What is the syntax to access a member function of a derived class named Truck in a vector of unique_ptrs that point to various Person instances?

Question

For a concrete example, say I am given

vector<shared_ptr<DClass>> cVector

and DClass (which is derived from MyClass) has the following interface

class DClass: public MyClass {
  public:
    DClass( string id, string name, int p, int v )
      : MyClass id, name, p ), var1_ ( v ) {}
    int GetDClassVar1() const { return var1_; }
  private:
    int var1_;
};

and MyClass (which DClass inherits from) has the following interface

class MyClass {
  public:
    MyClass( string id, string name, int p )
      : id_( id ), name_( name ), myclassvar1__( p ) {}
    string GetMyClassID() const { return id_; }
    string GetMyClassName() const { return name_; }
    int GetMyClassVar1() const { return myclassvar1__; }
    virtual int GetDClassVar1() const { return 0; }
  protected:
    string id_;
    string name_;
    int myclassvar1_;
};

How can I call upon the GetDClassVar1 function using cVector assuming that cVector is populated?

***EDIT I try using

cVector.at(1).GetDClassVar1.()

and I get the error

const value_type’ {aka ‘const class std::shared_ptr<MyClass>’} has no member 
named ‘GetDClassVar1'

lubgr · Answer 1 · 2019-04-26T19:42:32.620

How can I call upon the GetDClassVar1 function using cVector assuming that cVector is populated

for (const auto& d : cVector)
    std::cout << d->GetDClassVar1() << "\n";

Note that when calling a member function that is not const-qualified, you need to remove the const before auto&. For a single element:

cVector.front()->getDClassVar1();
cVector.at(42)->getDClassVar1();

This syntax is due to the fact that std::shared_ptr has an overloaded operator -> member function that forwards to the underlying pointer.

Also note that you want polymorphic base classes to have a virtual destructor, see here.

score 0 · Answer 2 · answered Apr 26 '19 at 19:43

To access the members of a pointer, you have to use -> instead of .. Other than that, it's pretty much the same!

int result = cVector[0]->GetDClassVar1();

Alternatively, you could dereference it first:

int result = (*cVector[0]).GetDClassVar1();

Or bind it to a reference, if you're doing multiple operations on it:

auto& element = *cVector[0];
int result = element.GetDClassVar1();

What is the syntax to access a member function of a derived class named Truck in a vector of unique_ptrs that point to various Person instances?

2 Answers2