What object is produced by ternary operator in C++?

Question

The following program

#include <optional>
#include <iostream>

int main()
{
    std::optional<int> a;
    constexpr bool x = true;

    const std::optional<int> & b = x ? a : std::nullopt;
    std::cout << (&a == &b);

    const std::optional<int> & c = x ? a : (const std::optional<int> &)std::nullopt;
    std::cout << (&a == &c);

    const std::optional<int> & d = x ? a : (const std::optional<int>)std::nullopt;
    std::cout << (&a == &d);
}

prints 010 even in case of compiler optimization: https://gcc.godbolt.org/z/asTrzdE3c

Could you please explain why it is so, and what is the difference between seemingly identical 3 cases?

Answer 1

First you have to undesund what is the type of ternary operator result.

x ? a : std::nullopt;

Here a is variable which can be reference and std::nullopt is something which is implicitly converted to optional of matching type (here std::optional<int>). So conversion of std::nullopt ends with creation of temporary value. To match type a is also copied.

So ternary operator deduces type to be a value of type std::optional<int> which becomes a temporary object. New instance of std::optional<int> is created.

Now const auto & is able to prolong lifetime of temporaries. So b is reference to std::optional<int> which is a temporary of prolonged lifetime.

d is same scenario, but it is more explicit.

c has (const std::optional<int> &)std::nullopt which is creates temporary object of std::optional<int> with prolonged lifetiem. Here ternary operator has as argument something what is std::optional<int>& and const std::optional<int>& so it is able to pass first arguments reference return type.

See what cppinsights generates with your code:

#include <optional>
#include <iostream>

int main()
{
  std::optional<int> a = std::optional<int>();
  constexpr const bool x = true;
  const std::optional<int> & b = x ? std::optional<int>(a) : std::optional<int>(std::nullopt_t(std::nullopt));
  std::cout.operator<<((&a == &b));
  const std::optional<int> & c = x ? a : static_cast<const std::optional<int>>(std::optional<int>(std::nullopt_t(std::nullopt)));
  std::cout.operator<<((&a == &c));
  const std::optional<int> & d = (x ? std::optional<int>(a) : static_cast<const std::optional<int>>(std::optional<int>(std::nullopt_t(std::nullopt))));
  std::cout.operator<<((&a == &d));
}