How to pass two-dimensional array of structures to function?

Question

There is a two-dimensional array of structures, I am passing an array pointer to a function:

result.capabilities = (Capabilities **)malloc(sizeof(Capabilities *)*6);
for(int i=0;i<6;i++){
    result.capabilities[i] = (Capabilities *)malloc(sizeof(Capabilities)*8);
}
init_capabilities(&result.capabilities);

Function call cause an error:

Unhandled exception at 0x003c10f9 in solution.exe: 0xC0000005: Access violation writing location 0xfdfdfdfd.

Here is my function:

void init_capabilities(Capabilities ***capabilities) {
    for(int i=0;i<6;i++){
        for(int j=0;j<8;j++){
            printf("%d %d\n",i,j);
            capabilities[i][j]->room_capabilities = new RoomCapability[rooms_count];
        }
    }
}

I thought that the dimension of the array capabilities - 6x8. It turned out that 1x6. An hour headache because of this. Show you how to change the type of argument or how to refer to elements of my array, so that everything fell into place?

Is this C or C++? Because I think I can help you if you're writing in C++ using new instead of malloc() which C++ provides (which gives you much cleaner, straightforward syntax and is safer than malloc() IIRC). — dmn
– dmn, Commented Oct 27, 2011 at 19:37
This is really bad C/C++ mix code. And when I say bad, I mean it. Don't use malloc in C++. Use references in C++ whenever possible and feasable. — Xeo
– Xeo, Commented Oct 27, 2011 at 19:40

Robᵩ · Accepted Answer · 2011-10-27 19:42:49Z

2

You are passing your pointer-to-pointer-to-Capabilities by pointer. That is why you have three asterisks instead of two.

Try this:

void init_capabilities(Capabilities ***capabilities) {
    for(int i=0;i<6;i++){
        for(int j=0;j<8;j++){
            printf("%d %d\n",i,j);
            // Note: extra dereference:
            (*capabilities)[i][j].room_capabilities = new RoomCapability[rooms_count];
        }
    }
}

Or this:

result.capabilities = (Capabilities **)malloc(sizeof(Capabilities *)*6);
for(int i=0;i<6;i++){
    result.capabilities[i] = (Capabilities *)malloc(sizeof(Capabilities)*8);
}
init_capabilities(result.capabilities); // Note NO address-of operator

// Note: two stars, not three
void init_capabilities(Capabilities **capabilities) {
    for(int i=0;i<6;i++){
        for(int j=0;j<8;j++){
            printf("%d %d\n",i,j);
            capabilities[i][j].room_capabilities = new RoomCapability[rooms_count];
        }
    }
}

Or, since you are coding in C++, not C:

// Assuming that result.capabilities and Capabilties::room_capabilities are declared
// vectors of the appropriate types ...
result.capabilities = std::vector<std::vector<Capabilities> >(std::vector<Capabilities>(8),6);
init_capabilities(result.capabilities);

void init_capabilities(std::vector<std::vector<Capabilities>& capabilities) {
    for(int i=0;i<capabilities.size();i++){
        for(int j=0;j<capabilties[i].size();j++){
            printf("%d %d\n",i,j);
            capabilities[i][j].room_capabilities.resize(rooms_count);
        }
    }
}

edited Oct 27, 2011 at 19:42

answered Oct 27, 2011 at 19:33

Robᵩ

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1 Comment

anatolyg Over a year ago

In init_capabilities, shouldn't it be . instead of ->?

andand · Accepted Answer · 2011-10-27 19:57:38Z

Declare

void init_capabilities(Capabilities ***capabilities)

as

void init_capabilities(Capabilities **capabilities)

and call by

init_capabilities(result.capabilities);

You just need to pass the pointer to your array structure, not a pointer to the pointer of your array structure.

Edit: And as others have pointed out, if you're going to use C++ you should really be using new as in:

result.capabilities = new (Capabilities *)[6];
for(int i=0;i<6;i++) {
  result.capabilities[i] = new Capabilities[8];
}
init_capabilities(result.capabilities);

...

void init_capabilities(Capabilities **capabilities) {
  for(int i=0;i<6;i++) {
    for(int j=0;j<8;j++) {
      capabilities[i][j].room_capabilities = new RoomCapability[rooms_count];
    }
  }
}

And don't forget to use delete[]. Better yet, do as Rob suggests and use the pre-defined C++ containers to handle this sort of thing. But if you really want to use unsafe pointers, what I have above should get it done for you.

Scott Hunter · Accepted Answer · 2011-10-27 19:43:10Z

0

You've got a whole host of problems here:

You allocate space for each individual Capability, then create a new one, turning the allocated space into garbage
You treat capabilities as a 2d array, when it is actually an array of pointers to arrays
You pass the address of this overall array, but do nothing to deference it in the function

Until you get straight what you're trying to do, HOW to do it will have to wait.

answered Oct 27, 2011 at 19:43

Scott Hunter

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