the procedure described at the top of this section can always be used without introducing ambiguities or confusion. 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 double const PI = 3.14159 – realistically longer, or overall compile-time parameters. A const automatic variable (non-static local variable) means that single assignment is happening, though a different value may be used each time, such as int const 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. C++ Methods In order to take advantage of the design by contract approach for user-defined types (structs and classes), which can have methods as well as member data, the programmer may tag instance methods as const if they don't modify the object's data members. Applying the const qualifier to instance methods thus is an essential feature for const-correctness, and is not available in many other object-oriented languages such as Java and C# or in Microsoft's C++/CLI or Managed Extensions for C++. While const methods can be called by const and non-const objects alike, non-const methods can only be invoked by non-const objects. The const modifier on an instance method applies to the object pointed to by the "this" pointer, which is an implicit argument passed to all instance methods. Thus having const methods is a way to apply const-correctness to the implicit "this" pointer argument just like other arguments. This example illustrates: class C { int i; public: int Get() const // Note the "const" tag { return i; } void Set(int j) // Note the lack of "const" { i = j; } }; void Foo(C& nonConstC, C const& constC) { int y = nonConstC.Get(); //