7 Standard conversions [conv]

7.1 Lvalue-to-rvalue conversion [conv.lval]

A glvalue of a non-function, non-array type T can be converted to a prvalue.56
If T is an incomplete type, a program that necessitates this conversion is ill-formed.
If T is a non-class type, the type of the prvalue is the cv-unqualified version of T.
Otherwise, the type of the prvalue is T.57
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 ex of the set of potential results of e, and ex names a variable x that is not odr-used by ex,
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 due to 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.
    [Note
    :
    Since no value is fetched from memory, there is no side effect for a volatile access ([intro.execution]), and an inactive member of a union may be accessed.
    end note
    ]
  • Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
  • Otherwise, if the object to which the glvalue refers contains an invalid pointer value ([basic.stc.dynamic.deallocation], [basic.stc.dynamic.safety]), the behavior is implementation-defined.
  • Otherwise, the value contained in the object indicated by the glvalue is the prvalue result.
[Note
:
See also [basic.lval].
end note
]
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.