Specialize how std::vector grows

Question

It is recommended by the C++ standard that std::vector grows exponentially in order to have an "amortized constant cost" regarding reallocation.

Although this type of growth is suitable for most scenarios, there may be a situation where I found I need vector to grow using a different algorithm.

Is there a way to customize how std::vector grows and what conditions it checks before re-allocating?

Answer 1

This depends on what you mean by "customize std::vector". The requirements on std::vector allow you to do what you want. However, you can only do that inside an implementation of std::vector, which requires you to be writing a compiler, or standard library implementation.

In user code, you are not allowed to write anything into std, or at least you can't modify the behavior of std::vector directly.

You can still achieve the desired behavior by manually managing a std::vector to do exactly what you want. Another option is to write your own user::vector class that has the desired behavior.

Answer 2

Just like HolyBlackCat's comment, you can't change it. There is the part of code of STL vector implementation which from VC++.

size_type _Calculate_growth(const size_type _Newsize) const {
    // given _Oldcapacity and _Newsize, calculate geometric growth
    const size_type _Oldcapacity = capacity();

    if (_Oldcapacity > max_size() - _Oldcapacity / 2) {
        return _Newsize; // geometric growth would overflow
    }

    const size_type _Geometric = _Oldcapacity + _Oldcapacity / 2;

    if (_Geometric < _Newsize) {
        return _Newsize; // geometric growth would be insufficient
    }

    return _Geometric; // geometric growth is sufficient
}