c++ std::pair, std::vector & memcopy

Question

Hi!

is it safe to memcopy myvect.size()*sizeof(foo) bytes from the memoryadress of the first element of a

std::vector<std::pair<T1, T2> > myvect

into an array of

struct foo{
    T1 first;
    T2 second;
}

if the array is allocated with the same number of elements as the vector's size?

thanks

Answer 1

No, a class containing T1 and T2 is not guaranteed the same layout or alignment as std::pair<T1, T2>, at least in C++98 (since std::pair is not a POD type). The story may be different in C++0x.

Answer 2

In general, no. On some platforms/compilers/STL implementations it might be, but don't do it anyway. You'd be relying on the implementation details of both pair<> and vector<>.

I myself have committed the sin of relying on vector<> being a contiguous array. For that, I deeply repent. But the pair<>... Just say no.

Answer 3

The answer to the question you didn't ask is probably std::transform:

struct pairToFoo {
    // optionally this can be a function template.
    // template<typename T1, typename T2>
    foo operator()(const std::pair<T1,T2> &p) const {
        foo f = {p.first, p.second};
        return f;
    }
};

std::transform(myvect.begin(), myvect.end(), myarray, pairToFoo());

Or std::copy, but give foo an operator= taking a pair as parameter. This assumes you can re-write foo, though:

struct foo {
    T1 first;
    T2 second;
    foo &operator=(const std::pair<T1,T2> &p) {
        first = p.first;
        second = p.second;
        return *this;
    }
};

std::copy(myvect.begin(), myvect.end(), myarray);