Windows apps
Collapse the table of content
Expand the table of content
Information
The topic you requested is included in another documentation set. For convenience, it's displayed below. Choose Switch to see the topic in its original location.

const (C++)

 

The new home for Visual Studio documentation is Visual Studio 2017 Documentation on docs.microsoft.com.

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.

  
      const declaration ;  
member-function const ;  

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.

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  
}  

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.

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:

Keywords

Show:
© 2017 Microsoft