Pointer to array of base class, populate with derived class

Question

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 BaseClasss methods through the DerivedClass, but by pointer and the pointer is an array of DerivedClasss?

Jon · Accepted Answer · 2019-11-14 13:27:44Z

14

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.

edited Nov 14, 2019 at 13:27

answered Oct 24, 2012 at 11:33

Jon

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5 Comments

j_random_hacker Over a year ago

The problem, which is called "object slicing", is not specific to arrays -- it happens any time you assign a derived class to a base class by value. E.g. BaseClass b; b = FirstDerivedClass(); is compilable C++ that nevertheless breaks silently (at best you might get a compiler warning).

comesuccingfuccslot Over a year ago

Link seems to be moved.

Aditya Kashi Over a year ago

@Jon You sir, just saved my day. I had declared an array of objects of an inherited class (with virtual functions) and expected it to work. In your answer, however, what does BaseClass[0] = new FirstDerivedClass; do? Did you mean base[0] = new FirstDerivedClass;?

user3015682 Over a year ago

Ya I haven't tried this yet, but it seems like is should be base[0]= not BaseClass[0] = . It's array of objects not an array of classes.

Jon Over a year ago

@user3015682 right, fixed. Not sure why it took all of us many years to spot!

Denis Ermolin · Accepted Answer · 2012-10-24 12:04:18Z

6

It is C++ use std::vector instead of simple array:

std::vector<BaseClass*> base;
base.push_back(new FirstDerivedClass());
base.push_back(new SecondDerivedClass());

As Kerrek SB noticed safest method is to use std::unique_ptr:

std::vector<std::unique_ptr<BaseClass> > base;
base.push_back( std_unique_ptr<BaseClass>(new FirstDerivedClass()) );
base.push_back( std_unique_ptr<BaseClass>(new SecondDerivedClass()) );

edited Oct 24, 2012 at 12:04

answered Oct 24, 2012 at 11:31

Denis Ermolin

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5 Comments

Denis Ermolin Over a year ago

What for? Speed? Reliability?

Al2O3 Over a year ago

What about: BaseClass* Base[2];Base[0] = new FirstDerivedClass; Base[1] = new SecondDerivedClass;

Jon Over a year ago

Using a vector is a good idea, but that's not the problem with the current code.

Kerrek SB Over a year ago

Even betterer is a std::vector<std::unique_ptr<BaseClass>>.

j_random_hacker Over a year ago

The crucial point here is to avoid object slicing by using pointers to BaseClass instead of BaseClass objects. Please emphasise that.

BЈовић · Accepted Answer · 2012-10-24 11:36:58Z

2

If your BaseClass contains pure virtual methods, this will fail to compile :

BaseClass* base = new BaseClass[2];

If it doesn't, you are going to get memory leak.

In c++, this is done by using std::vector or std::array, with some kind of smart pointer. For example :

std::vector< std::shared_ptr< BaseClass > > arr( 2 );
arr[0].reset( new FirstDerivedClass() );
arr[1].reset( new SecondDerivedClass() );

answered Oct 24, 2012 at 11:36

BЈовић

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2 Comments

j_random_hacker Over a year ago

An even more important problem with the OP's code is object slicing, which breaks programs silently.

BЈовић Over a year ago

@j_random_hacker That is why I suggested to use some kind of smart pointer. Storing objects of base type (if it has no pure functions) would cause slicing

Deukalion · Accepted Answer · 2012-10-24 11:43:24Z

0

This was the answer (from Rubby)

BaseClass* Base[2];

Base[0] = new FirstDerivedClass;
Base[1] = new SecondDerivedClass;

answered Oct 24, 2012 at 11:43

Deukalion

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2 Comments

Jon Over a year ago

This does something very different than what your original code was trying to do. This array is allocated on the stack, while originally it was on the heap.

Deukalion Over a year ago

Yes, I guess. Now the array is a pointer and there's pointers in the array.

user1203650 · Accepted Answer · 2012-10-24 11:45:49Z

0

Define a pointer array , the pointer type is BaseClass. And assign the pointer to the derivedclass to the elements of the array. just like:

BaseClass* base [2];
base[0] = new FirstDerivedClass;
base[1] = new SecondDerivedClass;

answered Oct 24, 2012 at 11:45

user1203650

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