How return a std::string from C's "getcwd" function

Sorry to keep hammering on this, but I'm trying to learn :). Is this any good? And yes, I care about memory leaks. I can't find a decent way of preallocating the char*, because there simply seems to be no cross-platform way.

const string getcwd()
{
    char* a_cwd = getcwd(NULL,0);
    string s_cwd(a_cwd);
    free(a_cwd);
    return s_cwd;
}

UPDATE2: without Boost or Qt, the most common stuff can get long-winded (see accepted answer)

If you want to remain standard, getcwd isn't required to do anything if you pass to it a NULL; you should instead allocate on the stack a buffer that is "large enough" for most occasions (say, 255 characters), but be prepared for the occasion in which getcwd may fail with errno==ERANGE; in that case you should allocate dinamically a bigger buffer, and increase its size if necessary.

Something like this could work (notice: not tested, just written by scratch, can be surely improved):

string getcwd()
{
    const size_t chunkSize=255;
    const int maxChunks=10240; // 2550 KiBs of current path are more than enough

    char stackBuffer[chunkSize]; // Stack buffer for the "normal" case
    if(getcwd(stackBuffer,sizeof(stackBuffer))!=NULL)
        return stackBuffer;
    if(errno!=ERANGE)
    {
        // It's not ERANGE, so we don't know how to handle it
        throw std::runtime_error("Cannot determine the current path.");
        // Of course you may choose a different error reporting method
    }
    // Ok, the stack buffer isn't long enough; fallback to heap allocation
    for(int chunks=2; chunks<maxChunks ; chunks++)
    {
        // With boost use scoped_ptr; in C++0x, use unique_ptr
        // If you want to be less C++ but more efficient you may want to use realloc
        std::auto_ptr<char> cwd(new char[chunkSize*chunks]); 
        if(getcwd(cwd.get(),chunkSize*chunks)!=NULL)
            return cwd.get();
        if(errno!=ERANGE)
        {
            // It's not ERANGE, so we don't know how to handle it
            throw std::runtime_error("Cannot determine the current path.");
            // Of course you may choose a different error reporting method
        }   
    }
    throw std::runtime_error("Cannot determine the current path; the path is apparently unreasonably long");
}

By the way, in your code there's a very wrong thing: you are trying to dellocate a_cwd (which presumably, in the nonstandard extension, is allocated with malloc or with some other memory allocation function, since getcwd is thought for C) with delete: you absolutely shouldn't do that, keep in mind that each allocation method has its deallocation counterpart, and they must not be mismatched.