Understanding how to correctly treat c++ class constants

Question

Consider the following:

namespace MyNamespace{
class MyClass {
public:
   // Public area
private:
   // Private area
protected:
   // Protected area
}; /* Class */
} /* Namespace */

And consider that I would like to define a constant which is specific for my class. I usually do the following:

namespace MyNamespace{
// Constants
const int MYINT = 12;
const std::string MYSTR = std::string("Hello");
// Class definition
class MyClass {
public:
   // Public area
private:
   // Private area
protected:
   // Protected area
}; /* Class */
} /* Namespace */

In this way I can get my variable in this way (somewhere in my code):

MyNamespace::MYINT;
MyNamespace::MYSTR;

Is this a good practice? Considering that constants can be treated in several ways (for example numeric constants are often treated using enum), what is the best approach to define a constant (related to a class, but that can be also useful somewhere else)?

Thankyou

Solution 1

If you want the constants specific to the class and also want them to be useful somewhere else as you said, possibly outside the class, then define them as static member data in the public section of the class:

//.h file
class MyClass 
{
 public:
   //constants declarations
   static const int MYINT;
   static const std::string MYSTR;
};

//.cpp file
//constants definitions 
const int MyClass::MYINT = 12;
const std::string MyClass::MYSTR = std::string("Hello");

Usage (or access):

std::cout << MyClass::MYINT << std::endl;
std::cout << MyClass::MYSTR << std::endl;

Output:

12
Hello

Online Demo: http://www.ideone.com/2xJsy

---

You can also use enum if you want to define many integral constants and all of them are somehow related, for example this:

class shirt
{
 public:
   //constants declarations
   enum shirt_size
   {
        small, 
        medium,
        large,
        extra_large
   };
};

But if the integral constants are not related, then it wouldn't make much sense to define them as enum, in my opinion.

Solution 2

There is no "best" solution as of course that is a very subjective term.

Considering that you mention the constants being used somewhere else, we can say that they should be declared in either the protected (if they are to be used exclusively by derived classes) or more likely the public section of the class.

Constants that are not of integer type should be defined as static const members (but you will have to be careful of the order of static initialization if there are any other static objects that refer to these constants).

Constants of integer type can either be declared as static const int or as enums, as you already mention. The discriminating factor here is whether two or more constants can be logically grouped together.

For example, this is probably a good idea:

class MyClass {
    public:
        enum {
            Color_Red,
            Color_Green,
            Color_Blue,
        };
};

While this is not:

class MyClass {
    public:
        enum {
            Color_Red,
            Vehicle_Car,
        };
};

Comments

[Waihon Yew]

@Andry: No, you cannot. You need to declare them in the header and define them in a separate .cpp file.

[Nawaz]

@Andry: No, you cannot. If you do, you'll get multiple definition error if you include the header file to many cpp file. So the best approach is : the definition should be in .cpp file, as I edited my answer.

[David Rodríguez - dribeas]

@Andry: Can I declare and defined them in header only? Short answer is no, but there is a longer answer... if the constant is of integral type, you can get away with just declaring and assigning the value in the header. The detail is that you can only do this if the integral constant is not used, where used basically means used as an lvalue. I.e. you can use it as a compile time constant or in any circumstance where an rvalue suffices: int x = 1 + type::constant;, but not where an lvalue is required: int const & k = type::constant;.

[David Rodríguez - dribeas]

@Nawaz: For integral constants, it is usually better to provide the value in the declaration, instead of the definition. That enables the compiler to use that value as a compile time constant. struct test { static const int a = 5; static const int b; }; int const test::a; int const test::b = 7; int array[ test::a ]; /*ok*/ int error[test::b]; /*not ok*/

[Nawaz]

@David Rodríguez: Nice. I do that usually. I didn't do that now, because there is const std::string also, which I can't initialize in the class itself. So for consistency, I did that. :-)