Using std::enable if for template with 3 generic arguments - c++

Question

I've already found a few threads to the topic of std::enable_if, but unfortunately I am not able to apply the examples to my code.
template<class From, class To, class Value>
struct convert
{
static Value apply(Value value)
{
return value;
}
};
I want this to be active only when From and To are the same, so I tried to use
std::enable_if<std::is_same<From,To>::value>::Value
But that doesn't work. How would I do this?
I also have these two specializations, to give you a better picture of my problem:
template<class From, class Value>
struct convert<From, kilometer, Value>
{
static Value apply(Value value)
{
doSomething;
}
};
template<class To, class Value>
struct convertImpl <kilometer, To, Value>
{
static Value apply(Value value)
{
doSomethingElse;
}
};
This is where my abguity problems come from. Right now I'm static_asserting the case that From and To are the same thus making the code to compile. But I would like to just return the value when those two are of the same type.

Answer 1

I don't think you want SFINAE for this, from your description I think you just want a specialization:
template<class From, class To, class Value>
struct convert
{
static Value apply(Value value)
{
Value somethingElse = ...;
return somethingElse;
}
};
template<class From, class Value>
struct convert<From,From,Value>
{
static Value apply(Value value)
{
return value; // SAME
}
};
I admit I honestly don't see the sense in this, as Value is an independent type from both From and To, but you probably know something about it which I don't. It would seem this is your desire if you want the full struct specialized. See it live.
Best of luck

Answer 2

template<class From, class To, class Value>
struct convert
{
template <typename F = From, typename T = To>
static auto apply(Value value) -> typename std::enable_if<std::is_same<F,T>::value, Value>::type
{
static_assert(std::is_same<F, From>::value && std::is_same<T, To>::value, "");
// no conversion
return value;
}
template <typename F = From, typename T = To>
static auto apply(Value value) -> typename std::enable_if<!std::is_same<F,T>::value, Value>::type
{
static_assert(std::is_same<F, From>::value && std::is_same<T, To>::value, "");
// do conversion
return value;
}
};
DEMO
Alternatively this can be implemented using a tag-based dispatching:
template<class From, class To, class Value>
struct convert
{
static Value apply(Value value)
{
using tag = std::integral_constant<bool, std::is_same<From, To>::value>;
return _apply(value, tag{});
}
private:
static Value _apply(Value value, std::true_type)
{
// no conversion
return value;
}
static Value _apply(Value value, std::false_type)
{
// do conversion
return value;
}
};
DEMO 2