How to Ensure Argument Type Homogeneity in Variadic Functions?

Ensuring Argument Type Homogeneity in Variadic Functions Without Structural Modifications

When defining functions with variadic arguments, it is often desirable to ensure that all provided arguments are of the same type. This helps maintain type safety and prevents potential errors.

Variadic functions themselves do not enforce type safety, leaving the programmer responsible for explicit checking. However, a combination of techniques can be used to implement this requirement effectively.

Option 1: Convert-Later Approach

One method is to accept the arguments as variadic and perform type conversion later. For instance, if you know the necessary steps to convert from std::array to dragon_list_t, the following template function can be used:

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

Option 2: SFINAE-Based Reject-Early Approach

Another option is to employ Substitution Failure Is Not an Error (SFINAE) to perform type checking at the function interface level. This technique allows for overload resolution to reject invalid argument types early on:

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 example, the f function will be instantiated only when all provided arguments are convertible to the ToType type.

Conclusion

By utilizing either the convert-later approach or the SFINAE-based reject-early approach, developers can specify one type for all arguments passed to variadic functions or variadic template functions without resorting to additional structures or overhead.

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