Question

I’m trying to use the std::shared_ptr in clang++(clang version 3.1 (trunk 143100)) using libstdc++(4.6.1). I have a little demo program:

#include <memory>

int main()
{
    std::shared_ptr<int> some(new int);
    std::shared_ptr<int> other(some);
    return 0;
}

which can be build using:

clang++ -std=c++0x -o main main.cpp

and gives the following error output:

main.cpp:6:23: error: call to deleted constructor of 'std::shared_ptr<int>'
    std::shared_ptr<int> other(some);
                         ^     ~~~~
/usr/include/c++/4.6/bits/shared_ptr.h:93:11: note: function has been explicitly marked
deleted here
class shared_ptr : public __shared_ptr<_Tp>

For some reason it needs the constructor which is deleted because a move constructor is provided (which is correct behaviour).
But why does it work compile with (g++ (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1.)? Somebody any ideas on how to fix this?

Answer 1

The implicitly-declared copy constructor for shared_ptr is deleted because shared_ptr has a move constructor or a move assignment operator (or both), per C++11 12.8p7:

If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4).

GCC 4.6.x does not implement this rule, which came into the C++11 working paper very late in the process as N3203=10-0193. The shared_ptr in libstdc++ 4.6.x was correct at the time it was written, but C++11 changed after that.. Boost had exactly the same issue with it’s shared_ptr, and this is one of the common incompatibilities between GCC and Clang.

Adding a defaulted copy constructor and copy assignment operator to shared_ptr will fix the problem.