How to Enforce Type Consistency in Variadic Functions Without Additional Structures?

Enforcing Type Consistency in Variadic Functions Without Additional Structures

Variadic functions and variadic template functions allow us to pass a variable number of arguments to a function. However, ensuring that all arguments have the same type can be challenging. This question explores a solution for this problem without using arrays, vectors, or structs.

Variadic Function Approach

The proposed solution is to accept arguments by the variadic template and let type checking verify validity when they are converted. However, this approach requires us to have a known conversion path to the desired type. In this case, there must be a known way to convert an array of Maiden to dragon_list_t.

Example:

template<typename ...Items>
dragon_list_t make_dragon_list(Items... maidens) {
    std::array<Maiden, sizeof...(Items)> arr = {{ maidens ... }};
    // here be dragons
}

SFINAE Approach

SFINAE (Substitution Failure Is Not An Error) can be used to enforce type consistency at the function interface level. This allows us to reject invalid arguments early in the overload resolution process.

Example:

template<typename R, typename...> struct fst { typedef R type; };

template<typename ...Args>
typename fst<void, 
  typename enable_if<
    is_convertible<Args, ToType>::value
  >::type...
>::type 
f(Args...);

Conclusion

Both approaches provide a way to specify one type for all arguments passed to variadic functions without using additional data structures. The variadic function approach relies on known conversion paths, while the SFINAE approach allows for overload resolution to reject invalid arguments. The choice between these approaches depends on the requirements of the specific use case.

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