C++: How do I make a function run only if the type is a tuple of vectors?

Question

I have the code here online and it works fine: http://paste.ubuntu.com/5793610/

The code is supposed to create a turn a std::tuple of types into a tuple of vectors of those types. Then it's supposed to fill the vectors with numbers and then print the tuple. The only problem I have with it is that theoretically I can pass in a tuple of any types to the printer method but I just want to restrict the implementation of tuple_printer::print to tuples of *vectors*. How would I do that?

I've tried this but it didn't work:

template <typename... Ts>

static void print(std::tuple<std::vector<Ts>...>& var, /* ... */)

{

// ...

}

I know why it doesn't work but do you see my intent? How would I be able to do that? Would I need some sort of variadic enable_if implementation that unpacks the parameter pack and checks if each type is a vector? That I would have a hard time doing though. :)

Thanks.

? · Accepted Answer

It's not that hard to make a variadic enable_if that checks if each type is a vector:

First, make an is_vector type trait:

template<typename T> struct is_vector : std::false_type {};

template<typename T> struct is_vector<std::vector<T>> : std::true_type {};

then, make a helper variadic template metafunction that returns is first (non-variadic) type unchanged

template<typename T, typename...> using head = T;

then, unpack your parameter pack into that discarded variadic template argument, enable_if'ing on the way:

template<typename ...Ts>

head<void, typename std::enable_if<is_vector<Ts>::value >::type...>

print(std::tuple<Ts...>& var)

{

// ...

}

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C++: How do I make a function run only if the type is a tuple of vectors?

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