struct vs class in C++

A common question about C++: how does a struct differ from a class in C++? …

I will start by defining a struct in C. I would see it as a customized, composite data type, which may be constructed from the existing built-in data types [int, char, etc.], bit fields [integers of specified bit size] and other structures…

This example shows a simple struct definition, along with the declaration of a variable of that type and access to one of the fields. A struct is a handy and flexible way to represent data. Similar facilities exist in most modern programming languages.

So, how does a C++ class differ from a C struct? There are a few differences. The key ones are:

• A class can also contain functions [called methods].
• The member variables and methods are hidden from the outside world, unless their declaration follows a public label.
• There can be a pair of special methods – the constructor and destructor – that are run automatically when an instance of the class [an object] is created and destroyed.
• Operators to work on the new data type can be defined using special methods [member functions].
• One class can be used as the basis for the definition of another [inheritance].
• Declaring a variable of the new type [an instance of the class; an object] requires just the name of the class – the keyword class is not required.

Most of these are illustrated in the example here:

But what about a struct in C++? The last example here gives a clue. The only difference between a struct and class in C++ is the default accessibility of member variables and methods. In a struct they are public; in a class they are private.

Having imparted this information, I urge you not to exploit it too heavily. A key priority when you are writing code is to ensure that it is readable [=maintainable]. Somebody – it could be you – might need to look at this code in a year’s time and understand what it does. I have heard advice as follows: Assume that the person that will maintain your code is an armed psychopath, who has limited patience and knows your home address.

This entry was posted on Wednesday, January 15th, 2020 at 6:53 am. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.