Re: std::vector and a bit-wise copy

From:

Joe Gottman <josephgottman@comcast.net>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 9 Feb 2011 20:45:31 CST

Message-ID:

<STF4p.222217$UC6.109894@en-nntp-08.dc1.easynews.com>

On 2/9/2011 6:24 PM, Jeremy wrote:

I have a question that I having trouble finding a straight answer to,
but thought it may be obvious to someone here.

If I have a struct with a vector as a member:

struct A
{
     std::vector<int> theVec;
};

And I make two instances of that vector x and y and populate the
vector in x. Is it undefined behavior to then set y= x ? Such as

int main()
{
     A x;
     A y;

     fill(x); // allocates the vector and populates it

     y = x;
}

Seeing A has no copy-constructor declared, we should do a bit-wise
copy from x to y. Since the vector in y has no memory allocated to it,
I'm assuming this should be just a case of memory trampling and
exhibit UB.

Is that wrong or am I missing something w.r.t. the bit-wise copy?

    If a class doesn't define a copy constructor, in most cases,
including this one, the compiler will define one for you. It will in
this case, and it will be defined as if you defined it as

A(const A &rhs)
  : theVec(rhs.theVec)
{}

This calls std::vector's copy constructor, which allocates the memory
for you. So there is no undefined behavior.

Joe Gottman

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