This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-04-13
1773. Out-of-lifetime lvalue-to-rvalue conversionSection: 7.3.2 [conv.lval] Status: C++14 Submitter: Canada Date: 2013-09-24[Moved to DR at the February, 2014 meeting.]
Consider an example like the following:
struct Base {
virtual int call() = 0;
};
Base *foo() {
constexpr int x = 0;
struct Local : Base {
virtual int call() { return x; }
};
static Local local;
return &local;
}
int main() {
return foo()->call();
}
While the likely intention is that the lvalue-to-rvalue conversion of the block-scope constant is implemented by using the value of the constant expression in place of reading from storage, it seems that the wording of 7.3.2 [conv.lval] paragraph 2 does not prevent this program from being subject to undefined behaviour caused by lifetime violation. In particular, it seems that a name expression that appears in a potentially-evaluated expression such that the object named is not odr-used (by that instance of the name) may still be evaluated, in theory, as an lvalue through which the object named or a subobject thereof is accessed.
Proposed resolution (September, 2013):
Change 7.3.2 [conv.lval] paragraph 2 as follows:
When an lvalue-to-rvalue conversion occurs in an unevaluated operand or a subexpression thereof ( Clause 7 [expr]) 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 (6.3 [basic.def.odr]),
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] In all other cases, the result of the conversion...
RetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4