CODEWITHSUNDEEP
c++variadic-templates
sunmat
sunmat

Reputation: 7248

Unpack C-style array along with parameter pack in C++

My question is somewhat similar to this one: I'm using C++14 and I'm trying to "unpack" a C-style array and call a function with its content. The difference is, I need to do that in a variadic function template and pass each element of the array into something that depends on the type parameter pack.

Here is the simplified code:

template<typename T>
struct Converter {
   static T convert(const char*) {
     ...
   }
};

template<typename ... T>
void myfunction( void(*f)(T...), char* args[]) {
    f(Converter<T...>::convert(args[i])...); // this is not correct but illustrates the idea
}

myfunction here takes a pointer to a function f accepting an arbitrary number of arguments of type T1, ... Tn. Each element of the args array should be converted to the corresponding type before being passed to f.

How can I do that in C++14?

Upvotes: 0

Views: 577

Answers (1)

cigien
cigien

Reputation: 60218

Here's a solution that creates a compile time integer sequence that you can use to index args:

template<typename ...T, size_t ...I>
void myfunction_impl(void(*f)(T...), char const* args[], std::index_sequence<I...>) {
    f(Converter<T>::convert(args[I])...); 
}

template<typename ... T, size_t N>
void myfunction(void(*f)(T...), char const *(&args)[N]) {
    static_assert(N == sizeof...(T), "");
    myfunction_impl(f, args, std::index_sequence_for<T...>());
}

Here's a demo.

This also uses the size of the array passed in, to static_assert that it's equal to the number of parameters of the function passed in.

Upvotes: 3

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