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Concept
Const (computer programming)
Summary
In some programming languages, const is a type qualifier (a keyword applied to a data type) that indicates that the data is read-only. While this can be used to declare constants, in the C family of languages differs from similar constructs in other languages in being part of the type, and thus has complicated behavior when combined with pointers, references, composite data types, and type-checking. In other languages, the data is not in a single memory location, but copied at compile time on each use. Languages which use it include C, C++, D, JavaScript, Julia, and Rust.
When applied in an object declaration, it indicates that the object is a constant: its value may not be changed, unlike a variable. This basic use – to declare constants – has parallels in many other languages.
However, unlike in other languages, in the C family of languages the const is part of the type, not part of the object. For example, in C, declares an object x of int const type – the const is part of the type, as if it were parsed "(int const) x" – while in Ada, declares a constant (a kind of object) X of INTEGER type: the constant is part of the object, but not part of the type.
This has two subtle results. Firstly, const can be applied to parts of a more complex type – for example, int const * const x; declares a constant pointer to a constant integer, while int const * x; declares a variable pointer to a constant integer, and int * const x; declares a constant pointer to a variable integer. Secondly, because const is part of the type, it must match as part of type-checking. For example, the following code is invalid:
void f(int& x);
// ...
int const i;
f(i);
because the argument to f must be a variable integer, but i is a constant integer. This matching is a form of program correctness, and is known as const-correctness. This allows a form of programming by contract, where functions specify as part of their type signature whether they modify their arguments or not, and whether their return value is modifiable or not.
Official source
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