Thread: 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:

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

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;
}

Doing something like this causes me to crash:

Code:

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

What am I missing? I tried adding

Code:

Text = "";

or

Code:

Text.resize(512);

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

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;
}

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

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

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

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:

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.

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

Code:

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++;
}