Why can't I pass operator to a function from an other in C++?

Question

In my C++ homework (where I have to short different arrays with different methods), I run into a problem. I can't pass comp() from one function to another.

Here is a simplified version of my code:

template <typename T, typename Compare = std::less<T>>
void fooFunction(T arr[], int arraySize, Compare comp = Compare{})
{
    int endElem=arraySize-1;
    int beginElem =0;
    fooFunction2(arr, beginElem , endElem, arraySize, comp()); //I am getting the errors here
}

template <typename T, typename Compare = std::less<T>>
void fooFunction2(T arr[], int beginElem, int endElem, int arraySize, Compare comp = Compare{})
{
    
}

struct string_size_less
{
    bool operator()(const std::string &a, const std::string &b) const
    {
        return a.size() < b.size();
    }
};

int main()
{
    int arrI[] = { 4, 5, 1, 4, 2};
    std::string arrS[] = {"Car", "Bicycle", "Metro", "Bike"};

    fooFunction(arrI, 5);
    fooFunction(arrS, 4, string_size_less());
    return 0;
}

At the moment I am getting:

error: no match for call to '(std::less<int>) ()'|
error: 'fooFunction2' was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]|

What would be the correct way to solve this?

Answer 1

The problem is that you're passing comp() as the last argument instead of passing comp.

To solve this pass comp instead of comp() as shown below:

struct string_size_less
{
    bool operator()(const std::string &a, const std::string &b) const
    {
        return a.size() < b.size();
    }
};

template <typename T, typename Compare = std::less<T>>
void fooFunction2(T arr[], int beginElem, int endElem, int arraySize, Compare comp = Compare{})
{
    
}
template <typename T, typename Compare = std::less<T>>
void fooFunction(T arr[], int arraySize, Compare comp = Compare{})
{
    int endElem=arraySize-1;
    int beginElem =0;
//----------------------------------------------------vvvv--->changed from comp() to comp
    fooFunction2(arr, beginElem , endElem, arraySize, comp);
}

Working demo

Moreover, before using a function you should have a declaration for that function. See in the working demo linked above.