README-reserve-candidates.md

reserve-candidates

Finds places that could use a reserve() call. Whenever you know how many elements a container will hold you should reserve space in order to avoid repeated memory allocations.

Trivial example missing reserve()

QList<int> ages;
// list.reserve(people.size());
for (auto person : people)
    list << person.age();

Example where reserve shouldn't be used:

QList<int> list;
for (int i = 0; i < 1000; ++i) {
    // reserve() will be called 1000 times, meaning 1000 allocations
    // whilst without a reserve the internal exponential growth algorithm would do a better job
    list.reserve(list.size() + 2);
    for (int j = 0; j < 2; ++j) {
        list << m;
    }
}

Supported containers

QVector, std::vector, QList, QSet and QVarLengthArray

Pitfalls

Rate of false-positives is around 15%. Don't go blindly calling reserve() without proper analysis. In doubt don't use it, all containers have a growth curve and usually only do log(N) allocations when you append N items.