Const

Parameters and variables

const can be declared both on function parameters and on variables (static or automatic, including global or local). The interpretation varies between uses. A const static variable (global variable or static local variable) is a constant, and may be used for data like mathematical constants, such as const double PI = 3.14159 – realistically longer, or overall compile-time parameters. A constautomatic variable (non-static local variable) means that single assignment is happening, though a different value may be used each time, such as const int x_squared = x * x. A const parameter in pass-by-reference means that the referenced value is not modified – it is part of the contract – while a const parameter in pass-by-value (or the pointer itself, in pass-by-reference) does not add anything to the interface (as the value has been copied), but indicates that internally, the function does not modify the local copy of the parameter (it is a single assignment). For this reason, some favor using const in parameters only for pass-by-reference, where it changes the contract, but not for pass-by-value, where it exposes the implementation.