Question

Lets say I have a structure

struct s
{
     std::deque<Object> q; //won't work  with C library
}

If the structure with std::deque is initialized using C library then it wouldn’t work.

struct s
{
     std::vector<Object> v; //would work with C library
}

However, this structure with std::vector would work with C library.
I think this is because the elements in deque are not contiguous whereas the elements in vector are contiguous. I think this might be a reason but not sure.

Answer 1

Neither version of S will work with a C library.
This is because C does not have any concept of how classes are layed out (or even what is in them).

Vector can be used with C library if you pass the address of an element. Because all the elements are in contiguous memory it looks like a normal pointer and thus any C function that takes a pointer will work.

struct Object { /* NORAML POD Object */ };

extern "C" void cFunctionCall(Object* data, size_t size);


std::vector<Object>   v;
// initialize v

cFunctionCall(&v[0], v.size()); // This will work because all members of v
                                // are in contiguous memory