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Difference between #define and const in C?

  • Difficulty Level : Easy
  • Last Updated : 16 Sep, 2021

#define is a preprocessor directive. Data defined by the #define macro definition are preprocessed, so that your entire code can use it. This can free up space and increase compilation times.

const variables are considered variables, and not macro definitions. 

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Long story short: CONSTs are handled by the compiler, where as #DEFINEs are handled by the pre-processor.

The big advantage of const over #define is type checking. #defines can’t be type checked, so this can cause problems when trying to determine the data type. If the variable is, instead, a constant then we can grab the type of the data that is stored in that constant variable. 

Since const are considered variables, we can use pointers on them. This means we can typecast, move addresses, and everything else you’d be able to do with a regular variable besides change the data itself, since the data assigned to that variable is constant.

In general, const is a better option if we have a choice and it can successfully apply to the code. There are situations when #define cannot be replaced by const. For example, #define can take parameters (See this for example). #define can also be used to replace some text in a program with another text. 

This article is contributed by Abhay Rathi and edited by Nolan Taft. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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