7 Expressions [expr]

7.3 Standard conversions [conv]

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

1
#
A glvalue of a non-function, non-array type T can be converted to a prvalue.43
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.44
2
#
When an lvalue-to-rvalue conversion is applied to an expression E, and either
  • (2.1)
    E is not potentially evaluated, or
  • (2.2)
    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 1: 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]
3
#
The result of the conversion is determined according to the following rules:
  • (3.1)
    If T is cv std​::​nullptr_t, the result is a null pointer constant ([conv.ptr]).
    [Note 1: 
    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]
  • (3.2)
    Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
  • (3.3)
    Otherwise, if the object to which the glvalue refers contains an invalid pointer value ([basic.compound]), the behavior is implementation-defined.
  • (3.4)
    Otherwise, if the bits in the value representation of the object to which the glvalue refers are not valid for the object's type, the behavior is undefined.
    [Example 2: bool f() { bool b = true; char c = 42; memcpy(&b, &c, 1); return b; // undefined behavior if 42 is not a valid value representation for bool } — end example]
  • (3.5)
    Otherwise, the object indicated by the glvalue is read ([defns.access]).
    Let V be the value contained in the object.
    If T is an integer type, the prvalue result is the value of type T congruent ([basic.fundamental]) to V, and V otherwise.
4
#
[Note 2: 
See also [basic.lval].
— end note]
43)43)
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].
44)44)
In C++ class and array prvalues can have cv-qualified types.
This differs from C, in which non-lvalues never have cv-qualified types.