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.57 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.58

When an lvalue-to-rvalue conversion is applied to an expression e, and either

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:

[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.