Constant (computer programming)

In computer programming, a constant is a value that is not altered by the program during normal execution. When associated with an identifier, a constant is said to be "named," although the terms "constant" and "named constant" are often used interchangeably. This is contrasted with a variable, which is an identifier with a value that can be changed during normal execution. To simplify, constants' values remains, while the values of variables varies, hence both their names.

Constants are useful for both programmers and compilers: for programmers, they are a form of self-documenting code and allow reasoning about correctness, while for compilers, they allow compile-time and run-time checks that verify that constancy assumptions are not violated,^[a] and allow or simplify some compiler optimizations.

There are various specific realizations of the general notion of a constant, with subtle distinctions that are often overlooked. The most significant are: compile-time (statically valued) constants, run-time (dynamically valued) constants, immutable objects, and constant types (const).

Typical examples of compile-time constants include mathematical constants, values from standards (here maximum transmission unit), or internal configuration values (here characters per line), such as these C examples:

const float PI = 3.1415927;  // maximal single float precision
const unsigned int MTU = 1500;  // Ethernet v2, RFC 894
const unsigned int COLUMNS = 80;

Typical examples of run-time constants are values calculated based on inputs to a function, such as this C++ example:

void f(std::string s) {
  const size_t l = s.length();
  // ...
}

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