Answer.

#define is a preprocessor directive that simply replaces all occurrences of an identifier with a value before compilation. It does not create variables, does not know about types, does not check boundaries, and does not respect scope.

const is a qualifier that creates a real variable, but it is locked for writing after initialization. A const variable has a type, scope, participates in debugging, and can be safer. Such variables are placed in the .rodata segment or on the stack/in memory.

When and why to use:

#define — for simple scalar values needed at compile-time, especially if used in preprocessor conditions or array sizes (#define SIZE 16).
const — for values that need to be passed in a type-safe way, debugged, and safely exported (in header files, inter-module).
For pointers to strings and arrays, it is preferable to use const.

Example:

#define PI 3.14159
const double G = 9.81;

int arr[PI]; // ERROR! PI is not an integer
int arr2[G]; // ERROR! G is not a compile-time value

Tricky question.

Question: Does const int a = 10; work as a compile-time constant for creating an array: int arr[a];?

Answer: No. In C, const is a qualifier, but the variable is created and initialized at runtime, not at compile-time, so the array size must be a literal or an expression known to the compiler. Use #define, or enum { SIZE = 10 };.

Example of error:

const int a = 5;
int arr[a]; // In C89/90 this won't work (VLAs appeared in C99, but not everywhere)

History

In a media server, they attempted to use const int to set buffer sizes, thinking it was a "compile-time constant". On one compiler (GCC, C99) everything worked, but on most—compilation error (VLA not supported), they urgently rewrote it to #define.

History

In platform-dependent code, they defined a string with #define NAME "MyApp", used it in several places, and forgot to put it in parentheses. After adding characters to the string without explicit parentheses, they got incorrect results, leading to strange bugs in the logs.

History

In a project with multiple modules, they defined a constant through #define in two places with different values (copy-paste). The result was that the modules worked with different constants, causing data nonconformities, which were only fixed through meticulous debugging.

What are the key differences between const and #define for defining constants in C? When and why to use each approach, with examples of common mistakes?

Answer.

When and why to use:

Tricky question.