When modifying a data declaration, the const keyword specifies that the object or variable is not modifiable.
Syntax
One of the easiest ways of doing this is to use the const qualifier. A qualifier, in C, is simply something that modifies, or adds a quality to, what follows. These behave much like an adjective in a sentence: 'the weary fellow', 'the old hag', 'the constant integer'. The easiest way to make use of the const modifier is in a simple variable declaration.
const values
The const keyword specifies that a variable's value is constant and tells the compiler to prevent the programmer from modifying it.
In C++, you can use the const keyword instead of the #define preprocessor directive to define constant values. Values defined with const are subject to type checking, and can be used in place of constant expressions. In C++, you can specify the size of an array with a const variable as follows:
In C, constant values default to external linkage, so they can appear only in source files. In C++, constant values default to internal linkage, which allows them to appear in header files.
The const keyword can also be used in pointer declarations.
A pointer to a variable declared as const can be assigned only to a pointer that is also declared as const.
You can use pointers to constant data as function parameters to prevent the function from modifying a parameter passed through a pointer.
For objects that are declared as const, you can only call constant member functions. This ensures that the constant object is never modified.
You can call either constant or nonconstant member functions for a nonconstant object. You can also overload a member function using the const keyword; this allows a different version of the function to be called for constant and nonconstant objects.
You cannot declare constructors or destructors with the const keyword.
const member functions
Declaring a member function with the const keyword specifies that the function is a 'read-only' function that does not modify the object for which it is called. A constant member function cannot modify any non-static data members or call any member functions that aren't constant.To declare a constant member function, place the const keyword after the closing parenthesis of the argument list. The const keyword is required in both the declaration and the definition.
C and C++ const differences
Dev C++ Constant
When you declare a variable as const in a C source code file, you do so as:
You can then use this variable in another module as follows:
But to get the same behavior in C++, you must declare your const variable as:
If you wish to declare an extern variable in a C++ source code file for use in a C source code file, use:
to prevent name mangling by the C++ compiler.
Remarks
When following a member function's parameter list, the const keyword specifies that the function does not modify the object for which it is invoked.
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