Template in which one argument depends on another argument - c++

Question

I have the following situation. Let say we want to implement a sorted array data structure which keeps the array sorted upon insertion. At first attempt, I would do something like:
template<typename T, typename Comparator, Comparator comparator>
SortedArray {
public:
void find(T value);
void insert(T value);
void remove(T value);
}
The argument T is of course for the type of the elements in the array. Then I need a comparator to tell how to compare objects of type T so that I can keep the elements sorted. Since I want to allow for both function pointers (as in classical qsort) as well as function objects and maybe lambda as well, I need to add the template parameter for the comparator.
Now the problem is that I want the compiler to automatically deduce the 2nd Comparator argument based on the 3rd argument. Right now, a typical usage will be exploiting decltype like
int compare_int(int x, int y) {
return x - y;
}
SortedArray<int, decltype(compare_int), compare_int> myArray;
but this doesn't work with lambda and certainly I would love to just write
SortedArray<int, compare_int> myArray;
instead.
Any idea or is it actually possible in C++ at the moment?

Answer 1

You can non type template parameters as follows:
template<typename T, auto C >
class SortedArray
{
private:
std::vector<T> v;
public:
void sort(){ std::sort( v.begin(), v.end(), C );}
void print() { for( auto& el: v ) std::cout << el << std::endl; }
void push(T t){ v.push_back(t);}
};
bool compare_int( int a, int b )
{
return a<b;
}
int main()
{
SortedArray<int, compare_int> sa1;
sa1.push(5);
sa1.push(3);
sa1.push(7);
sa1.sort();
sa1.print();
SortedArray<int, [](int a, int b){ return a<b;} > sa2;
sa2.push(5);
sa2.push(3);
sa2.push(7);
sa2.sort();
sa2.print();
}
As you can see, you can also use a lambda as template parameter.
There is no need to do any template gymnastic with derived template parameters anymore.