Code with Templates not Linking 1

[netprik1]

I'm having a problem getting templates to work in a program. I haven't used them for a while, so maybe I'm confused somehow as to how to use them correctly. I looked it up and it seems that I have the right syntax though. I do not get a compile error. I get a linker error. I'm using Deviant C++ by the way, but I don't think this is an issue specific to the compiler. Here's the code that's giving me problems...

In a file called arrays.cpp, I have this:

#include "arrays.h"

template <typename T>  GenericArray2DStruct<T>* allocate2DArray (unsigned int xs, unsigned int ys) {
    int i;
    //allocate the struct itself
    GenericArray2DStruct <T> *prod =  (GenericArray2DStruct <T>*) malloc(sizeof (GenericArray2DStruct <T>));
    if (prod == NULL) {
        printf ("Insufficient memory to execute!");  }
    //allocate the array of pointers to arrays
    prod->data =(T**) malloc(sizeof(T*)*ys);
    if (prod->data == NULL) {
        printf ("Insufficient memory to execute!");  }
    //allocate each of the arrays
    for (i=0;i<ys;i++) {
        prod->data [i] = (T*) malloc(sizeof(T)*xs);
        if (prod->data [i] == NULL) {
        printf ("Insufficient memory to execute!");  }}
    prod->xSize = xs;
    prod->ySize = ys;
    return prod;    }

The arrays.h header has this:

template <class T>
typedef struct GenericArray2DStruct{
	int xSize;
	int ySize;
	T **data;	};

template <class T>
GenericArray2DStruct<T>* allocate2DArray (unsigned int xs, unsigned int ys);

Now the code that gives me problems looks like this:

#include "arrays.h"

using namespace std;

image1Array = allocate2DArray <unsigned char> (100, 100);

The linker gives me an error saying:
[Linker error] undefined reference to `GenericArray2DStruct<unsigned char>* allocate2DArray<unsigned char>(unsigned int, unsigned int)'

Can anyone help me out? Why is this happening and how do I fix it? Thanks in advance!

 

[Salem]

Why are you using malloc() in a C++ program?
It should be [tt]new[/tt] and [tt]delete[/tt] all the way.

AFAIK, the whole of the template code should be in the header file, so the compiler can create the appropriate instance whenever you reference it.



--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[netprik1]

That doesn't fix anything though...I've even tried shoving all the code into a single file, but that still doesn't fix anything. It looks like it doesn't understand that this:

allocate2DArray <unsigned char> (100, 100);

is trying to access this:

template <class T>
GenericArray2DStruct<T>* allocate2DArray (unsigned int xs, unsigned int ys);

It doesn't seem to see that these are one and the same, and that the first is a reference to the second.

And as far as using malloc goes, thanks a lot for the suggestion! I was converting this code to C++ from C (the templates are new additions), and I forgot that C++ doesn't typically use malloc anymore. Still not what's stopping my code from working, though.

 

[netprik1]

Alright, so the problem's solved. Apparently, the fact that all template declarations needed to be in the header file was the only problem. I didn't REBUILD the code, I only BUILT it, and that's why my changes had no effect at first. I rebuilt the code, and it worked. Now the other problem I'm now having is my code just got slower by a factor of 2 - anyone know why?

 

[Salem]

Slower compared to what?
You need to describe (or show us) how version 1 was implemented as well.

Changing
[tt]unsigned char image1array[100][100];[/tt]
to
[tt]image1Array = allocate2DArray <unsigned char> (100, 100);[/tt]
for example?

Or just making [tt]allocate2DArray()[/tt] a template function?

Or did you just time the debug version where you previously timed the release version?



--
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.

 

[netprik1]

Ah, yes, I see...that's what happened. I'm traditionally a Java developer, so I forget about things like that in C++. I ran the code in debug mode the second time. No problem now - the code's just as fast as it was before. Thanks for all the help!