shoosh shoosh - 3 months ago 9
C++ Question

C++: getting the address of the start of an std::vector?

Sometimes it is useful to use the starting address of an

std::vector
and temporarily treat that address as the address of a regularly allocated buffer.

For instance replace this:

char* buf = new char[size];
fillTheBuffer(buf, size);
useTheBuffer(buf, size);
delete[] buf;


With This:

vector<char> buf(size);
fillTheBuffer(&buf[0], size);
useTheBuffer(&buf[0], size);


The advantage of this is of course that the buffer is deallocated automatically and I don't have to worry about the
delete[]
.

The problem I'm having with this is when size == 0. In that case the first version works ok. An empty buffer is "allocated" and the subsequent functions do nothing size they get size == 0.

The second version however fails if size == 0 since calling
buf[0]
may rightly contain an assertion that
0 < size
.

So is there an alternative to the idiom
&buf[0]
that returns the address of the start of the vector even if the vector is empty?

I've also considered using
buf.begin()
but according to the standard it isn't even guaranteed to return a pointer.

Answer

I guess you'd just have to check.

Perhaps a utility function:

template <class T, class Alloc>
T* data(std::vector<T, Alloc>& vec)
{
    return vec.empty() ? 0 : &vec[0];
}

template <class T, class Alloc>
const T* data(const std::vector<T, Alloc>& vec)
{
    return vec.empty() ? 0 : &vec[0];
}