How do you pass a template name to a method? - c++

Question

Lets say I have two template functions for iterating through a container:
template <typename I, typename C>
It Prev(I i, const C& c) noexcept {
Expects(i != c.end());
if (i == c.begin()) return c.end();
return i - 1;
}
and
template <typename I, typename C>
It Next(I i, const C& c) noexcept {
Expects(i != c.end());
return i + 1;
}
And a class with a GetNextElement and GetPrevElement that use these templates:
struct MyClass {
std::vector<int> elements;
int* GetNextElement(std::vector<int>::iterator i) {
auto next = Next(i, elements);
if (next == elements.end()) return nullptr;
return &*it;
}
int* GetPrevElement(std::vector<int>::iterator i) {
auto prev = Prev(i, elements);
if (prev == elements.end()) return nullptr;
return &*it;
}
};
These two methods do the same thing, except for calling a different template. How do I turn these into a single member function that might be called like
MyClass mc;
// ...
auto it = mc.elements.begin();
auto next = mc.GetElement<Next>(it);
// Or maybe mc.GetElement(it, Next);

Answer 1

#define OINVOKE(...) __VA_ARGS__(decltype(args)(args)...)
#define OVERLOADS_OF(...) [](auto&&...args)\
noexcept(noexcept(OINVOKE(__VA_ARGS__)))\
->decltype(OINVOKE(__VA_ARGS__))\
{ return OINVOKE(__VA_ARGS__); }
auto fNext=OVERLOADS_OF(Next);
auto fPrev=OVERLOADS_OF(Prev);
Then:
template<auto* pf>
int* GetElement(std::vector<int>::iterator i) {
auto next = (*pf)(i, elements);
if (next == elements.end()) return nullptr;
return &*it;
}
auto it = mc.elements.begin();
auto next = GetElement<&fNext>(it);
requires C++17.

Answer 2

This problem is greatly simplified if you can wrap your functions in struct.
struct Prev
{
template <typename I, typename C>
I operator()(I i, const C& c) noexcept {
Expects(i != c.end());
if (i == c.begin()) return c.end();
return i - 1;
}
};
struct Next
{
template <typename I, typename C>
I operator()(I i, const C& c) noexcept {
Expects(i != c.end());
return i + 1;
}
};
You can then simply pass them as template arguments.
struct MyClass
{
public:
auto begin() { return elements.begin(); }
auto end() { return elements.end(); }
template<typename T, typename I>
I GetElement(I iter)
{
return T()(iter, this->elements);
}
private:
std::vector<int> elements;
};
int main()
{
MyClass mc;
auto it = mc.begin();
auto next = mc.GetElement<Next>(it);
auto prev = mc.GetElement<Prev>(it);
}

Answer 3

You can try this code:
template <std::vector<int>::iterator (*FOO)(std::vector<int>::iterator, const std::vector<int>&)>
std::vector<int>::iterator GetElement(std::vector<int>::iterator i)
{
return FOO(i, elements);
}