Question

I have a class Foo defined like this:

class Foo
{
  public:
    Foo(int num);
    Foo(const Foo& other);
    ~Foo();

    Foo& operator=(const Foo& other);
    ...

  private:
    string *fooArray;
    void clearMemory();
    ...
}

Foo::Foo(int num)
{
    fooArray = new string[num];
}

Foo::Foo(const Foo& other)
{
    *this = other;
}

Foo::~Foo()
{
    clearMemory();
}

Foo& operator=(const Foo& other)
{
    clearMemory();
    fooArray = new string[other.size]; // size is a private variable
    memcpy(fooArray, other.fooArray, other.size * sizeof(string));
}

void Foo::someOtherFooFuncion(int newNum)
{
    clearMemory(); // crash with Visual Studio, does not crash with g++, but g++ also
                   // crashes when destructor is called
    fooArray = new string[newNum];
}

void Foo::clearMemory()
{
    if(fooArray != NULL)
    {
        delete[] fooArray; // sometimes it crashes here with g++; it always crashes when
                           // compiling in Visual Studio
        fooArray = NULL;
    }
}

As noted in the code comments, it is giving me crashes at times. I have tried following the steps in GDB, I got as far as

destructing Foo:
@0x7fff5fbff9b0
$26 = {
  fooArray = 0x1001009d8, 
  ...
}

And then delete[] fooArray is reached, and all of a sudden

Foo(49858) malloc: *** error for object 0x100100990: pointer being freed was not allocated

Have no idea where the 0x100100990 came from.

I realize the code is very incomplete, but I really don’t know even where to start hunting for the bug right now, and would like some tips as to what possible conditions could cause delete[] errors.

EDIT:

Added c’tor, d’tor, and assignment operator. I am away from PC so code may not be 100% accurate. Assigning values to fooArray and accessing them works just fine though.

Also, I would greatly appreciate a general list of problems that could potentially cause delete[] to throw an error, so that I could at least have some idea of where to look in the code.

EDIT 2:

So I followed Xeo’s advice to use std::uninitialized_copy, now the code works fine and compiles under g++. The destructor works fine in Visual Studio as well, but somehow deleting fooArray in someOtherFooFuncion makes it crash.

Any other ideas?

Answer 1

Do not use memcpy in C++

I can’t stress that point enough. When memcpying a class object in C++ (non-POD to be exact), you’re breaking that class’ invariants defined by its constructors, since you are circumventing exactly those. Every time you memcpya a std::string class, you get a new object referring to the same memory as another object. You’ll get a double delete with this, which is causing your crash.

Use std::uninitialized_copy from <algorithm> like this:

//                      src_begin             src_end           dest
std::uninitialized_copy(other.array, other.array + other.size, array);

(untested because I’m writing this from my iPod)

Or even better, just use std::vector instead of raw memory. You’ll have no need for a destructor and copy ctor / assignment operator anymore.