Question

Let’s have a function func which allocates memory for vec. Also let’s func is called many times in a loop.

void func ()
{
    std::vector<int> vec;
    vec.resize(someNumber);
}

for (int i=0; i<someNumber; ++i)
{
    func ();
}

How I can allocate memory for vec only once in a loop?

Answer 1

As Beta said, every time the function func() is called a new vector is created and is destroyed when the function exits. If you want to have a single vector which gets resized which each call, you may want to do something like

std::vector<int> vec;
for (int i=0; i<someNumber; ++i)
{
    vec.resize(i);
}

or you can direcly initialise your vector with the max memory you will need as

std::vector<int> vec(someNumber);