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Constrained genericity: Difference between revisions
Added C++ solution
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-- We have declared Banana and Tomato as a Food.</lang>
The Tomato_Box can only contain tomatoes; the Banana_Box can only contain bananas. You can only create boxes of eatable objects.
=={{header|C++}}==
{{works with|C++11}}
Uses static assertion to disallow instantiations on incorrect types
<lang c++>
template<typename T> //Detection helper struct
struct can_eat //Detects presence of non-const member function void eat()
{
private:
template<typename U, void (U::*)()> struct SFINAE {};
template<typename U> static char Test(SFINAE<U, &U::eat>*);
template<typename U> static int Test(...);
public:
static constexpr bool value = sizeof(Test<T>(0)) == sizeof(char);
};
struct potato
{ void eat(); };
struct brick
{};
template<typename T>
class FoodBox
{
//Using static assertion to prohibit non-edible types
static_assert(can_eat<T>::value, "Only edible items are allowed in foodbox");
//Rest of class definition
};
int main()
{
FoodBox<potato> lunch;
//Following leads to compile-time error
//FoodBox<brick> practical_joke;
}</lang>
=={{header|C sharp|C#}}==
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