This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

684. Constant expressions involving the address of an automatic variable

[Voted into the WP at the September, 2008 meeting (resolution in paper N2757).]

The expressions that are excluded from being constant expressions in 7.7 [expr.const] paragraph 2 does not address an example like the following:

    void f() {
       int a;
       constexpr int* p = &a;    // should be ill-formed, currently isn't
    }

Suggested resolution:

Add the following bullet to the list in 7.7 [expr.const] paragraph 2:

• an id-expression that refers to a variable with automatic storage duration unless a permitted lvalue-to-rvalue conversion is applied (see above)

Proposed resolution (June, 2008):

1. Change 6.9.3.2 [basic.start.static] paragraph 1 as follows:

Objects with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (3.7.2) shall be zero-initialized (9.4 [dcl.init]) before any other initialization takes place. A reference with static or thread storage duration and an object of trivial or literal type with static or thread storage duration can be initialized with a constant expression (7.7 [expr.const]); this is called constant initialization. Constant initialization is performed:

• if an object of trivial or literal type with static or thread storage duration is initialized with a constant expression (7.7 [expr.const]), or

• if a reference with static or thread storage duration is initialized with a constant expression that is not an lvalue designating an object with thread or automatic storage duration.

Together, zero-initialization and constant initialization...

2. Add the following in 7.7 [expr.const] paragraph 2:

• an lvalue-to-rvalue conversion (4.1) unless it is applied to...

• an array-to-pointer conversion (7.3.3 [conv.array]) that is applied to an lvalue that designates an object with thread or automatic storage duration

• a unary operator & (7.6.2.2 [expr.unary.op]) that is applied to an lvalue that designates an object with thread or automatic storage duration

• an id-expression that refers to a variable or data member of reference type;

• ...

(Note: the change to 6.9.3.2 [basic.start.static] paragraph 1 needs to be reconciled with the conflicting change in issue 688.)