How can I make a template resolve type only by first parameter?

Question

How can I have a function like this

template<typename TValue>
TValue GridSnap(TValue Value, TValue Grid)

That resolves template type from only the first parameter like this

GridSnap(ADouble, AFloat)  => TValue should resolve to double
GridSnap(AFloat,  ADouble) => TValue should resolve to float
GridSnap(AFloat,  AInt)    => TValue should resolve to float

basically I don't want to manually cast the second argument, it would be nice if it could just do implicit cast base on the first parameter.

The type of both the function parameters(`Value` and `Grid`) is `TValue` so the passed arguments `ADouble` and `AFloat` must be of the same type otherwise you'll get error saying conflicting types deduced. — Jason, Sep 03 '22 at 04:52
@JasonLiam that's what I'm trying to avoid without using explicit casts everywhere. — M.kazem Akhgary, Sep 03 '22 at 05:02
@M.kazemAkhgary I see. So the [answer](https://stackoverflow.com/a/73589722/12002570) below should solve that. — Jason, Sep 03 '22 at 05:04

NathanOliver · Answer 1 · 2022-09-03T05:17:51.417

3

What you want to do is to have the template parameter of the second function parameter to be in a non-deduced context. You can do this using std::type_identity like

template <typename TValue>
TValue GridSnap(TValue Value, std::type_identity_t<TValue> Grid)

If you can't use C++20, then you can just write your own type_identity and type_identity_t like

template <typename T>
struct type_identity 
{
    using type = T;
};

template <typename T>
using type_identity_t = typename type_identity<T>::type;

edited Sep 03 '22 at 05:17

answered Sep 03 '22 at 04:53

NathanOliver

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Thanks a lot for the answer, It works. but I found a shorter solution `TValue GridSnapAuto(TValue Value, decltype(Value) Grid)` – M.kazem Akhgary Sep 03 '22 at 05:08
@M.kazemAkhgary Nice solution. Didn't even think of that. You should post that as an answer. – NathanOliver Sep 03 '22 at 05:09

score 1 · Accepted Answer · answered Sep 03 '22 at 05:14

1

@NathanOliver solution worked nicely, But I also found this solution

TValue GridSnapAuto(TValue Value, decltype(Value) Grid)

answered Sep 03 '22 at 05:14

M.kazem Akhgary

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How can I make a template resolve type only by first parameter?

2 Answers2