How to Enforce Type Uniformity in Variadic Functions?

Ensuring Argument Type Uniformity in Variadic Functions

In this scenario, we aim to create a function that can accept a variable number of arguments, but ensure that all arguments are of the same type. We seek to achieve this without resorting to additional data structures like arrays or vectors.

Variadic Functions

Variadic functions by themselves do not provide type safety for their arguments. To enforce type uniformity, consider employing variadic template functions.

Variadic Template Functions

A variadic template function can be defined as follows:

template<typename... Args>
return_type function_name(Args...);

In this template, Args... represents the variable number of arguments that can be passed to the function.

Type Checking Enforcement

To guarantee that all arguments are of the same type, we can use SFINAE (Substitution Failure Is Not An Error). SFINAE allows us to create a template that detects whether a given condition is true or false during compilation.

Here's an example:

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

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

In this template, f() will only be allowed to accept arguments that are convertible to the type ToType (which represents the desired argument type). This configuration rejects any arguments that are not convertible to ToType.

Use Case Solution

For your specific use case, you can use the following:

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

This template converts the variable arguments into an array of type std::array. You can then use any available methods to convert this array into your dragon_list_t type.

Combination with SFINAE

Combining this approach with the SFINAE technique described earlier, you can create a template that rejects any arguments that are not convertible to Maiden.

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