https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92124
Bug ID: 92124
Summary: std::vector copy-assigning when it should move-assign.
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: cassio.neri at gmail dot com
Target Milestone: ---
Consider two vectors a and rv. In the situation below a = std::move(rv)
copy-assigns elements of rv into a, violating
[container.requirements.general]/4, Table 83: "All existing elements of a are
either move assigned to or destroyed" (See [1].)
It happens with std::vector<X, A<X>> such that:
1) X's move-constructor might throw (though I'm assigning and not
constructing);
2) A<X> does not propagate on move-assignment and allocators used by source and
target vectors do not compare equal.
The following MCVE contains some boiler plate and the most important parts are
indicated by comments.
#include <cstdio>
#include <vector>
#include <memory>
#include <type_traits>
struct X {
X() = default;
X(const X&) = default;
// Move constructor might throw
X(X&&) noexcept(false) {} // "= default" changes reported behaviour
// Tracking calls to assignment functions
X& operator=(const X&) {
putchar('c'); return *this;
}
X& operator=(X&&) noexcept(true) {
putchar('m'); return *this;
}
};
unsigned counter = 0;
template <typename T>
struct A : std::allocator<T> {
template <typename U>
struct rebind { using other = A<U>; };
A() : std::allocator<T>(), id(++counter) {}
// Does not propagate
using propagate_on_container_move_assignment = std::false_type;
// Does not always compare equal
using is_always_equal = std::false_type;
bool operator ==(const A& o) { return id == o.id; }
bool operator !=(const A& o) { return id != o.id; }
unsigned id;
};
int main() {
std::vector<X, A<X>> a(2), rv(2);
a = std::move(rv);
}
Running the code above outputs "cc" (instead of "mm") confirming the two
elements of rv are copy-assigned into a.
See relevant discussion in [2] (with link to possible culprit lines of code in
libstdc++) and life example above in [3]
[1]
https://timsong-cpp.github.io/cppwp/n4659/container.requirements#tab:containers.container.requirements
[2]
https://stackoverflow.com/questions/58378051/issue-when-compiling-libstdc-with-clang?noredirect=1#comment103136248_58378051
[3] https://godbolt.org/z/EgkPrP
