[conv.lval] (original) (raw)
7 Expressions [expr]
7.3 Standard conversions [conv]
7.3.1 Lvalue-to-rvalue conversion [conv.lval]
A glvalue of a non-function, non-array type Tcan be converted to a prvalue.52
If T is an incomplete type, a program that necessitates this conversion is ill-formed.
If Tis a non-class type, the type of the prvalue is the cv-unqualified version of T.
Otherwise, the type of the prvalue is T.53
When an lvalue-to-rvalue conversion is applied to an expression E, and either
- E is not potentially evaluated, or
- the evaluation of E results in the evaluation of a member of the set of potential results of E, and names a variable x that is not odr-used by ([basic.def.odr]),
the value contained in the referenced object is not accessed.
[ Example
:
struct S { int n; }; auto f() { S x { 1 }; constexpr S y { 2 }; return [&](bool b) { return (b ? y : x).n; }; } auto g = f(); int m = g(false); // undefined behavior: access of x.n outside its lifetime int n = g(true); // OK, does not access y.n
— end example
]
The result of the conversion is determined according to the following rules:
- If T is cv std::nullptr_t, the result is a null pointer constant ([conv.ptr]).
[ Note
:
Since the conversion does not access the object to which the glvalue refers, there is no side effect even if T is volatile-qualified ([intro.execution]), and the glvalue can refer to an inactive member of a union ([class.union]).
— end note
] - Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
- Otherwise, the object indicated by the glvalue is read ([defns.access]), and the value contained in the object is the prvalue result.
For historical reasons, this conversion is called the “lvalue-to-rvalue” conversion, even though that name does not accurately reflect the taxonomy of expressions described in [basic.lval].
In C++ class and array prvalues can have cv-qualified types.
This differs from ISO C, in which non-lvalues never have cv-qualified types.