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const float pi = 3.14159f;
float degrees;
float radians;
/* ... */
radians = degrees * pi / 180;

Non-Compliant Code Example

In this example, adapted from \[[Dewhurst 02|AA. C++ References#Dewhurst 02]\] Gotcha #25, a macro is used to define a maximum value, but there is no type information associated with the macro.

#define MAX (1<<16)
// ...
void f(int);
void f(long);
// ...
f(MAX);  // which f?

The value 1 << 16 could be an int or a long depending on the platform. As a result, this code becomes platform dependent.

Compliant Solution

Using a constant associates type information with the value and eliminates the possibility of confusion.

int const max = 1<<16;
// ...
void f(int);
void f(long);
// ...
f(max);

Exceptions

DCL00-EX1: It is acceptable to define valueless macros to serve as 'inclusion guards'. That is, the macro serves to control the multiple inclusion of header files, as in the following example:

#ifndef SOME_HEADER_H
#define SOME_HEADER_H
...  // content of header file
#endif

See PRE06-C. Enclose header files in an inclusion guard for more information on inclusion guards.

Risk Assessment

Failing to const-qualify immutable objects can result in a constant being modified at runtime.

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