Reputation: 190659

"no match for ‘operator=’" error with g++

I have a class AP

template<typename T>
class AP : public std::auto_ptr<T>
{
    typedef std::auto_ptr<T> Super;
public:
    AP() : Super() { }
    AP(T* t) : Super(t) { }
    AP(AP<T>& o) : Super(o) { }
};

And a function to return it.

namespace AIR {
namespace Tests {

namespace
{

    AP<A> CreateGraph()
    {
    AP<A> top(A::Create("xyz").release());
        ...
    return top;
    }

    AP<A> top; 
    top = CreateGraph();

When I compile the code

AP<A> top; 
top = CreateGraph();

I got this error message

no match for ‘operator=’ in ‘top = AIR::Tests::<unnamed>::CreateGraph()()’

I added this operator to the AP class, but it doesn't work.

AP<T>& operator=(AP<T>& o) { (*(Super*)this) = o; return *this; }

What's wrong with the class?

EDIT

top.reset(CreateGraph().release()) solved this issue.

Upvotes: 1

Answers (3)

James McNellis

Reputation: 354979

CreateGraph() returns by value, thus a CreateGraph() function call is an rvalue.

Because the std::auto_ptr copy assignment operator takes its argument by non-const reference, the implicitly declared AP copy assignment operator takes its argument by non-const reference.

A non-const reference can only bind to an lvalue, hence the error.

As I explained in an answer to one of your previous questions, if you want to have std::auto_ptr-like copying (where the actual copy constructor takes its argument by non-const reference), you also need to implement something similar to std::auto_ptr_ref.

I explain how std::auto_ptr uses this helper class to allow copying of rvalues in the accepted answer to How could one implement std::auto_ptr's copy constructor?

Upvotes: 4