[Tutorial] Pipes Part 4 - Additional Information

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Constants vs #define vs enum


When writing code in C / C++ - or anyone other language for that matter – your code often becomes littered with hard-coded values that only you, the programmer, understands. These values are often referred to as ‘magic numbers’ as their use and derivation is a mystery to other programmers. The use of a constant in your code allows you to assign a more textual and descriptive value to your magic number. Which is more readable, the value `52` or the constant `NUMBER_OF_CARDS_IN_DECK` ?

Constants in C/ C++ can be handled a number of ways, each with their own pros and cons. These include:

  • the const qualifier for a variable
  • the #define preprocessor directive
  • the enum keyword

The Const Qualifier


Adding the `const` qualifier to a variable declaration tells the compiler that the variable cannot be modified by code. Any attempt to update the value of the variable will result in a compile time error. Declaring a variable as a constant allows the compiler to optimise the code as it knows in advance that no updates are possible. 

const int cards_in_deck = 52;
cards_in_deck = 53;               < this will result in a compile time error.

The `const` keyword can also be used when specifying parameters to a function. Adding the `const` keyword is used to ensure that the parameter is not updated within the function. Again, the use of the `const` keyword allows the compiler to optimize the code knowing that the variable is immutable. 

int cards_in_deck = 52;
 
shuffleCards(cards_in_deck);
cards_in_deck = 53;      // This will compile correctly as the cards_in_deck variable is not a constant.
 
void shuffleCards(const int cards_to_shuffle) {
 
  cards_to_shuffle--;     // This will fail to compile as the parameter is defined as a constant.
 
}

The #define Preprocessor Directive


Before compiling an application, the C / C++ compiler runs a preprocessing task that looks for compiler directives and acts on them first. Preprocessing directives can be used to define constants and to include / exclude sections of code based on the platform the compilation is targeting or to remove debugging code from a release build.

More information on preprocessing directives can be found here http://www.cplusplus.com/doc/tutorial/preprocessor/

The simple #define directive instructs the preprocessor to look for the first operand and replace it within the code using the second operand. In the example below, I have created constants for three animal types, as numeric values, and three name constants as strings. One thing to note though is that the preprocessor will not check the validity of the substitutions it makes and can result in compilation errors.

#define CAT           1
#define DOG           2
#define MOUSE         3
 
#define CAT_NAME      "Leo"
#define DOG_NAME      "Fido"
#define MOUSE_NAME    "Mickey"
 
int myAnimal = CAT;
String myAnimalName = CAT_NAME;
 
String myOtherCat = DOG;           < This will fail to compile as the variable is defined as a string but an attempt is being made to initialize it with a numeric value.

One nice thing about the #define directive is that it allows you to combine values together, like those shown below. 

#define SMALL           1
#define MEDIUM          2
#define LARGE           3
 
#define CAT             10
#define DOG             20
#define MOUSE           30
 
#define CAT_SMALL       CAT + SMALL
#define CAT_MEDIUM      CAT + MEDIUM
#define CAT_LARGE       CAT + LARGE
 
#define DOG_SMALL       DOG + SMALL
#define DOG_MEDIUM      DOG + MEDIUM
#define DOG_LARGE       DOG + LARGE
 
#define MOUSE_SMALL     MOUSE + SMALL
#define MOUSE_MEDIUM    MOUSE + MEDIUM
#define MOUSE_LARGE     MOUSE + LARGE
 
int myCat = CAT;
int myLargeCat = CAT_LARGE;

Enumerations


In C / C++, an enumeration is a data type consisting of a set of named values of type integer. Enumerations can be named or anonymous as shown below. Note in the first example, I have nominated that the first element, cat, is assigned the numeric value 10. Dog will automatically be assigned the value of `11` and mouse, `12`. In the second example where no starting number is specified, the items are numbered from zero onwards. 

enum Pet {
  cat = 10,
  dog,
  mouse
};

enum {
  horse,
  cow,
  sheep
};

The following declarations are all valid.

int aHorse = horse;
Pet aCat = cat;
Pet aDog = Pet::dog;

Named enumerations can be used when defining parameters to a function. As you can see from the sample calls, the declaration will not prevent you from passing any other enumeration type or even an integer.

void printPetDetails(Pet thePet) {

  switch (thePet) {

    case Pet::cat:
      printf("cat\n");
      break;

    case Pet::dog:
      printf("dog\n");
      break;

    case Pet::mouse:
      printf("mouse\n");
      break;

    default:
      printf("other\n");
      break;

  }

}

printPetDetails(aCat);			< prints “cat”
printPetDetails(aDog); 		        < prints “dog”
printPetDetails(aHorse); 		< prints “other” but is illogical as it isn’t a Pet
printPetDetails(2); 			< prints “other” but is illogical as it isn’t a Pet

Enumerations can also include the class keyword as shown below. This overcomes the limitations in the previous example by ensuring that references to a Pet are checked in both the declarations and passing of variables to a function.

enum class Pet {
  cat = 10,
  dog,
  mouse
};

enum class FarmAnimal {
  horse,
  cow,
  sheep
};


Pet aCat = Pet::cat;
Pet aDog = dog;			< Will not compile.  The elements of Pet must be specified as Pet::{element}
FarmAnimal aHorse = FarmAnimal::horse;

printPetDetails(aCat);		< prints “cat”
printPetDetails(aDog); 		< prints “dog”
printPetDetails(aHorse); 	< will not compile as the compiler cannot convert a FarmAnimal to a Pet.
printPetDetails(2); 		< will not compile as the compiler cannot convert an integer to a Pet.

However, enumerations declared with the class scope are not viewed as simple integers anymore.

You can use any or all three of these techniques in your code. Use whatever you feel comfortable with!

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