Question

I have an interface which is used like the following:

if (SUCCEEDED(pInterface->GetSize(&size))
{
    wchar_t tmp = new wchar_t[size];
    if (SUCCEEDED(pInterface->GetValue(tmp, size)))
    {
        std::wstring str = tmp;
        // do some work which doesn't throw
    }
    delete[] tmp;
}

Is it safe and portable to do this instead?

if (SUCCEEDED(pInterface->GetSize(&size))
{
    std::wstring str;
    str.resize(size-1);
    if (SUCCEEDED(pInterface->GetValue(&str[0], size)))
    {
        // do some work
    }
}

Now, obviously this works (doesn’t crash/corrupt memory) or I wouldn’t have asked, but I’m mostly wanting to know if there’s a compelling reason not to do this.

Edit: Actually I had change this to .resize(size-1), as apparently the null character is taken into account for you (by VS 2010 anyway). Using .resize(size) ended up where appending to the end of the string was resulting in:

str.resize(size);
pInterface->GetValue(&str[0], size);
str contains L"foo\0";
str += L"bar";
str contains L"foo\0bar";

Trying to use the resulting str.c_str ends up looking like L”foo” due to the null in the middle.

Answer 1

As AraK points out, the string storage might not be contiguous, though this is unlikely. You could also consider using a vector:

if (SUCCEEDED(pInterface->GetSize(&size))
{
    std::vector <wchar_t> vtmp( size );
    if (SUCCEEDED(pInterface->GetValue( & vtmp[0], size)))
    {
        std::wstring str = & vtmp[0];
        // or maybe don't bother with the string - just use the vector
    }
}

which is more likely to be exception safe.