No copy constructor available or copy constructor is declared 'explicit'

Question

Could somebody please explain why I'm getting a compile error here - error C2558: class 'std::auto_ptr<_Ty>' : no copy constructor available or copy constructor is declared 'explicit'

#include <memory>
#include <vector>
#include <string>
template<typename T>
struct test
{
    typedef std::auto_ptr<T> dataptr;
    typedef std::auto_ptr< test<T> > testptr;
    test( const T& data ):
    data_( new T(data) )
    {
    };
    void add_other( const T& other )
    {
        others_.push_back( testptr( new test(other) ) );
    }
private:
    dataptr data_;
    std::vector< testptr > others_;
};

int main(int argc, char* argv[])
{
    test<std::string> g("d");

    //this is the line that causes the error.
    g.add_other("d");

    return 0;
}

Answer 1

    others_.push_back( testptr( new test(other) ) );

You are trying to push an auto_ptr into a std::vector

auto_ptr does not define an implicit copy constructor and is not compatible as a value in the stl container classes.

See this question for more info: StackOverflow: Why is it wrong to use stdauto ptr with stl containers

Answer 2

You cannot create standard library containers of auto_ptr, as you are trying to do here:

std::vector< testptr > others_;

as they do not have the correct semantics. You will have to use ordinary pointers or a different flavour of smart pointer, such as shared_ptr.

Answer 3

Basically a std::auto_ptr cannot be used in this way.

others_.push_back( testptr( new test(other) ) );

Requires that a copy constructor that takes a const& exists and no such constructor exists for std::auto_ptr. This is widely viewed as a good thing since you should never use std::auto_ptr in a container! If you do not understand why this is, then read this article by Herb Sutter, particularly the section entitled "Things Not To Do, and Why Not To Do Them" about 3/4 of the way through.

Answer 4

You probably want std::unique_ptr or std::shared_ptr from the upcoming C++0x standard which will replace auto_ptr if you have access to a compiler that has these implemented (gcc 4.5+)

http://www2.research.att.com/~bs/C++0xFAQ.html#std-unique_ptr http://www2.research.att.com/~bs/C++0xFAQ.html#std-shared_ptr