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 T can be converted to a prvalue.53
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.54
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 ([basic.def.odr]),
the value contained in the referenced object is not accessed.
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:
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.