How can you ensure that all arguments passed to a variadic function or template function are of the same type without using additional data structures?

Specifying One Type for Variadic Arguments

This article explores a method to ensure that all arguments passed to a variadic function or variadic template function are of the same type, without using additional data structures such as arrays, vectors, or structs.

Variadic Functions and Template Functions

Variadic functions and variadic template functions allow a function to accept an unknown number of arguments. However, they do not enforce any type constraints on the arguments.

Enforcing Type Safety

To guarantee that all arguments are of the same type, we can take the following steps:

1. Accept Arguments by Variadic Template: Define the function or template function to accept arguments by a variadic template parameter.
2. Use SFINAE for Convertibility Check: Use the Substitution Failure Is Not an Error (SFINAE) technique on the function interface to check if the arguments can be converted to a specific type. This technique allows us to reject invalid arguments early.
3. Define a Helper Type: Create a helper type using a variadic template fst (First Type) to determine the type of the first argument.
4. Apply SFINAE to the Function Signature: Apply enable_if to the function signature to check if the arguments are convertible to the desired type. If the check fails, the compiler will generate an error.

Example:

The following code demonstrates how to implement this technique:

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...);

Usage:

This code ensures that all arguments passed to f can be converted to the ToType type. If any argument is not convertible, the compiler will issue an error.

Convert-Later Approach:

Alternatively, if you know the steps to convert from an array to the desired type, you can use the following approach:

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

Conclusion:

By utilizing variadic template functions and SFINAE, we can enforce type safety on variadic arguments, ensuring that all arguments are of the same type. This allows us to create functions that operate on homogeneous data without the need for additional data structures.

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