Why are there defined constants and declared constants in CPP?
Why are there two ways to "declare" constants in CPP?
Which is better, or should I write, which of them should I use when?#define MYCON 100
const int MYCON=100
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Short rule: For conditional compilation (like different code fragments for DEBUG and RELEASE) use #define
. For all other cases use const
construction.
Using #define
produces a preprocessor symbol: it has no existence at all after preprocessing has occurred and is equivalent to having typed "100" into the file.
Features of preprocessor symbols:
- You can use them in preprocessor directive like
#ifdef
- It has lexical scope
- You can not take their address (and therefore cannot use them as arguments where a
type*
is expected)
Using const type
declares a c++ variable.
- You can not use this thing in preprocessor directives
- It follows the usual c++ scope rules
- You can takes it's address
It is widely considered better to use const
for "in program" constants and #define
only for conditional compilation (which represents a change from the (very!) old days when you could not always rely on c compiler to handle const
intelligently and using #define
was preferred). If nothing else this gives you better control of the symbol's scope.
In the original version of C, #define
was the only method available to declare a constant value. This was done at compile time rather than at run time: The compiler hardcoded the value for each instruction. const
is a feature of C++ (later added to C in limited fashion), and in C++ it is recommended that you use const rather than #define. const
variables actually exist in memory, and can be initialized at construction during runtime.
#define
is, however, still frequently used in C++, mainly for communication with the compiler.
Some examples:
Class header wrappers to prevent multiple class declarations:
#ifdef VAL_H
#define VAL_H
// Define class header
#endif // VAL_H
Conditional Compilation:
// Comment this line to disable debug output
#define DEBUG
// Some code
#ifdef DEBUG
// Only gets compiled if DEBUG is defined.
cerr << "Debug output here" << endl;
#endif
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