std::string.assign segfault

I use a std::string inside of a class I allocate dynamically, and I crash or get segfaults whenever I try to use the string. I assume I'm not initializing it right somehow, but I don't know what to do.

Here is the class:

Code:

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class cInterface
{
    public:
        std::string Text;
        bool done;
        cInterface()
        {
            done=false;
        }
        ~cInterface()
        {
            Text.clear();
        }
};
cInterface* ckInterface;
int numInterfaces;

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Here is the allocation of cInterface:

Code:

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int newInterface()
{
    int ID = -1;
    if(numInterfaces == 0)
    {
        ID = 0;
        ckInterface = (cInterface*)malloc(sizeof(cInterface) < 1 ? 1 : sizeof(cInterface));
        numInterfaces++;
    }
    else
    {
        for(int i=0; i<numInterfaces; i++)
        {
            if(ckInterface[i].done == true) { ID = i; break; }
        }
    }
    if(ID == -1)
    {
        ID = numInterfaces;
        ckInterface = (cInterface*)realloc(ckInterface, (numInterfaces+1)*sizeof(cInterface));
        numInterfaces++;
    }
    return ID;
}

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Doing something like this causes me to crash:

Code:

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int ID = newInterface();
ckInterface[ID].Text.assign("Segfault happens here");

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What am I missing? I tried adding

Code:

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Text = "";

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or

Code:

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Text.resize(512);

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to the constructor of the class, but that didn't fix the issue :/

Quote:

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Originally Posted by Alexander Edgar

Here is the allocation of cInterface:

Code:

---

int newInterface()
{
    int ID = -1;
    if(numInterfaces == 0)
    {
        ID = 0;
        ckInterface = (cInterface*)malloc(sizeof(cInterface) < 1 ? 1 : sizeof(cInterface));
        numInterfaces++;
    }
    else
    {
        for(int i=0; i<numInterfaces; i++)
        {
            if(ckInterface[i].done == true) { ID = i; break; }
        }
    }
    if(ID == -1)
    {
        ID = numInterfaces;
        ckInterface = (cInterface*)realloc(ckInterface, (numInterfaces+1)*sizeof(cInterface));
        numInterfaces++;
    }
    return ID;
}

---

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Red flags!
Using C memory allocation in C++ is walking through a minefield as non-pod objects are not constructed.
Here, std::string's allocators do not get the scope to reserve memory for the string.

Read this thoroughly:
[16] Freestore management Updated! , C++ FAQ

So I switched to new and delete, but I didn't find a good replacement for realloc. Is there a way to do something similar without having to rewrite my entire program to use std::vector? I tried using this, which I thought would be ok because cInterface has no pointers in it:

Code:

---

    if(ID == -1)
    {
        ID = numInterfaces;
        //ckInterface = (cInterface*)realloc(ckInterface, (numInterfaces+1)*sizeof(cInterface));
        cInterface* oldface = new cInterface[numInterfaces];
        oldface = ckInterface;
        delete [] ckInterface;
        ckInterface = new cInterface[numInterfaces+1];
        ckInterface = oldface;
        delete [] oldface;
        numInterfaces++;
    }

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But I still segfault the second time I try to make a new interface (the first one gets created successfully now, though).

Quote:

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Originally Posted by Alexander Edgar

But I still segfault the second time I try to make a new interface (the first one gets created successfully now, though).

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A simple and quick but not elegant solution is to use C style strings here.
(or you could allocate the total amount of memory required from scratch, and copy over the original, finally freeing it)
You can also experiment a bit with allocators..but that would be reinventing the wheel and the end result will (probably) be something not as good as a standard container.

The else logic in your newInterface function looks a bit fishy to me, but I'm not sure about that.

Your custom destructor for cInterface is also completely unnecessary. The default destructor would destroy the Text object anyway.

I've just about never seen a beginner on here use realloc correctly, and you weren't either I'm afraid. Hint, it's about handling failure correctly.Just switch to a vector, which will make it a whole lot simpler and more efficient.

Quote:

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Originally Posted by Alexander Edgar

So I switched to new and delete, but I didn't find a good replacement for realloc. Is there a way to do something similar without having to rewrite my entire program to use std::vector?

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Do something like this:

Code:

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cInterface *newface = new cInterface[++numInterfaces]; // allocate new interfaces
std::copy(ckInterfaces, ckInterfaces + numInterfaces - 1, newface); // place holder for whatever code will make deep copies
//
// ... whatever you intend to do with the new object @ckInterfaces[numInterfaces - 1]
//
delete [] ckInterfaces; // delete the old
ckInterfaces = newface; // assign the new

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This is how it's usually moved to new memory when you reallocate.

Quote:

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This is how it's usually moved to new memory when you reallocate.

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You should really consider using `std::swap_ranges(???, ???, ???);'.

If the class follows a compatible swap protocol it is usually a lot nicer.

Soma

Thanks whiteflags, that worked! Also, in regards to iMalc:

Quote:

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I've just about never seen a beginner on here use realloc correctly, and you weren't either I'm afraid. Hint, it's about handling failure correctly.

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Would the right way to use realloc look something like this?

Code:

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if(ID == -1)
{
    ID = numInterfaces;
    cInterface* newface = (cInterface*)realloc(ckInterface (numInterfaces+1)*sizeof(cInterface));
    if(newface != NULL) { ckInterface = newface; }
    else { printf("Error allocating memory in newInterface(), but there is apparently nothing I can do about it so I don't know what the point of this is\n"); free(ckInterface); exit(1);  }
    numInterfaces++;
}

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