Question

I’m writing an application where I will have several derived classes accessed through pointers to a base class. I wish for one of these derived classes to contain a vector of pointers it’s neighbors (also of the base class type) in the application, like so:

    #include <vector>

    class BaseClass
    {
    public:
        BaseClass() { }
        virtual ~BaseClass() { }
    };

    class DerivedClass : virtual public BaseClass
    {
    public:
        DerivedClass() : BaseClass() { }
        ~DerivedClass() { }
    private:
        vector<BaseClass*> Neighbors;
    };

However, get the following compiler error:

example.cpp:16: error: ISO C++ forbids declaration of ‘vector’ with no type
example.cpp:16: error: expected ‘;’ before ‘<’ token

Is this even possible? If it is possible please someone point out my mistake! The compiler should know what type a BaseClass is as it has just been declared, in fact declaring a member of type BaseClass Foo; works, so I don’t understand why the vector cannot recognise BaseClass*.

Cheers!

Answer 1

You didn’t put either using namespace std;, using std::vector, or std::vector<...>...

#include <vector>
using std::vector; //choice 1
using namespace std; //choice 2 

class BaseClass
{
public:
    BaseClass() { }
    virtual ~BaseClass() { }
};

class DerivedClass : virtual public BaseClass
{
public:
    DerivedClass() : BaseClass() { }
    ~DerivedClass() { }
private:
    std::vector<BaseClass*> Neighbors; //choice 3
};