c++ - dynamic_cast 的问题

Question

我有这段代码:

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";
            }
        }

我有类 Line 并从它派生 FamilyLine 和 RegularLine，所以我想找到 FamilyLine

我的程序在第 15 行失败，我收到一个错误

cannot dynamic_cast target is not pointer or reference

有人可以帮忙吗，在此先感谢

已编辑

我试过这个:

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

同样的错误

已编辑

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";
        }
    }

收到这个错误

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不会隐式转换为指针 - 它是一个类类型对象 - 并且 dynamic_cast , static_cast和 const_cast都只对指针进行操作。

虽然您可以使用 dynamic_cast在 shared_ptr<T>::get() ，最好用dynamic_pointer_cast<FamilyLine>()相反，否则您可能会不小心引入双 delete小号:

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.

例如:

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