memset structure with std::string contained

Question

I have a massive structure where the entire contents is scalar variables, enumerations, and arrays of scalars (stack-based) with the exception of one std::string variable.

Now, here's my question...

Can I memset the structure to 0 for its whole size (like I would if it was simply all scalars), or is that not possible with the std::string being in there? I'm not sure what memset would do to its internal representation.

And if you're going to say its good/bad please explain why - I'd like to know why it is the way it is :)

Answer 1

No, you cant, it would overwrite the internal state of the string and make bad things happen. You could wrap all the POD stuff in a seperate struct and put that in your current one, that way you could memset that and let the string default construct.

Edit: Just to clarify, the string will almost certainly be storing a pointer to the memory its allocated for storage. The string's constructor will always have run before you can memset it (even if you memset this in the constructor of your type, the string constructor would run first). So you would be overwriting this pointer value, and instead of pointing to its storage, it would a pointer to NULL, or some other almost definitely invalid value.

Answer 2

Here's an exotic idea: Suppose your class Foo has lots of primitive members which remain uninitialized in Foo's constructor, with the exception of one string:

class Foo
{
  int a;
  double b;
  std::string s;
};

The constructor Foo::Foo() will correctly initialize the string, but it won't care for anything else. So, let's zero out the memory before we construct!

void * addr = ::operator new(sizeof(Foo));
std::memset(addr, 0, sizeof(Foo));
Foo * p = new (addr) Foo;

// later

p->~Foo();
::operator delete(addr);

Of course it would be cleaner to just initialize all the members to zero in the constructor, but perhaps you have your own reasons that you don't want to create a custom constructor.