C ++：指针向量与固定大小的数组性能

Question

Performance-wise, which is faster?

A vector of object pointers allocated by the new operator?

std::vector<Object *> array;

Or an array allocated with new in the constructor?

Object[] objects;
objects = new objects[64];

The idea is that in every frame, the program loops through each element reading/writing values for each element.

Edit:

The second snippet was pulled from an XNA book. I am not using XNA to write my framework, and I'm trying to figure out the best method for using containers in an application that requires speed.

Answer 1

Definitely the second one.

• With a vector of pointers, each individual element of that vector can be allocated anywhere on the heap.

• With an array of objects, all elements are stored sequentially. This means the processor can cache chunks of memory more effectively as you iterate through the array.

The concept is called cache locality , referring to how well organised your data is with respect to memory access patterns and caching.

As pointed out in the comments, neither of your examples are correct. I assume you meant something like this:

std::vector<Object*> vector_of_pointers(size);

Object *array_of_objects = new Object[size];

However, I fear you may not have phrased your question the way you intended. You're not comparing two similar things. A vector is basically just an array that can grow if necessary. It makes all the same guarantees as an array, and so if it's storing the same data type, you shouldn't notice any difference between the two.

// Bad cache locality:
Object **A = new Object*[size];
std::vector<Object*> B(size);

// Good cache locality:
Object *C = new Object[size];
std::vector<Object> D(size);