I get compilation errors on g++ (GCC) 4.7.2
but not on MSVC-2012
when trying to std::vector::push_back
a non-copyable (private copy constructor) but moveable object. To me my example looks identical to many other examples on SO and elsewhere. The error message makes it looks like a problem with the struct not being 'direct constructible' - I don't know what this means so am doubly unsure about why an object needs to be 'direct constructible' to be pushed back.
#include <vector>
#include <memory>
struct MyStruct
{
MyStruct(std::unique_ptr<int> p);
MyStruct(MyStruct&& other);
MyStruct& operator=(MyStruct&& other);
std::unique_ptr<int> mP;
private:
// Non-copyable
MyStruct(const MyStruct&);
MyStruct& operator=(const MyStruct& other);
};
int main()
{
MyStruct s(std::unique_ptr<int>(new int(5)));
std::vector<MyStruct> v;
auto other = std::move(s); // Test it is moveable
v.push_back(std::move(other)); // Fails to compile
return 0;
}
Gives errors
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/type_traits: In instantiation of ‘struct std::__is_direct_constructible_impl<MyStruct, const MyStruct&>’:
... snip ...
/usr/lib/gcc/x86_64-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_vector.h:900:9: required from ‘void std::vector<_Tp, _Alloc>::push_back(std::vector<_Tp, _Alloc>::value_type&&) [with _Tp = MyStruct; _Alloc = std::allocator<MyStruct>; std::vector<_Tp, _Alloc>::value_type = MyStruct]’
main.cpp:27:33: required from here
main.cpp:16:5: error: ‘MyStruct::MyStruct(const MyStruct&)’ is private
Simple workaround from various answers:
- Use
MyStruct(const MyStruct&) = delete;
instead ofprivate ctor
hack - Inherit
boost::noncopyable
(or another class with private ctor)