How to Enforce a Single Argument Type for Variadic Functions or Templates Without Using Containers?

Specifying a Single Argument Type for Variadic Functions or Templates Without Containers

When working with variadic functions or template functions, it can be desirable to ensure that all arguments adhere to a specific type, while exhibiting a clear compile-time error upon incorrect usage.

Variadic Functions

Variadic functions, such as the printf family, accept a variable number of arguments of varying types. However, they lack type safety, making it difficult to enforce a specific type for all arguments.

Variadic Template Functions

Variadic template functions can provide type safety through template type parameters. However, specifying an array or vector of the desired type within the template parameters adds unnecessary complexity to the function signature.

Solution: Template Metaprogramming (SFINAE)

To achieve type enforcement without using containers, you can leverage template metaprogramming and the Substitution Failure Is Not An Error (SFINAE) technique. Here's a simplified example:

template<typename ToType, typename Arg>
struct is_convertible { static constexpr bool value = false; };

template<typename ToType, typename FromType>
struct is_convertible<ToType, FromType> : std::is_convertible<FromType, ToType> {};

template<typename ...Args>
void f(Args...) {
    // Check if all arguments are convertible to the desired type
    typename std::enable_if<
        is_convertible<ToType, Args>::value && ...
    >::type...;
}

In this example, the is_convertible template metafunction checks if each argument can be converted to the ToType type. The f function then uses SFINAE to enable its use only if all arguments meet this condition.

Usage

To enforce the type constraint for your make_dragon_list function, you can use SFINAE as follows:

template<typename ToType, typename ...Args>
typename std::enable_if<
    is_convertible<ToType, Args>::value && ...
>::type
make_dragon_list(Args...);

With this approach, the compiler will reject any attempt to pass arguments of incorrect types, providing clear and early error messages.

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