C++ - What is this doing if the constructor is private?

Question

In the code below, why does the compiler not complain for mClass2?

class CMyClass{
private:
    CMyClass(){}
};

void TestMethod(){
    CMyClass mClass1;   //Fails.
    CMyClass mClass2(); //Works.
}

related http://stackoverflow.com/questions/4296090/some-problems-while-learning-stl/4296118#4296118 — Mihran Hovsepyan, Jul 21 '11 at 08:17
Also known as: Most Vexing Parse (see http://en.wikipedia.org/wiki/Most_vexing_parse) — Matthieu M., Jul 21 '11 at 08:54

score 16 · Accepted Answer · answered Jul 21 '11 at 08:14

16

Because you've just declared a function mClass2 of zero arguments that returns a CMyClass. That's a valid option since there could be, say, a static CMyClass instance which that function has access to. Note that CMyClass still has a public copy constructor.

(To convince yourself, compile this module to assembler and observe that commenting out the line CMyClass mClass2(); produces the same output.)

answered Jul 21 '11 at 08:14

Fred Foo

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Also, a valid move constructor. – Arafangion Jul 21 '11 at 08:18

score 10 · Answer 2 · edited May 23 '17 at 11:47

10

Because it is declaring a function and not calling the constructor as you think.

This is called as the Most Vexing Parse in c++.

CMyClass mClass2();

declares a function mClass2() which takes no parameter and returns CMyClass

edited May 23 '17 at 11:47

Community

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answered Jul 21 '11 at 08:15

Alok Save

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**Most vexing parse** - can't say I've ever heard of that before, I'll have a look at that, thank you. :) – R4D4 Jul 21 '11 at 08:22

score 1 · Answer 3 · answered Jul 21 '11 at 08:15

1

The second one is a function declaration.

answered Jul 21 '11 at 08:15

Karoly Horvath

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score 0 · Answer 4 · answered Jul 21 '11 at 17:34

0

People ought to move to the uniform syntax initialization in C++0x/C++11 using the {} brackets instead which removes this issue.

Class C{};

http://www2.research.att.com/~bs/C++0xFAQ.html#uniform-init

answered Jul 21 '11 at 17:34

David

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C++ - What is this doing if the constructor is private?

4 Answers4