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In the C++ programming language, a reference is a simple reference datatype that is less powerful but safer than the pointer type inherited from C.The name C++ reference may cause confusion, as in computer science a reference is a general concept datatype, with pointers and C++ references being specific reference datatype implementations.
The first two of these, const and volatile, are also present in C++, and are the only type qualifiers in C++. Thus in C++ the term "cv-qualified type" (for const and volatile) is often used for "qualified type", while the terms "c-qualified type" and "v-qualified type" are used when only one of the qualifiers is relevant.
Conversely, an operator returning a reference type based on the lvalue-ness of the expression was deemed too confusing. The initial proposal to the C++ standards committee outlined a combination of the two variants; the operator would return a reference type only if the declared type of the expression included a reference.
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.
There must exist argument values such that, after argument substitution, the expression in the return statement produces a constant expression. Before C++11, the values of variables could be used in constant expressions only if the variables are declared const, have an initializer which is a constant expression, and are of integral or ...
For instance, a container<T>::reference must be a true lvalue of type T. This is not the case with vector<bool>::reference , which is a proxy class convertible to bool . [ 12 ] Similarly, the vector<bool>::iterator does not yield a bool& when dereferenced .
In the third line, x is reassigned the value of 23. Finally, y is assigned the value of 32.4. For an assignment operation, it is necessary that the value of the expression is well-defined (it is a valid rvalue) and that the variable represents a modifiable entity (it is a valid modifiable (non-const) lvalue).
R-values can be l-values (see below) or non-l-values—a term only used to distinguish from l-values. Consider the C expression 4 + 9. When executed, the computer generates an integer value of 13, but because the program has not explicitly designated where in the computer this 13 is stored, the expression is a non l-value.