How Can I Prevent Implicit Type Conversions in Non-Constructing C Functions?

Avoiding Implicit Conversions in Non-Constructing Functions

In the provided code, the function function is declared to accept an integer parameter. However, it also inadvertently accepts characters, booleans, and longs due to implicit casting. To prevent this undesirable behavior, we seek methods to enforce strict type matching and prohibit implicit conversions.

Using Function Templates to Enforce Type Mismatch

One approach to avoid implicit conversions is to define a function template that matches all types except the desired type. For instance, we can create a template for other types and mark it as deleted, effectively prohibiting its compilation:

void function(int); // this will be selected for int only

template<class T>
void function(T) = delete; // C++11

In this way, the non-template function with a direct match, in this case void function(int), will always be selected for integers. Any attempt to call the function with other types will trigger an error due to the deleted template.

Pre-C 11 Approach Using a Delete Overload

Prior to C 11, a different method was necessary to achieve type-strict function calls without implicit conversions. This involved creating a DeleteOverload class and using it to disable overload selection for non-desired types:

// because this ugly code will give you compilation error for all other types
class DeleteOverload
{
private:
    DeleteOverload(void*);
};

template<class T>
void function(T a, DeleteOverload = 0);

void function(int a)
{}

C 23 Static Assertion for Enforcing Type Constraints

C 23 introduces a more user-friendly approach using static_assert(false, msg). This allows for clearer error messages upon type mismatch:

void function(int) {} // this will be selected for int only

template<class T>
void function(T) {
    // since C++23
    static_assert(false, "function shall be called for int only");
}
int main() {
    function(1);
    // function(1l);
    // ^^^^^^^^^^^^ produces error:
    // error: static assertion failed: function shall be called for int only
}

Conclusion

By employing these techniques, we can avoid implicit conversions in non-constructing functions, ensuring stricter type checking and eliminating potential errors caused by inadvertent type mismatches.

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