problems with dynamic_cast

Question

hello, I have this snippet of the code:

void addLineRelative(LineNumber number, LineNumber relativeNumber) {
            list<shared_ptr<Line> >::iterator i;
            findLine(i, number);
            if(i == listOfLines.end()){
                throw "LineDoesNotExist";
            }

   line 15  if(dynamic_cast<shared_ptr<FamilyLine> >(*i)){
                cout << "Family Line";
            } else {
                throw "Not A Family Line";
            }
        }

I have class Line and derived from it FamilyLine and RegularLine, so I want find FamilyLine

my program fails on the line 15, I receive an error

cannot dynamic_cast target is not pointer or reference

can somebody please help, thanks in advance

edited

I tried this one:

shared_ptr<FamilyLine> ptr(dynamic_cast<shared_ptr<FamilyLine> >(*i));
if(ptr){
    //do stuff
}

the same error

edited

void addLineRelative(LineNumber number, LineNumber relativeNumber) {
        list<shared_ptr<Line> >::iterator i;
        findLine(i, number);
        if(i == listOfLines.end()){
            throw "LineDoesNotExist";
        }

        shared_ptr<FamilyLine> ptr(dynamic_pointer_cast<FamilyLine>(*i));
        if (ptr){
            cout << "Family Line";
        } else {
            throw "Not A Family Line";
        }
    }

receive this error

Multiple markers at this line
    - `dynamic_pointer_cast' was not declared in this 
     scope
    - unused variable 'dynamic_pointer_cast'
    - expected primary-expression before '>' token

Answer 1

shared_ptr does not implicitly convert to a pointer - it is a class-type object - and dynamic_cast, static_cast and const_cast all operate on pointers only.

While you could use dynamic_cast on shared_ptr<T>::get(), its better to use dynamic_pointer_cast<FamilyLine>() instead as you might otherwise accidentally introduce double-deletes:

Returns:
* When dynamic_cast<T*>(r.get()) returns a nonzero value, a shared_ptr<T> object that stores a copy of it and shares ownership with r;
* Otherwise, an empty shared_ptr<T> object.
[...]
Notes: the seemingly equivalent expression

shared_ptr<T>(dynamic_cast<T*>(r.get()))

will eventually result in undefined behavior, attempting to delete the same object twice.

E.g.:

shared_ptr<FamilyLine> ptr(dynamic_pointer_cast<FamilyLine>(*i));
if (ptr) {
    // ... do stuff with ptr
}