Copy constructor is not called for copy-initialization or optimized?

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栀梦
栀梦 2021-01-25 05:30

If copy constructor is made private then in

Case 1: No error, the compiler doesn\'t care if the copy constructor was defined in class.

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  •  别那么骄傲
    2021-01-25 05:44

    Case 2 comes under 12.8/31 in N3225:

    A program is ill-formed if the copy/move constructor or the copy/move assignment operator for an object is implicitly odr-used and the special member function is not accessible.

    Just because the copy ctor is elided doesn't mean it isn't odr-used. 3.2/2:

    A member of a set of candidate functions is odr-used if it is selected by overload resolution when referred to from a potentially-evaluated expression. [Note: this covers calls to named functions (5.2.2), oper- ator overloading (Clause 13), user-defined conversions (12.3.2), allocation function for placement new (5.3.4), as well as non-default initialization (8.5). A copy constructor or move constructor is odr-used even if the call is actually elided by the implementation. —end note ]

    Beware of course that MSVC is not fully C++0x-compliant, because (a) C++0x isn't a standard yet, and isn't finalized; and (b) MSVC hasn't implemented everything up to date anyway. But this stuff isn't substantially changed from C++03, so I'm fairly confident the explanation still holds.

    Case 1 would come under this too, except that on the two C++03 compilers I've checked it doesn't get that far because there's no possible conversion from a string literal to T. I can't be bothered to check whether there are any additional conversion sequences allowed in C++0x, there could be a new clause anywhere :-)

    It's still a mystery to me why MSVC allows case 1 ever, even with a public copy ctor. Does it allow it in strict C++03 mode?

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