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

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

vector<CustomClass *> content(20000);

or

vector<CustomClass *> content;
...
content.reserve(20000);

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.