If I have a base class, with only virtual methods and 2 derived classes from the base class, with those virtual methods implemented.
How do I:
// causes C2259
BaseClass* base = new BaseClass[2];
BaseClass[0] = new FirstDerivedClass;
BaseClass[1] = new SecondDerivedClass;
or:
// causes "base is being used without being initialized"
BaseClass* base;
// causes CC59 again
BaseClass* base = new BaseClass;
base[0] = FirstDerivedClass();
base[1] = SecondDerivedClass();
(or something similar)
...so that I can access the BaseClass
s methods through the DerivedClass
,
but by pointer and the pointer is an array of DerivedClass
s?
Derived class pointer cannot point to base class.
A base class's private members are never accessible directly from a derived class, but can be accessed through calls to the public and protected members of the base class.
The derived class inherits all members and member functions of a base class. The derived class can have more functionality with respect to the Base class and can easily access the Base class. A Derived class is also called a child class or subclass.
Pointer to Derived Class in C++ The member function “display” will print the value of data member “var_base” We created another class “DerivedClass” which is inheriting “BaseClass” and contains data member “var_derived” and member function “display”.
Your array is of the wrong type: it stores BaseClass
object instances instead of pointers to them. Since BaseClass
seems to be abstract, the compiler complains that it cannot default-construct instances to fill your array.
Even if BaseClass
were not abstract, using arrays polymorphically is a big no-no in C++ so you should do things differently in any case.
Fix this by changing the code to:
BaseClass** base = new BaseClass*[2];
base[0] = new FirstDerivedClass;
base[1] = new SecondDerivedClass;
That said, most of the time it is preferable to use std::vector
instead of plain arrays and smart pointers (such as std::shared_ptr
) instead of dumb pointers. Using these tools instead of manually writing code will take care of a host of issues transparently at an extremely small runtime cost.
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