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C# null coalescing operator equivalent for c++

Answer 1

+6 A:

There isn't a way to do this by default in C++, but you could write one:

in C# the ?? operator is defined as

a ?? b === (a != null ? a : b)

So, the C++ method would look like

Coalesce(a, b) // put your own types in, or make a template
{
    return a != null ? a : b;
}

McKay 2009-11-23 19:34:20

In this method b is evaluated even if a isn't null. This might be a problem if b has side effects.

Amnon 2009-11-23 19:52:16

@Amnon: Indeed. Consider: `p = Coalesce(p, new int(10));`. Also, it seems in C# the right-hand operand cannot be NULL (can't be checked at compile-time, since there are no nullable types in C++?). On the other hand: is there that much need for it in C++, so that you can't just type `a ? a : b;`?

UncleBens 2009-11-23 21:34:01

Answer 2

+2 A:

How about this?

#define IFNULL(a,b) ((a) == null ? (b) : (a))

Justin Grant 2009-11-23 19:34:59

beware of IFNULL(a++, b)Yeah, I realize you'd want to beware of that anyway if you think a might be null.IFNULL(SomeClass.DoSomethingReallyLong(), "")causes problems too.

McKay 2009-11-23 19:40:18

Good point-- serves me right for being lazy. Using a template method is definintely the way to go here, since it avoids side effects and will have equivalent performance (or better). I'm going to add a +1 on @McKay's solution now. :-)

Justin Grant 2009-11-23 20:27:16

A function template doesn't necessarily have equal or better performance. This "short-circuits" (`b` is not evaluated unless `a` is null), whereas a function's arguments are always evaluated.

Steve Jessop 2009-11-23 22:02:09

@Steve Correct Justin's code would "perform" better in the case ofIFNULL("", SomeClass.DoSomethingReallyLong());

McKay 2009-11-23 22:25:18

Also a good point. But, I think the most common use case (at least in C# when I've used the ?? operator) is where a is more expensive (e.g. database call, XML parsing) than b. Often b is a constant, like 0 or "". So I suspect the template would win out most of the time. Plus, unless you know how the macro is implemented, the double-execution wouldn't be obvious to the macro's caller, while function-call semantics are well-known so anyone with an expensive b would know to skip the function and just manually create a temporary variable. Funny, I'm arguing against my own answer... :-)

Justin Grant 2009-11-23 22:37:05

And I was arguing against my own. Both have their advantages and drawbacks.

McKay 2010-10-26 22:52:12

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C# null coalescing operator equivalent for c++

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