C++ Abstract class construction and destruction

Question

class base
{
    base () { }
    virtual ~base () { }
}

class middleBase
{
    middleBase () { }
    middleBase (int param) { }
    ~middleBase () { }
}

class concrete : public middleBase
{
    concrete () { }
    concrete (int param) { // process }
    ~concrete () { // delete something }
}

Error is : undefinded reference to "middleBase::middleBase (int param)"

and is this suitable for best practice for parameterized constructors?

Answer 1

class base
{
  public: // constructor should be accessible by derived class
    base () { }
    virtual ~base () { }
}; // add semicolon

class middleBase : public base // you missed the declaration
{
  public: 
    middleBase () { }
    middleBase (int param) { }
    virtual ~middleBase () { }
};

class concrete : public middleBase
{
  public:
    concrete () { }
    concrete (int param) : middleBase(param) { /* process */ }
    virtual ~concrete () { /* delete something */ }
};

Answer 2

c++ needs semicolons after class definitions:

class { … };

Answer 3

If you don't put access specifier (public/private/protected) before class' member functions or variables, the default is private specifier for C++ classes. So middleBase's constructor is private and cannot be access by concrete class' constructor, causing the error you mentioned.