vector of arrays of pointers to structures

Hi,

I have a function as below:

Code:

---

void f ( std::vector<Blob> *blobsPtr )
{
        // create the vector to store every pair of pointers to Blobs
        vector<Blob*[2]> pairs;

        // create a vector of pairs of Blob pointers, total of (num of Blobs choose 2)
        for (vector<Blob>::size_type i = 0; i < blobsPtr->size() - 1; i++)
                for (vector<Blob>::size_type j = 0; j < blobsPtr->size(); j++)
                {
                        const Blob *b[2];
                        b[0] = &(blobsPtr->at(i));
                        b[1] = &(blobsPtr->at(j));
                        pairs.push_back(b);
                }

        // examine each pair and remove ones that fail the test
}

---

Where Blob is a simple structure.
The goal of the function is to take all of the elements in the vector referenced by blobsPtr, and first create a vector of pairs of pointers to Blobs. In other words, create a vector that has every pairwise combination of Blobs (as pointers). Then I want to do some tests and remove the elements of 'pairs' that fail the tests. Finally, I want to delete all the Blobs that failed the test from the vector pointed to by 'blobsPtr'.

Whew!

The error I am getting with the current version is "cannot convert parameter 1 from 'const Blob *[2]' to 'Blob *const (&)[2]" when I try to do the push_back.
I'm starting to get confused, so any help would be appreciated. Or perhaps I'm going about this totally wrong. The thing is I don't want to copy all the Blobs when I'm creating the pairwise vector, because it's just a waste of memory.

Thanks.

If you want a pair of things, then use a std:: pair, that's what it's for. Not those array[2] things.

Cool, thanks. I'll look up std:: pair and hack around for a bit. That should simplify things.

Well, std:: pair seemed to help out. Here's what I came up with, which seems to work.

Code:

---

void filterPairwise(const IplImage *im, vector<Blob> *blobsPtr)
{
        // create the vector to store every pair of pointers to Blobs
        vector< pair<Blob*, Blob*> > pairs;

        // fill in the vector with every pairwise combination ( total of blobsPtr->size() choose 2 )
        for (vector<Blob>::size_type i = 0; i < blobsPtr->size() - 1; i++)
                for (vector<Blob>::size_type j = 0; j < blobsPtr->size(); j++)
                        pairs.push_back( pair<Blob*, Blob*>(&blobsPtr->at(i), &blobsPtr->at(j)) );

        // examine each pair and remove ones that fail the test
}

---

Glad it helped.

Quote:

---

Code:

---

pairs.push_back( pair<Blob*, Blob*>(&blobsPtr->at(i), &blobsPtr->at(j)) );

---

---

You can simplify this line of code by making use of the std::make_pair convenience function:

Code:

---

pairs.push_back( make_pair(&blobsPtr->at(i), &blobsPtr->at(j)) );

---

That avoids having to type Blob* over and over again.

Incidentally, have you considered passing the vector<Blob> by reference instead of passing a pointer to vector<Blob>?

How would I use a reference? Do you mean in the parameters of the function?

Would it be:

Code:

---

void f ( std::vector<Blob> &blobsPtr )

---

?

And would calling the function be different? Right now I have:

Code:

---

vector<Blob> B;
f(&B);

---

Using a reference, I could still directly modify the vector in my function right? It's just simplifying the syntax a bit?

Code:

---

vector<Blob> B;
f(B);

---

>Using a reference, I could still directly modify the vector in my function right? It's just simplifying the syntax a bit?

yep, pretty much.

http://www.cprogramming.com/tutorial/references.html

Hmm... I've just realized that I need to add a third parameter to each vector element - a double: the "score" of each pair. Then I want to sort the vector based on the score.
Is there such a thing as a std:: triplet? Or should I use a pair of pairs?

There's std::tr1::tuple, but not many compilers support it yet. You could use Boost.Tuple.

But actually, maybe you should just make a small struct.

Could someone remind me why this doesn't work? I don't know much about constructors and such.

Code:

---

typedef struct
{
        int a;
        // etc... all primitive types
}  Blob;


typedef struct
{
        Blob &blobA;
        Blob &blobB;
        double d;
} BlobPair;

---

I get

Code:

---

'BlobPair' : no appropriate default constructor available
binary '=' : no operator found which takes a right-hand operand of type 'Blob *__w64 ' (or there is no acceptable conversion)
etc...

---

I just want the bare minimum for compiling. I don't know how to make all the constructors, deconstructors, operators etc... I will have to learn that at a later date.

I think it's because references need to have a value. Not like pointers, which can have an undefined value. So you need a constructor to assign them, but it isn't there.

Great, thanks. I'll just switch them to pointers and keep it simple for now.

Indeed, robwhite is right. References are most easily used as function arguments where a argument is required. Otherwise you need to use a constructor that initializes the references.
And you also seem to be assigning like = &var, which is right. It should = var.