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This article mainly explains why inheritance levels and arrays are not mixed in C++. The explanation in this article is simple and clear and easy to learn and understand. Please follow the editor's train of thought. Let's study and learn why inheritance levels and arrays are not mixed in C++.

T.81: do not mix inheritance levels and arrays

Reason (reason)

An array of derived classes can implicitly "decay" to a pointer to a base class with potential disastrous results.

An array of derived classes can be implicitly reduced to pointers to the base class that can have disastrous consequences.

Example (sample)

Assume that Apple and Pear are two kinds of Fruits.

Assume that Apple and Pear are two kinds of Fruit.

Void maul (Fruit* p)

{

* p = Pear {}; / / put a Pear into * p

P [1] = Pear {}; / / put a Pear into p [1]

}

Apple aa [] = {an_apple, another_apple}; / / aa contains Apples (obviously!)

Maul (aa)

Apple& a0 = & aa [0]; / / a Pear?

Apple& A1 = & aa [1]; / / a Pear?

Probably, aa [0] will be a Pear (without the use of a cast!). If sizeof (Apple)! = sizeof (Pear) the access to aa [1] will not be aligned to the proper start of an object in the array. We have a type violation and possibly (probably) a memory corruption. Never write such code.

It is quite possible that aa [0] is a Pear (no type conversion is required). If sizeof (Apple)! = sizeof (Pear), the access location for aa [1] will not start correctly with the next object. We will encounter type violations and possible (almost certain) memory corruption. Never write code like this.

Note that maul () violates the a T * points to an individual object rule.

Alternative: Use a proper (templatized) container:

Note that maul () has violated the principle of using T* or onwer to specify a unique object. Other options: use the appropriate (templated) container.

Void maul2 (Fruit* p)

{

* p = Pear {}; / / put a Pear into * p

}

Vector va = {an_apple, another_apple}; / / va contains Apples (obviously!)

Maul2 (va); / / error: cannot convert a vector to a Fruit*

Maul2 (& va [0]); / / you asked for it

Apple& a0 = & va [0]; / / a Pear?

Note that the assignment in maul2 () violated the no-slicing rule.

Note that the assignment in maul2 () violates the principle of not splitting objects.

Enforcement (implementation recommendations)

Detect this horror!

Detect such a terrible problem!

Thank you for your reading. The above is the content of "Why don't you mix inheritance levels and arrays in C++?" after the study of this article, I believe you have a deeper understanding of why C++ should not mix inheritance levels and arrays, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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