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

Question

#include 

#define FORWARD(arg)\\
std::forward(arg)

template
constexpr bool AndL(Args&&... args)         


        

Answer 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
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 ::value, int> = 0>
bool unwrap(T&& val) { return std::forward(val)(); }

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

template
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.