Force the user to pass a template parameter of a known type which is itself templated

Question

Suppose I have a class

template <typename T>
struct Foo {
    T data;
};

where I want to enforce the fact that T is a known templated type. For instance, suppose that I want T to be a std::vector of something. Now one way to handle this problem would be to redefine the class so that the template parameter is also the template parameter of the data function:

template <typename S>
struct Foo {
    std::vector<S> data;
};

I do not want to do that. In this specific case, it feels more natural to require the user to construct a foo object like this:

Foo<T<S>> f;

which, in the case where we want T to be a vector, looks like

Foo<std::vector<S>> f;

Is there anyway to keep the first form of the class, which requiring that T be of a specific class? Perhaps a static_assert?

I don't see how a template template parameter helps here because it would not let me force the template template parameter to be a specific type (e.g. a vector). All that would let me do is perhaps suggest a default type. To my understanding... — bremen_matt, May 15 '19 at 14:02

score 0 · Answer 1 · answered May 15 '19 at 14:08

0

Got it

template <typename T>
struct Foo {
    static_assert( std::is_same<T,std::vector<T::value_type>>::value, "T is not a vector you idiot" );
};

answered May 15 '19 at 14:08

bremen_matt

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What if `T` doesn't have `value_type`? Edit : I guess it will fail anyway, but you won't get the nice error message. – François Andrieux May 15 '19 at 14:08
You'll need to write `std::vector` instead of `std::vector`. – François Andrieux May 15 '19 at 14:10
I just saw the question you linked to. You are absolutely right this question is a duplicate. The answer to that question is much more general – bremen_matt May 15 '19 at 14:12

Force the user to pass a template parameter of a known type which is itself templated

1 Answers1