How to Define a Type Trait for Function Availability in C ?

Metaprogramming: C Function Definition Failure as Separate Function Definition

In C , it's possible to define metafunctions based on type traits. For example, one can define a template to convert a value to a string based on whether the value's type is arithmetic:

template<typename T> 
enable_if_t<is_arithmetic<T>::value, string> stringify(T t){
    return to_string(t);
}
template<typename T> 
enable_if_t<!is_arithmetic<T>::value, string> stringify(T t){
    return static_cast<ostringstream&>(ostringstream() << t).str();
}

This approach, however, relies on the is_arithmetic property of the type. A more desirable approach would be based on the availability of the std::to_string function for the type. However, it's challenging to express the following requirement as a type trait:

"If std::to_string is defined for the type, use it."

It's possible to declare the criteria for std::to_string availability:

template<typename T> 
decltype(to_string(T{})) stringify(T t){
    return to_string(t);
}

But it's unclear how to construct the opposite criteria.

Using Walter Brown's void_t type trait, it's easy to define the desired type trait:

template<typename T, typename = void>
struct has_to_string
: std::false_type { };

template<typename T>
struct has_to_string<T, 
    void_t<decltype(std::to_string(std::declval<T>()))>> 
: std::true_type { };

This trait evaluates whether the std::to_string function is defined for a given type, allowing for more flexible template selection based on function availability.

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