Using STL Allocator with STL Vectors

Question

Here's the basic problem. There's an API which I depend on, with a method using the following syntax:

void foo_api (std::vector<type>& ref_to_my_populated_vector);

The area of code in question is rather performance intensive, and I want to avoid using the heap to allocate memory. As a result, I created a custom allocator which allocates the memory required for the vector on the stack. So, I can now define a vector as:

// Create the stack allocator, with room for 100 elements
my_stack_allocator<type, 100> my_allocator;

// Create the vector, specifying our stack allocator to use
std::vector<type, my_stack_allocator> my_vec(my_allocator);

This is all fine. Performance tests using the stack allocated vector compared to the standard vector show performance is roughly 4x faster. The problem is, I can't call foo_api! So...

foo_api(my_vec); // Results in an error due to incompatible types.
// Can't convert std::vector<type> to std::vector<type, allocator>

Is there a solution to this?

Answer 1

You have to use the default allocator just as the function expects. You have two different types, and there's no way around that.

Just call reserve prior to operating on the vector to get the memory allocations out of the way.

Think about the bad things that could happen. That function may take your vector and start adding more elements. Soon, you could over-flow the stack space you've allocated; oops!

If you're really concerned about performance, a much better route is to replace operator new and kin with a custom memory manager. I have done so and allocations can be hugely improved. For me, allocating sizes of size 512 or less is about 4 operations (move a couple pointers around); I used a pool allocator)