Class member template function call not being deducted-CodePudding

I am having trouble understanding why the following does not compile. I have the following code like so (some code ommited):

Header:

template <typename KeyType, typename ElementType>
class TUnorderedMap
{
public:
    ElementType* Find(const KeyType Key);
    const ElementType* Find(const KeyType Key) const;
};

struct Foo
{
};

Source file:

void Lookup()
{
    TUnorderedMap <Foo*, Foo> NodeToWidgetLookup;
    
    Foo TempFoo;
    const Foo* SelectionTarget = &TempFoo;

    // Issue here in this call
    NodeToWidgetLookup.Find(SelectionTarget);

    // Issue here in this call
    NodeToWidgetLookup.Find(SelectionTarget);
}

Error message: What is the issue here? Why is neither of the Find functions accepted?

CodePudding user response：

What is the issue here? Why is neither of the Find functions accepted?

With the given instantiation

TUnorderedMap <Foo*, Foo> NodeToWidgetLookup;

The function Find expects a const pointer Foo *const:

const ElementType* Find(Foo* const Key) const;

While you are trying to pass a non-const pointer to const argument const Foo*:

const Foo* SelectionTarget

You can either change the template arguments like this:

TUnorderedMap <const Foo*, Foo> NodeToWidgetLookup;

Or make your argument point to a non-const instance:

Foo* SelectionTarget;

If you want to take all pointer overloads, you may want to declare an overload that is a function template itself:

template<typename Key>
const ElementType* Find(const Key* key) const {
    static_assert(std::is_same<Key*, KeyType>::value, "The types don't match");
    ...
}

The template ignores cv-qualifiers of the pointed to object of and the (outermost) pointer part of the argument, but it won't be applicable for non-pointer types.

Thanks to overload resolution rules you also can mix it with a non-template overload:

ElementType* Find(const KeyType Key);
const ElementType* Find(const KeyType Key) const;
template<typename Key> const ElementType* Find(const Key* key) const;

But be advised, that non-template functions in this scenario precede the function template in the overload resolution candidate list.