How Can I Prevent Implicit Conversions for Non-Matching Function Types in C ?

Preventing Implicit Conversions for Non-Matching Function Types

In C , functions can take parameters of various types. This includes user-defined types, which can be passed as function arguments by implicit casting. However, in certain scenarios, it may be desirable to prevent this implicit casting and restrict function calls to only those arguments that match the declared function signature.

To avoid implicit conversions for non-constructing functions, the explicit keyword can be used. However, this approach only works for constructing functions, not for non-constructing functions.

One solution to this problem is to define a function template that matches all other types. This technique prioritizes the template function for non-matching types, preventing them from being passed to the original function with the exact function signature.

Example:

// Original function signature requires an int
void function(int);

// Function template matches all other types
template <class T>
void function(T) = delete; // C++11 

This approach ensures that the original function with the exact signature function(int) is only invoked when an integer argument is passed. Anything else, such as characters or longs, triggers the function template and results in a compilation error.

Pre-C 11 Method:

For C versions prior to C 11, a different technique can be used:

// Creating a class to handle overload deletion
class DeleteOverload {
private:
    DeleteOverload(void*);
};

// Function template with overload deletion
template <class T>
void function(T a, DeleteOverload = 0);

// Original function with exact signature
void function(int a) {}

In this approach, the DeleteOverload class cannot be instantiated, effectively forbidding its use as a template argument. This forces all non-matching types to trigger the template function and prohibits them from reaching the original function with the exact signature.

C 23 Version:

C 23 introduces the static_assert feature, which can be utilized for greater clarity in this scenario:

void function(int); // Chosen for ints

template <class T>
void function(T) {
    static_assert(false, "function should only be called for ints");
}

int main() {
    function(1);
    // function(1l); // Error: static assertion failed
}

By employing this method, the error message clearly conveys that the function is intended solely for integers, enhancing the readability and comprehension of the code.

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