Avoid code duplication between two functions, but one returns in the middle of the code

Question

Lets say i have two functions inside a class:

vector<int> getAllPossibleNumbers() {
    vector<int> nums;
    for (int i=0; i < MAX; i++)
        if (isAPossibleNumber(i))
            nums.push_back(i);
    return nums;
}

bool hasAtLeastOnePossibleNumber() {
    for (int i=0; i < MAX; ++i)
        if (isAPossibleNumber(i))
            return true;
    return false;
}

I could rewrite the second function as

bool hasAtLeastOnePossibleNumber() {
    return getAllPossibleNumbers().size() > 0;
}

But it's obvious, especially with a very high MAX number, how the first is much more efficent.

So, how can I avoid code duplication of this kind?

I would prefer a general solution, that can work with more complicated functions and can compile with C++11.

Answer 1

One option would be to move most of the logic into a third function with a callback to the outer functions which gives the numbers. The result of the callback can be used to determine whether to continue the loop:

template <typename Callback>
void checkNumbers(Callback callback)
{
    for (int i=0; i < MAX; i++)
    {
        if (isAPossibleNumber(i))
        {
            if (!callback(i))
            {
                break;
            }
        }
    }
}

std::vector<int> getAllPossibleNumbers() {
    std::vector<int> nums;
    checkNumbers([&](int i){ nums.push_back(i); return true; });
    return nums;
}

bool hasAtLeastOnePossibleNumber() {
    bool result = false;
    checkNumbers([&](int i){ result = true; return false; });
    return result;
}

The extra function should be optimised away by modern compilers.