The definition identifies the code or data associated with the name of the variable, function, class, and so on. The compiler necessarily requires the definition to allocate the storage space for the declared entity. When a variable is defined, it contains an amount of memory made up of multiple bytes for that variable. A function definition produces code for the function. We can define a program element only once in a program because the definition is a single specification of a program element. The relationship between statement and definition can be one too many.
In some situations, a program element cannot be defined but declared, for example when a function is never invoked or its address is never used, even if it is declared. Another example is where the class definition is not used while it must be declared.
A declaration is used to specify the names of the program, such as the name of a variable, function, namespace, classes, and so on. You cannot use a name in a program without its declaration. Program elements can be declared multiple times, unlike definition. Multiple declarations can only be achieved when the different declarations are made using the identical format. The declaration is the means of providing visibility to the program element from the compiler’s perspective.
The statement serves the purpose of definition, only in certain cases, the condition is not implied as given below.
- When the static data member is declared inside a class declaration, in that case, it is not a declaration. Because it generates a single copy for all objects in the class, and the static data members are the components of the objects of a provided class type.
- A variable is declared without an initializer or function body but includes external specifiers. This indicates that the definition could be for the other function and provides the external binding name.
- The declaration of the class name without including a definition such as class T;
Usually, the declaration is carried out in a scope. The scope decides the visibility of the declared name and the duration of the defined object.
Key differences between definition and statement
- The definition of a program element determines the value associated with that element. On the other hand, the declaration of a program element specifies its name and type to the compiler.
- The definition of the program element reserves a certain amount of memory while the declaration does not imply memory allocation.
- A program element can be declared multiple times. Conversely, the definition incorporates a unique specification with the name of the program element that could be distinguished by any code or data.
- The scope in the declaration describes the visibility of the variable, function, object, class, enum, etc. In contrast, in the definition scope is related to duration.
Definition and Declaration are very confusing terms if you are new to programming. The two concepts are different in some way, as the definition involves allocating memory to variables while declaration memory is unallocated. The declaration can be made more than once; conversely, an entity can be defined exactly once in a program.
The definition is automatically a statement in most scenarios. Now let’s understand the difference between definition and statement with the detailed comparison table.
|Basis for comparison||Definition||Statement|
|BASIC||Determines the value stored in variable, function, or class.||Specifies the name and type of variable, function, class, and so on.|
|Memory allocation||It happens||It does not take place.|
|Repetition||Declarations cannot be defined again if once it is already defined.||Redeclaration can easily be possible.|
|Scope||Duration is determined||Visibility is specified|