[cfe-dev] Guaranteed copy elision and GNU ?: operator

[Ronald Wampler via cfe-dev]

Hello,

I recently came across an issue when using the gnu conditional
operator (?:) in C++17. In C++14, the code below when compiled with
Clang 5 outputs 42 as expected, but in C++17 the output is 0. In
C++17, I believe the copy constructor for shared_ptr is being elided
and so the ref count is not incremented and the underlying ptr gets
deleted. If the first operand is cast to an rvalue (e.g.,
std::move(makeSharedIntWrapper(42,true)), the behavior is as expected
(since we are disabling the copy elision). Since this is an GNU
extension, is it OK to not elide the copy in this case?

I didn't test to see what gcc does since I don't have ready access to gcc7.

#include <memory>
#include <iostream>

struct int_wrapper {
    int_wrapper(const int& x) : x(new int(x)) {}
    ~int_wrapper() { delete x; }
    int *x;
};

static std::shared_ptr<int_wrapper> makeSharedIntWrapper(int x, bool t) {
    return t ? std::make_shared<int_wrapper>(x) :
std::shared_ptr<int_wrapper>();
}

int main(int argc, const char * argv[]) {
    auto s = makeSharedIntWrapper(42, true) ?:
std::make_shared<int_wrapper>(17);
    std::cout << *s.get()->x << std::endl;

    return 0;
}