Is there a way to avoid implicit conversion to void*?

Question

I'm using an API that accepts void* in certain functions. I frequently accidentally pass the wrong pointer type to the function, and of course it compiles fine, but doesn't work at runtime.

Is there a way to disable implicit conversion to void* for pointers to a certain class?

Answer 1

Is there any way to disable implicit conversion to void* for pointers to a certain class?

No, you can't prevent the implicit conversion, but you could wrap the API function(s) in proxy functions that checks types at compile time and approve/disapprove them there.

Example:

#include <iostream> #include <string> #include <type_traits>  void api(void* p) { // your original API     std::cout << "void* " << p << '\n'; }  template<class T> void api_wrapper(T* p) { // your wrapper     // let constness fail in the original api instead of in the static_assert:     using type = std::remove_const_t<T>*;      static_assert(         // add your approved types here         std::is_convertible_v<type, std::ostream*> ||         std::is_convertible_v<type, std::string*>,         "Not an approved type"     );     api(p); }  int main() {     std::string foo;     api_wrapper(&std::cout);     api_wrapper(&foo);     //api_wrapper(&std::cin); // compile time error "Not an approved type" } 

If the set of pointer types that you would like to disapprove is very small, then instead of listing all the approved types in the static_assert, just list the disapproved types and adjust the boolean logic:

    static_assert(         // add your disapproved types here         not std::is_convertible_v<type, std::ostream*> &&         not std::is_convertible_v<type, std::string*>,         "Not an approved type"     );