Can I use an std::vector as a facade for a pre-allocated (raw) array?

Question

I have acquired a memory location from DirectX where my vertex information is stored. An extremely convenient way to deal with vertex information is to use a std::vector<> of a struct containing vertex info.

Given that I have a pointer to a large buffer, could I use a std::vector to manage the elements in the buffer? Constructing a std::vector regularly causes it to have its own address, which isn't really what I want. Could I use operator placement new somehow?

Answer 1

Yes you can. Use custom allocator. In this allocator return address of your DirectX memory.

Here is a complete examlpe based on an answer from Compelling examples of custom C++ STL allocators?. This solution uses placement new in the allocator.

#include <memory>
#include <iostream>
#include <vector>

using namespace std;

template <typename T>
class placement_memory_allocator: public std::allocator<T>
{
    void* pre_allocated_memory;
public:
    typedef size_t size_type;
    typedef T* pointer;
    typedef const T* const_pointer;

    template<typename _Tp1>
    struct rebind
    {
            typedef placement_memory_allocator<_Tp1> other;
    };

    pointer allocate(size_type n, const void *hint=0)
    {
            char* p = new(pre_allocated_memory)char[n * sizeof(T)];
            cout << "Alloc " << n * sizeof(T) << " bytes @" << hex << (void*)p <<endl;
            return (T*)p;
    }

    void deallocate(pointer p, size_type n)
    {
            cout << "Dealloc " << n << " bytes @" << hex << p << endl;
            //delete p;
    }

    placement_memory_allocator(void* p = 0) throw(): std::allocator<T>(), pre_allocated_memory(p) { cout << "Hello allocator!" << endl; }
    placement_memory_allocator(const placement_memory_allocator &a) throw(): std::allocator<T>(a) {pre_allocated_memory = a.pre_allocated_memory;}
    ~placement_memory_allocator() throw() { }
};

class MyClass
{   
    char empty[10];
    char* name;
public:
    MyClass(char* n) : name(n){ cout << "MyClass: " << name << " @" << hex << (void*)this << endl; }
    MyClass(const MyClass& s){ name = s.name; cout << "=MyClass: " << s.name << " @" << hex << (void*)this << endl; }
    ~MyClass(){ cout << "~MyClass: " << name << " @" << hex << (void*)this <<  endl; }
};

int main()
{
    // create allocator object, intialized with DirectX memory ptr.
    placement_memory_allocator<MyClass> pl(DIRECT_X_MEMORY_PTR);
    //Create vector object, which use the created allocator object.
    vector<MyClass, placement_memory_allocator<MyClass>> v(pl);
    // !important! reserve all the memory of directx buffer.
    // try to comment this line and rerun to see the difference
    v.reserve( DIRECT_X_MEMORY_SIZE_IN_BYTES / sizeof(MyClass));

    //some push backs.
    v.push_back(MyClass("first"));
    cout << "Done1" << endl;
    v.push_back(MyClass("second"));
    cout << "Done1" << endl;
    v.push_back(MyClass("third"));
    cout << "Done1" << endl;

}

Answer 2

The elements of std::vector are allocated dynamically on the heap (with new by std::vector itself) in such a way that they are always contiguous in memory. So if your structure is vertex, using std::vector<vertex> is not what you want as the elements of the vector will not be located in your large buffer.

You could use std::vector<vertex*> however, like for example:

vertex* bigBuffer;     // provided by DirectX
size_t bigBufferLen;   // provided by DirectX

std::vector<vertex*> array;
for (size_t i = 0; i < bigBufferLen; ++i)
{
    array.push_back(bigBuffer + i * sizeof(vertex));
}