their memory is freed (for variables, not for dynamically allocated objects stored as pointers!). Their values will be forgotten. In your case, you are passing an SDL_Texture * as a parameter. This means, a copy is made of the address stored in PandaTexture. This address is copied over to thisTexture. You then write the return value of SDL_CreateTextureFromSurface to thisTexture. Next the function ends, and thisTexture goes out-of-scope. As a result, the location of your SDL_Texture (the SDL_Texture * pointer) is lost forever. You actually want to store the address to pointer PandaTexture, but as you can see, the address is only written to thisTexture.
Solution: How to Fix your Function
We can fix this by passing a pointer, to your pointer called PandaTexture. A "pointer to a pointer" is written as:
SDL_Surface** thisTexture;
We want to pass the address of pointer PandaTexture to this. This way, we can write to PandaTexture from inside your method! After all, we know where PandaTexture stores its pointer in memory, allowing us to change it. To actually put the address of PandaTexture in it, we need to use the & character in the function call as such:
LoadMedia(&PandaTexture, PandaImage);
Next, inside of our function, we want to change the value of PandaTexture. However, we were passed &PandaTexture and not PandaTexture itself. To write the value of &PandaTexture (the address where our texture will be stored), we need dereferencing, as such:
(*thisTexture) = SDL_CreateTextureFromSurface(gRenderer, TempSurface);
This works because: "thisTexture is a pointer to a pointer to an SDL_Texture (aka an SDL_Texture**). By dereferencing it, we obtain a pointer to an SDL_Texture (aka an SDL_Texture*). Here we can store the return value of the SDL_CreateTextureFromSurface function.
Why do we not run into out-of-scope issues here? Parameter thisTexture will still go out of scope, and its value will be forgotten. But! We didn't write to thisTexture, instead we wrote our SDL_Texture * pointer to the address that thisTexture points to! This bit of memory is not cleared due to scoping, so we can view the results from outside the function!
In summary, you can solve your problem using a pointer to a pointer. I hope the above clears up the concepts of pointers, variables, addresses and dereferencing a bit!