[cfe-dev] Compilation error in clang ( related to copy constructor )

[Nikola Smiljanic]

Here's what I think is happening:

function foo(B) requires B to have a copy constructor because it's passing
by value.

foo(0) invokes B's int constructor. It's working as implicit conversion
constructor from int because it isn't marked as explicit. This temporary is
an r-value and you can't bind a non-const reference to it. So const copy
constructor works, non-const doesn't.

Now I have no idea what gcc is doing, but I don't think that having A makes
any difference at all. Note that I don't really know the exact rules here,
but the error clang reports looks reasonable.







On Tue, May 5, 2015 at 11:07 PM, Chakshu Grover <chakshu.gro at samsung.com>
wrote:

>  Hi all,
> This is regarding this program
>
> #include <string>
> #include <iostream>
>
> struct A {
>   A(){};
>   A(const A &) { std::cout << "A's Copy Constructor" << std::endl; }
> };
>
> struct B : A
> {
>   B(int) { std::cout << "Constructor" << std::endl; }
>   B(const B &) { std::cout << "Copy Constructor" << std::endl; }
>   B(A) { std::cout << "Special Constructor" << std::endl; }
> };
>
> void foo(B){}
>
> int main()
> {
>   foo(0);
>   return 0;
> }
>
>
>
> The output of this program is
>
> Constructor
>
>
>
> using both gcc and clang
>
> But if the program is slightly different ("const" missing from B's copy
> constructor)
>
> #include <string>
> #include <iostream>
>
> struct A {
>   A(){};
>   A(const A &) { std::cout << "A's Copy Constructor" << std::endl; }
> };
>
> struct B : A
> {
>   B(int) { std::cout << "Constructor" << std::endl; }
>   B(B &) { std::cout << "Copy Constructor" << std::endl; }
>   B(A) { std::cout << "Special Constructor" << std::endl; }
> };
>
> void foo(B){}
>
> int main()
> {
>   foo(0);
>   return 0;
> }
>
> Output of gcc is
>
> Constructor
>
> A's Copy Constructor
>
> Special Constructor
>
>
> Clang doesn't compile giving error
>
> "no viable constructor copying parameter of type 'B' "
>
> My previous understanding was that gcc tried to use B's copy constructor
> but as it has non constant reference argument so it uses A's copy
> constructor but even after adding the "const", I see that "Copy
> Constructor" is still not printed.
> Clang is unable to compile that means it is not even trying to do it using
> B(A) constructor when const is not present. When it is, then also
> "Copy Constructor" is not printed.
>
> Please suggest where I might be wrong and why I am observing this behavior.
> Any leads will be helpful
>
> Regards,
> Chakshu
>
>
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> cfe-dev at cs.uiuc.edu
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>
>
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