no match for operator= using a std::vector

问题:

I've got a class declared like this:

class Level
{
    private:
        std::vector<mapObject::MapObject> features;
    (...)
};

and in one of its member functions I try to iterate through that vector like this:

vector<mapObject::MapObject::iterator it;
for(it=features.begin(); it<features.end(); it++)
{
    /* loop code */
}

This seems straightforward to me, but g++ gives me this error:

src/Level.cpp:402: error: no match for ‘operator=’ in ‘it = ((const yarl::level::Level*)this)->yarl::level::Level::features.std::vector<_Tp, _Alloc>::begin [with _Tp = yarl::mapObject::MapObject, _Alloc = std::allocator<yarl::mapObject::MapObject>]()’
/usr/include/c++/4.4/bits/stl_iterator.h:669: note: candidates are: __gnu_cxx::__normal_iterator<yarl::mapObject::MapObject*,std::vector > >& __gnu_cxx::__normal_iterator<yarl::mapObject::MapObject*,std::vector > >::operator=(const __gnu_cxx::__normal_iterator<yarl::mapObject::MapObject*, ``std::vector<yarl::mapObject::MapObject, std::allocator<yarl::mapObject::MapObject> > >&)

Anyone know why this is happening?

回答1:

I'd guess that this part of the error describes your problem:

(const yarl::level::Level*)this

Is the member function in which this code is found a const-qualified member function? If so, you'll need to use a const_iterator:

vector<mapObject::MapObject>::const_iterator it;

If the member function is const-qualified, then only the const-qualified overloads of begin() and end() on the member vector will be available, and both of those return const_iterators.

回答2:

Did you close your right-angle bracket here?

vector<mapObject::MapObject::iterator it;

If you want a vector of objects, your object needs an operator=. Does MapObject have one? If not, consider a vector of pointers to MapObject.

回答3:

If I were you, I would check to see that the mapObject::MapObject has a default constructor and a public assignment operator.

Somewhere in the class header, you should see something like:

public:
    MapObject();
    operator=(const MapObject &mapObject);

Which means that the class has a default constructor and an assignment operator.

A std::vector cannot be instantiated using a class without a default constructor, and you cannot iterate through the class as above without an assignment operator.

So, add an assignment operator to your class definition and your iteration will compile.

回答4:

@James McNellis's answer (the "green checked" best answer) fixed my error with a similar description, however my error was caused by what @tmarthal mentioned his answer post (not defining an assignment operator). His suggested fix is to include an assignment operator but I just wanted to add that I was also able fix this error by using std::vector<>::const_iterator instead of std::vector<>::iterator for classes where no assignment operator is defined. I'm not sure whether this truly a correct fix or just something that kept the compiler from complaining however.