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How does constant defined by const differ from the constant defined by the preprocessor statement?
Wiki User
∙ 6y ago
The preprocessor #define directive creates a macro, not a constant. While you can define a literal constant, and call it a constant, this is not the same as an actual constant. Constants (those defined with the const keyword) are really no different to variables; they have a size and type, they can be referenced, pointed at, and they have scope. In fact they only differ from variables in that you cannot alter their value once instantiated (the very definition of a constant). Macros have none of these properties, they are nothing more than text-replacements. And since they are inline expanded before compilation, the compiler never sees them, so you not only lose type-safety and scope, you also lose the help of the debugger. And on top of all that you also cannot point nor refer to a macro, nor can a macro refer to an object. You can only #define literal values, string literals and functions.
Whenever you have the option of using const or #define, go with const every time. It is type-safe, thus you automatically enlist the compiler to help spot any problems that would otherwise be invisible to you via a macro, and therefore difficult to track down. The #define directive should only ever be used when there is no suitable alternative within the C++ language itself, or when a macro helps resolves a complication that would be difficult to accomplish any other way. Keep in mind that macros are not actually part of the language, they are nothing more than simple text-replacements, akin to an automated copy/paste if you like. But when used correctly, such as when assembling top-level constructs from many pieces of boilerplate, they can greatly simplify your code. Just remember that the debugger can't help you debug a macro -- you're completely on your own.
Wiki User
∙ 10y ago
Wiki User
∙ 6y agoPreprocessor directives (macros) are not type safe. Always express ideas in code, never use a macro in place of a type safe language construct (like const). Remember that the compiler never sees the macro it only sees the code generated from the macro by the preprocessor; the macro itself is stripped out of the source along with user-comments because the compiler cannot interpret symbols that aren't part of the language specification. Macros are intended to modify source code in ways that would be difficult or impossible to achieve with the language specification alone. In C++ there are very few reasons to prefer a macro over a language feature.
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