const (C++)

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01/03/2018

The latest version of this topic can be found at const (C++).

When modifying a data declaration, the const keyword specifies that the object or variable is not modifiable.

Syntax

  
      const declaration ;  
member-function const ;

const values

The const keyword specifies that a variable's value is constant and tells the compiler to prevent the programmer from modifying it.

// constant_values1.cpp  
int main() {  
   const int i = 5;  
   i = 10;   // C3892  
   i++;   // C2105  
}

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:

// constant_values2.cpp  
// compile with: /c  
const int maxarray = 255;  
char store_char[maxarray];  // allowed in C++; not allowed in C

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.

// constant_values3.cpp  
int main() {  
   char *mybuf = 0, *yourbuf;  
   char *const aptr = mybuf;  
   *aptr = 'a';   // OK  
   aptr = yourbuf;   // C3892  
}

A pointer to a variable declared as const can be assigned only to a pointer that is also declared as const.

// constant_values4.cpp  
#include <stdio.h>  
int main() {  
   const char *mybuf = "test";  
   char *yourbuf = "test2";  
   printf_s("%s\n", mybuf);  
  
   const char *bptr = mybuf;   // Pointer to constant data  
   printf_s("%s\n", bptr);  
  
   // *bptr = 'a';   // Error  
}

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.

birthday.getMonth();    // Okay  
birthday.setMonth( 4 ); // Error

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.

// constant_member_function.cpp  
class Date  
{  
public:  
   Date( int mn, int dy, int yr );  
   int getMonth() const;     // A read-only function  
   void setMonth( int mn );   // A write function; can't be const  
private:  
   int month;  
};  
  
int Date::getMonth() const  
{  
   return month;        // Doesn't modify anything  
}  
void Date::setMonth( int mn )  
{  
   month = mn;          // Modifies data member  
}  
int main()  
{  
   Date MyDate( 7, 4, 1998 );  
   const Date BirthDate( 1, 18, 1953 );  
   MyDate.setMonth( 4 );    // Okay  
   BirthDate.getMonth();    // Okay  
   BirthDate.setMonth( 4 ); // C2662 Error  
}

C and C++ const Differences

When you declare a variable as const in a C source code file, you do so as:

const int i = 2;

You can then use this variable in another module as follows:

extern const int i;

But to get the same behavior in C++, you must declare your const variable as:

extern const int i = 2;

If you wish to declare an extern variable in a C++ source code file for use in a C source code file, use:

extern "C" const int x=10;

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.

For more information on const, see the following topics:

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