C++ alternatives to std::array when the size is fixed, but not a constexpr?

Question

What is the best replacement for std::array<...> if I don't want to have to provide constexpr size? I figured it would be best to just use std::vector and do reserve(...) on it, but maybe I'm overlooking something?

Answer 1

如果需要在运行时确定大小， std::vector应该是正确选择的容器。

Answer 2

Yes, use std::vector .

So if your code is

std:array<int, 42> my_array;

Replace it by

std:vector<int> my_array(42);

Note: you probably don't want to use reserve , because it leaves the vector empty. If you are using std::array , your code doesn't have the concept of empty array, so it's best represented by a std::vector instance that is filled at construction, and never resized.

Answer 3

std::vector<> is probably your answer. I just wouldn't assume reserve() guarantees any speedup.

Bjarne Stroustrup:

People sometimes worry about the cost of std::vector growing incrementally. I used to worry about that and used reserve() to optimize the growth. After measuring my code and repeatedly having trouble finding the performance benefits of reserve() in real programs, I stopped using it except where it is needed to avoid iterator invalidation (a rare case in my code). Again: measure before you optimize.

http://www.stroustrup.com/bs_faq2.html [See bottom of "Why are the standard containers so slow?"]