Does put_money hold its argument by value or reference?

问题

Does the following invoke undefined behavior?

#include <iostream>
#include <iomanip>
#include <algorithm>
#include <experimental/iterator>

int main() {
    long double values[] = {1, 2, 3};
    std::transform(
        std::begin(values), std::end(values),
        std::experimental::make_ostream_joiner(std::cout, ", "),
        [](long double v) {
            return std::put_money(v + 1);
        }
    );
    return 0;
}

My worry is that return std::put_money(v + 1) returns a reference to the temporary v + 1.

回答1:

The standard ([ext.manip]/6) only defines this specific expression:

out << put_money(mon, intl);

It is unspecified how mon is stored in the mean time, and it is definitely possible for it to become a dangling reference and be UB.

An "easy" fix is making your own class to know you store the value:

struct money_putter {
    long double value;

    template<class charT, class traits>
    friend std::basic_ostream<charT, traits>& operator<<(std::basic_ostream<charT, traits>& os, const money_putter& mon) {
        return os << std::put_money(mon.value);
    }
};


int main() {
    int values[] = {1, 2, 3};
    std::transform(
        std::begin(values), std::end(values),
        std::experimental::make_ostream_joiner(std::cout, ", "),
        [](int i)  {
            return money_putter{i};  // or i + 1
        }
    );
    return 0;
}

回答2:

You could test it, though this wont tell you anything about whether it is guaranteed, but then as the return type of put_money is not specified, you cannot assume that the returned value does not hold a reference.

...anyhow let's test it:

#include <iostream>
#include <iomanip>
#include <algorithm>
#include <experimental/iterator>

int main() {
    int i = 42;
    std::cout << std::put_money(i) << "\n";
    auto x = std::put_money(i);
    i = 43;
    std::cout << x;    
    return 0;
}

Output with clang:

42
43

So actually the answer is positive. With clang the returned value does hold a reference and the output is the same with gcc. Hence, yes your code has UB.

回答3:

This answer does a great job at answering my question, but I thought I'd supply a more generic solution to the problem of ensuring the object output to an ostream_joiner takes no dangling references, one that uses a lambda to capture those references:

#include <type_traits>
#include <ostream>

template<typename F>
class put_invocation_t {
public:
    constexpr put_invocation_t(F const& f) : f(f) {}
    constexpr put_invocation_t(F&& f) : f(std::move(f)) {}
    template<class charT, class traits>
    friend std::basic_ostream<charT, traits>& operator<<(
        std::basic_ostream<charT, traits>& os, put_invocation_t const& pi
    ) {
        return pi.f(os);
    }
private:
    F f;
};

// or use a deduction guide in C++17
template<typename F>
put_invocation_t<std::decay_t<F>> put_invocation(F&& f) {
    return put_invocation_t<std::decay_t<F>>(std::forward<F>(f));
}

Used as

std::transform(
    std::begin(values), std::end(values),
    std::experimental::make_ostream_joiner(std::cout, ", "),
    [](long double v) {
        return put_invocation([=](auto& os) -> auto& {
            return os << std::put_money(v + 1);
        });
    }
);

This has the bonus of also scaling to outputing multiple values, by using something like the following within the transform:

return put_invocation([=](auto& os) -> auto& {
    return os << "Value is: " << std::put_money(v + 1);
});

来源：https://stackoverflow.com/questions/56499120/does-put-money-hold-its-argument-by-value-or-reference

标签

c++

undefined-behavior

iomanip