C++: std::vector::reserve not reserving when containing pointers

Question

When I call std::vector::reserve when the identifier is of type std::vector<Foo*> reserve(...) does nothing:

std::vector<int*> bar;
bar.reserve(20);

//I expect bar.size to return 20...
std::size_t sz = bar.size();
for(std::size_t i = 0; i < sz; ++i) {
    //Do Stuff to all items!
}

The aforementioned for loop runs exactly zero times and bar.size() returns zero. I do not remember if this is also true for all other STL containers, but if so, including the behavior for std::vector: WHY?

Answer 1

.reserve() doesn't change the size of a vector. The member function you are looking for is .resize(). reserve() is simply an optimization. If you are going to add a bunch of things to a vector one-by-one using push_back() then telling it how many you will add using reserve() can make the code run a little bit faster. But just calling reserve() doesn't change the size.

Answer 2

reserve changes the capacity of the vector, not the size. You probably want resize

Answer 3

vector::reserve() changes the capacity of a vector, not its size .

capacity is how much memory has been allocated internally to hold elements of the vector. size is how many elements have actually held by the vector. vector::resize() affects the latter.