Why no constructors are called?

Question

Here C() is a temporary object which should have been created with no-arg constructor and then I expected a call to move constructor. Yet none of them happened. Can someone exmplain why?

#include <iostream>

using namespace std;
class C{
    public:
        C(){std::cout<<"No arg\n";}
        C(const C& r){std:cout<<"Copy Cons\n";}
        C(C&& r){std:cout<<"Move Cons\n";}
};

int main() {
    C c(C());
}

@Nicol It's the most vexing parse actually. The OP's expectation is still wrong, but they would reasonably expect the default ctor once at least. — Asteroids With Wings, Nov 05 '20 at 16:43
Close reason is wrong (or only partially wrong). This is most vexing parse. https://stackoverflow.com/questions/5926103/most-vexing-parse — Mike Vine, Nov 05 '20 at 16:44
I think this one is worth answering: `C c(C());` needs some analysis. — Bathsheba, Nov 05 '20 at 16:46
@Bathsheba Is there something that's missing from the dupe target you think? — cigien, Nov 05 '20 at 16:47
@cigien: It took me well into a minute to figure this one out. — Bathsheba, Nov 05 '20 at 16:48
@Bathsheba That's fair, but the dupe target covers what you have in your answer, right? — cigien, Nov 05 '20 at 16:49
Only difference in dup that it is not the same type, but I doubt that difference is significant — Slava, Nov 05 '20 at 16:50

score 3 · Answer 1 · edited Nov 05 '20 at 16:53

3

The statement C c(C()); is actually a function declaration for a function called c that returns C and takes a function (unnamed) as a parameter that returns a C and takes no parameters.

In other words, it's purely declarative and has no effect on the program.

This is an example of the most vexing parse.

Even if it we fixed that (with C c((C())) or C c{C()}) , pre-C++17 most compilers used permission granted by the language to optimise away that "extra" temporary object (and since C++17 they must do so), so at best you'd probably only see the output from a single default constructor invocation.

edited Nov 05 '20 at 16:53

Asteroids With Wings

17,071
2
21
35

answered Nov 05 '20 at 16:47

Bathsheba

231,907
34
361
483

We call them _declarations_ in C++, not prototypes (which are subtly different things) – Asteroids With Wings Nov 05 '20 at 16:48
@AsteroidsWithWings: It's a wiki answer - let's all edit it. – Bathsheba Nov 05 '20 at 16:48
I'd rather harass you in comments :P – Asteroids With Wings Nov 05 '20 at 16:48
@AsteroidsWithWings How about swords or duelling pistols? – Bathsheba Nov 05 '20 at 16:49
Sure, that can work! – Asteroids With Wings Nov 05 '20 at 16:50
@AsteroidsWithWings: I'm a pretty good shooter, but my swordsmanship is wanting. – Bathsheba Nov 05 '20 at 16:51
Swords it is then. – Asteroids With Wings Nov 05 '20 at 16:51
@AsteroidsWithWings: Damn. My weakness is to reveal my weakness. Costly. – Bathsheba Nov 05 '20 at 16:52
2

You walked right into that one... – Asteroids With Wings Nov 05 '20 at 16:53
I see that you've added an answer that says what would happen if the OP's code didn't suffer from MVP. It seems that you could post the answer on the dupe target as well. May I ask you to do that instead, so as not to have good quality answers to the same question spread out across the site? – cigien Nov 05 '20 at 17:02
1

@cigien No, because the "dupe target" doesn't mention, not even once, an expectation of a move ctor or copy ctor being invoked. So, the explanation you're referring to has nothing at all to do with that other question. That's why _this is not a duplicate_. Can we just relax and be happy that this OP got a nice, complete answer to their question? There are more important things to worry about. – Asteroids With Wings Nov 05 '20 at 17:07
1

@Bathsheba Ignore my previous comment, I wasn't thinking about this correctly. I see now that your answer clarifies the misunderstanding the OP has, and is useful in that regard at least, even though the underlying cause is covered in the dupe target. Thanks for taking the effort to write out an answer. – cigien Nov 05 '20 at 17:50

Why no constructors are called?

1 Answers1