Question

What would be the correct way to convert from unsigned char std::vector to char?

void SendSocket(const std::vector<BYTE> &buffer)
{
    int ret;
    const BYTE* bufferPtr = &buffer[0];
    ret = send(_socket, (const char*)bufferPtr, buffer.size(), 0);
}

Answer 1

Assuming that BYTE is a typedef for char (with or without signed or unsigned), then your code is fine, but slightly more verbose than necessary. The storage used by a vector is required to be a contiguous array, so taking the address of the first element gives you a pointer to that array. Any kind of char has the same layout and alignment as any other kind of char, so the pointer conversion is valid.

The argument to send is const void*, and (more or less) any pointer can be converted to that implicitly, so there’s no need to cast:

ret = send(_socket, &buffer[0], buffer.size(), 0);

However, you should check (or otherwise ensure) that buffer isn’t empty before using [].