CODEWITHSUNDEEP
c++switch-statementconstantscompile-time-constant
Zhro
Zhro

Reputation: 2614

Is there a way to define a switch-compliant constant for static object members?

I understand that I can't use the result of getValue() in a switch statement because the case tree is built at compile time.

What I have is a class which contains static const members that have a constant value set at runtime from a constructor. The value to be set is always known at compile time.

Is it possible to use templates or some other solution to define these const objects in a type safe manner and keep them as static members of a class?

Note that I do NOT want an enum in this case as I want to switch on different types such as an int in the following example.

Example:

#include <iostream>

using namespace std;

class Some_Class {
private:
   int _value;

public:
   Some_Class(int value) {
      _value = value;
   }

   int getValue() const {
      return _value;
   }

   static const Some_Class ONE;
   static const Some_Class TWO;
}; // class

const Some_Class Some_Class::ONE(1);
const Some_Class Some_Class::TWO(2);

int main() {
   int value = 1;

   switch (value) {
      case Some_Class::ONE.getValue():
         cout << "Do thing 1" << endl;
         break;
      case Some_Class::TWO.getValue():
         cout << "Do thing 2" << endl;
   }

   return 0;
}

This does not work for the aforementioned problem:

main.cpp(29) : error C2051: case expression not constant
main.cpp(32) : error C2051: case expression not constant

Upvotes: 0

Views: 1054

Answers (1)

Weak to Enuma Elish
Weak to Enuma Elish

Reputation: 4637

Something similar can be done, provided the constructor can be constexpr and the class can be inherited. Since we can't have static constexpr Some_Class as a member of Some_Class, we get around that with a derived class.

#include <iostream>

namespace detail
{
    class Base_Class //Has the functionality
    {
    private:
        int _value;
    public:
        constexpr Base_Class(int value) : _value(value) {}
        constexpr int getValue() const
        {
            return _value;
        }
    };
}

//Inherits functionality, has static members
class Some_Class : public detail::Base_Class
{
public:
    using Base_Class::Base_Class;
    static constexpr Base_Class ONE{1};
    static constexpr Base_Class TWO{2};
};

int main()
{
    int value = 1;
    switch (value)
    {
    case Some_Class::ONE.getValue():
        std::cout << "Do thing 1" << std::endl;
        break;
    case Some_Class::TWO.getValue():
        std::cout << "Do thing 2" << std::endl;
    }
    return 0;
}

Upvotes: 4

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