Can you use keyword explicit to prevent automatic conversion of method parameters?

Question

I know you can use C++ keyword \'explicit\' for constructors of classes to prevent an automatic conversion of type. Can you use this same command to prevent the conversion

Answer 1

No, you can't use explicit, but you can use a templated function to catch the incorrect parameter types.

With C++11, you can declare the templated function as deleted. Here is a simple example:

#include <iostream>

struct Thing {
    void Foo(int value) {
        std::cout << "Foo: value" << std::endl;
    }

    template <typename T>
    void Foo(T value) = delete;
};

This gives the following error message if you try to call Thing::Foo with a size_t parameter:

error: use of deleted function
    ‘void Thing::Foo(T) [with T = long unsigned int]’

---

In pre-C++11 code, it can be accomplished using an undefined private function instead.

class ClassThatOnlyTakesBoolsAndUIntsAsArguments
{
public:
  // Assume definitions for these exist elsewhere
  void Method(bool arg1);
  void Method(unsigned int arg1);

  // Below just an example showing how to do the same thing with more arguments
  void MethodWithMoreParms(bool arg1, SomeType& arg2);
  void MethodWithMoreParms(unsigned int arg1, SomeType& arg2);

private:
  // You can leave these undefined
  template<typename T>
  void Method(T arg1);

  // Below just an example showing how to do the same thing with more arguments
  template<typename T>
  void MethodWithMoreParms(T arg1, SomeType& arg2);
};

The disadvantage is that the code and the error message are less clear in this case, so the C++11 option should be selected whenever available.

Repeat this pattern for every method that takes the bool or unsigned int. Do not provide an implementation for the templatized version of the method.

This will force the user to always explicitly call the bool or unsigned int version.

Any attempt to call Method with a type other than bool or unsigned int will fail to compile because the member is private, subject to the standard exceptions to visibility rules, of course (friend, internal calls, etc.). If something that does have access calls the private method, you will get a linker error.

Answer 2

No. explicit prevents automatic conversion between specific classes, irrespective of context. And of course you can't do it for built-in classes.