How to make `short-circuit evaluation` also available in `fold expressions`?

问题

#include <type_traits>

#define FORWARD(arg)\
std::forward<decltype(arg)>(arg)

template<typename... Args>
constexpr bool AndL(Args&&... args)
{
    return (... && FORWARD(args));
}

template<typename... Args>
constexpr bool AndR(Args&&... args)
{
    return (FORWARD(args) && ...);
}

int main()
{
    bool* pb = nullptr;

    false && (*pb = true);       // ok at runtime.
    AndL(false, (*pb = true));  // error at runtime!
    AndR(false, (*pb = true));  // error at runtime!
}

The traditional && operator supports short-circuit evaluation, so false && (*pb = true) will be ok at runtime, but the following two cases are not.

How to make short-circuit evaluation also available in fold expressions?

回答1:

The problem here is just a misconception of what's actually happening.

How to make short-circuit evaluation also available in fold expressions?

It is available in fold expressions. (args && ... ) follows the exactly the same rules as (a && b && c && d). That is, d will only be evaluated if a, b, and c all evaluate to truthy.

That's not the actual difference between your two cases.

false && (*pb = true);       // ok at runtime.
AndL(false, (*pb = true));   // error at runtime!

While fold expressions do exactly the same thing as their non-fold counterparts, there's one important difference between these two statements. The first is just a statement-expression, the second is a function call. And all function arguments must be evaluated before the start of the body begins.

So the second is equivalent to:

auto&& a = false;
auto&& b = (*pb = true);
(FORWARD(a) && FORWARD(b));

It's that ordering that is causing the problem, not the fold expression (note: b could be evaluated before a).

In order to make this transparent, what you really need are lazy arguments. This is a feature in several languages (e.g. Scala), but not in C++. If you need laziness, the best you could do is wrap everything in a lambda:

template<typename... Args>
constexpr bool AndL(Args&&... args)
{
    return (... && FORWARD(args)());
}

AndL([]{ return false; }, [&]{ return *pb = true; });

You could then make this arbitrarily complex - maybe only "unwrap" those types that are callable, otherwise assume that they're bool:

template <class T, std::enable_if_t<std::is_invocable<T>::value, int> = 0>
bool unwrap(T&& val) { return std::forward<T>(val)(); }

template <class T, std::enable_if_t<std::is_convertible<T, bool>::value, int> = 0>
bool unwrap(T&& val) { return std::forward<T>(val); }

template<typename... Args>
constexpr bool AndL(Args&&... args)
{
    return (... && unwrap(FORWARD(args)));
}

AndL(false, [&]{ return *pb = true; });

But really, the main point is that function argument evaluation precedes the function body, and the issue is not the fold expression itself.

来源：https://stackoverflow.com/questions/43047858/how-to-make-short-circuit-evaluation-also-available-in-fold-expressions

标签

c++

standards

c++17

short-circuiting

fold-expression