CODEWITHSUNDEEP
c++boosttype-traitsc++03
Caduchon
Caduchon

Reputation: 5201

Is there a typetrait checking inclusion between types?

I'm looking for a typetrait able to know if the range of a type is included in another. A typetrait where is_included_in<T,U>::value is true when each value of type T can be stored as a value of type U. Example :

is_included_in<float,double>::value; // true
is_included_in<double,float>::value; // false
is_included_in<int,double>::value; // true
is_included_in<bool,long int>::value; // true
is_included_in<long long int,float>::value; // false

Is there somthing in Boost able to do this ? Or have I to write it myself ?

Note: I don't use C++11 for compatibility reason.

Upvotes: 1

Views: 102

Answers (2)

Daniel Langr
Daniel Langr

Reputation: 23497

For fundamental integral and floating-point types, you can compare their number of digits as follows:

template <typename T, typename U>  
struct is_included_in
  : boost::integral_constant<bool,
      std::numeric_limits<T>::digits <= std::numeric_limits<U>::digits> { };

It works for all you exemplary cases. The only problem is that it yields true for, e.g., <float, long>. A partial specialization helps here:

template <typename T, typename U,
  bool = boost::is_floating_point<T>::value && boost::is_integral<U>::value>
struct is_included_in
  : boost::integral_constant<bool,
      std::numeric_limits<T>::digits <= std::numeric_limits<U>::digits> { };

template <typename T, typename U>
struct is_included_in<T, U, true> : boost::false_type { };

Live demo: https://wandbox.org/permlink/NBXFOUK8fX9sxyfm.

Upvotes: 1

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

No, you'd have to make it yourself.

That being said, if you want to use this to make arithmetic conversions value-safe, Boost already has numeric_cast that achieves this.

So, depending on your goal, you may not need a trait.

At the very least you could probably examine the numeric_cast implementation and use its principles to build your own trait(s).

Upvotes: 2

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