Question

detail discription of constant in aspects of language​

Answer 1

In computer programming, a constant is a value that cannot be altered by the program during normal execution, i.e., the value is constant.[a] 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, i.e., the value is variable. 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 constancy assumptions are not violated, 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();

// ...

}