[c++] How to set initial size of std::vector?

I have a vector<CustomClass*> and I put a lot of items in the vector and I need fast access, so I don't use list. How to set initial size of vector (for example to be 20 000 places, so to avoid copy when I insert new)?

This question is related to c++ stl

The answer is

You need to use the reserve function to set an initial allocated size or do it in the initial constructor.

vector<CustomClass *> content(20000);


vector<CustomClass *> content;

When you reserve() elements, the vector will allocate enough space for (at least?) that many elements. The elements do not exist in the vector, but the memory is ready to be used. This will then possibly speed up push_back() because the memory is already allocated.