How to represent constants in UML class diagram?

Question

I am working on developing a UML class diagram from C++ code. I have a classless header file that is basically a definitions file (it has structs, enums and defined constants). I know how to represent structs and enums in general, but I am unclear about how to represent (or if I should represent) constants in the class diagram. Additionally I am curious if the structs, enums and constants should be part of some larger container? (Like referencing the namespace helper or the definitions.h)

#ifndef DEFINITIONS_H
#define DEFINITIONS_H

#include <stdint.h>
#include <string>

namespace helper
{

  enum Colors
  {
    Red = 0,
    Blue,
    Green   
  };

  struct volunteer
  {
    std::string FirstName;
    std::string MiddleName;
    std::string LastName;
    std::string Occupation;
  };


  typedef const std::string Constant;
  Constant Foo = "Foo";
  Constant Bar = "Bar";
  Constant Bat = "Bat";
 }

#endif

score 5 · Accepted Answer · answered Jun 26 '17 at 16:41

The goal of a class diagram is to document relationships between classes as well as how objects of those classes can change:

In software engineering, a class diagram in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among objects.

^{_{Source: Wikipedia}}

The key elements here are:

Relationships between classes: given a class, what other classes does it use?
Operations that can be performed on a class: what methods/functions belong to a class?
State belonging to a class: what data does a class encapsulate?

A constant does not fit into any of these elements. It is not a relationship, and certainly not an operation. The closest element is state, but a constant is pretty much the opposite: it is static, and not tied to an object. While a class diagram documents classes, it is focused on elements of classes used by objects of those class types.

In your specific example, I would do the following:

The enum would be a class in the diagram, but would likely be empty. The only state is the integer that represents each enum value, but that is essentially a surrogate key and not referenced in code. Note that if you use a C++ enum class instead, you might have state worth documenting.
The struct should be a class with no behavior, but the state documented as public.
Those constants should not be documented in a UML class diagram. For one, they do not belong to a class, making them completely irrelevant. Two, constants do not belong in a class diagram anyway for reasons I outlined above.
I would create a separate UML package diagram for namespace helper (as an aside, I recommend picking a more descriptive name for this namespace) showing that the enum and struct are in the namespace. This diagram type also allows for documenting static members in general and constants specifically.

UML has an own metatype `Enumeration` (see p. 165 of the UML 2.5 specs) and this should be used, not an empty class. A `struct` should preferable be stereotyped with `<>`. — , Jul 22 '17 at 21:19

score 3 · Answer 2 · answered Jun 26 '17 at 16:37

3

A class diagram is supposed to show an entity level model. A constant is a minor implementation detail and logically, it is not a member of the class, it bears no meaning in the OO view of things. So the answer is "you don't".

answered Jun 26 '17 at 16:37

Martin Maat

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score 1 · Answer 3 · answered Jul 14 '17 at 14:04

1

Some of your helper information could be modeled as such:

The notion of a constant in UML exists, but it's usually inside a class and has a type. See the Employee class in the diagram above.

answered Jul 14 '17 at 14:04

Fuhrmanator

1,435
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score 0 · Answer 4 · answered Jun 26 '17 at 16:36

I would represent them as objects (or "instances" in IBM Rational terminology) of class Constant or std::string (whichever seems more appropriate). The object symbol can have the value added below the name. You could mark the value as {read-only} if desired.

How to represent constants in UML class diagram?

4 Answers4