C++ typedef struct vs class

Question

I am not very familiar with C++ , and while I am trying some test programms I came to a question regarding the best if I may say so way to define some primitive elements in C++ code.

Let's take a class that describes rectangles. It would create them, draw them , rotate, resize, etc... now in most cases we have to deal with points on the canvas. The rectangle its self is described by 2 points: Upper Left and Lower Right corner. Also in order to Rotate it, you need an angle, and a point(anchor point). Or maybe to move it you need a new anchor point for the given rectangle. I guess I made my point in using points . So what is more efficient ? to define this primitive point as a class or as a struct?

class cPoint
{
public: 
 int X;
 int Y;
};

or

typedef struct
{
 int X;
 int Y;
}sPoint;

Answer 1

Niether are more efficient. On a technical level, there is no difference between a class and a struct aside from default visibility of members (public in struct, private in class) and default inheritance model (public in struct, private in class).

They typedef struct {} name model is not idiomatic in C++. In fact, it's an abomination -- a holdover from C. Don't use this model. Use this struct name {}; instead. Using the typedef struct {} name; model doesn't gain you anything in C++ (it was needed in C), and may cost you sojmething in terms of maintainability. For instance, it might be harder to grep for typedef struct declarations. But since it doesn't gain you anything by doing this, there's no compelling reason not to simply do struct name {}; in C++.

Aside from technical issues, the differences between struct and class are semantic ones. It is traditional and expected that objects declared as structs are simple objects which consist of only public: data members (so-called PODs). If it has private or protected data, is expected to be derived from, or has any methods, it is declared as a class.

This guideline is open to interpretation, and is just that -- a guideline. There is nothing to prevent you from declaring an abstract base class as a struct, for example. However you may want to consider following this guideline in order to follow the Principle of Least Surprise, making your code easier to understand and maintain.

Answer 2

Both are nearly equivalent. More precisely, struct { is the same as class {public:

An optimizing compiler would probably generate exactly the same code. Use MELT or simply pass -fdump-tree-all (beware, that option produces hundreds of dump files) to g++ (assuming you use a recent GCC compiler) -preferably with some optimization like -O - to find out (or look at the produced assembler code with g++ -O -fverbose-asm -S ...)

Answer 3

typedef struct is actually the C way to do this. In C++ the two versions would look very similar: Your class as written, and the struct as follows:

struct sPoint
{
 int X;
 int Y;
};

The two forms are functionally identical but you can provide your future maintainers with significant information by picking and sticking to some convention about how they're used. For example one approach is that if you intend to make the data elements private and give it useful methods (for example if you use inline accessors you can insert print calls every time the methods are used) then by all means make it a class. If you intend to have the data be public and access them as members then make it a struct.

Answer 4

There's no performance difference between a class and a struct

A class defaults to private access, whilst a struct defaults to public access. If interoperability with C is an issue for you then you'll have to use struct, and obviously it can't have any member functions.

As an aside, there's no std::is_struct in the standard library. Instead the std::is_class method returns true if the type is a class or a structure.

Answer 5

Simply put, the first way is more C++, and the second way is more C. Both work, while the first way is more 'standard' now.

Answer 6

A struct in C++ is like a class that would have public members by default^*

There is no other formal difference, though your code would probably look confusing if you started using structs as classes, especially the inheritance mechanisms where data privacy is a major benefit.

If you are about to declare private/protected members, there is really little point in using a struct, though your code will still be 100% legal.

---

^*including inherited members, since the zealots and nitpickers around seem to think the point is of a capital importance and only ignorant heatens would fail to mention it.
Except for the fact that this fine doctrine point is defined (or rather hinted, since the inference that base classes are simply defining inherited members is left to the sagacity of the reader) in another verse of the Stoustrup Holy Bible, there is really nothing to fuss about IMHO.

Answer 7

To properly declare the struct in your example, use

struct sPoint {
    int X;
    int Y;
};

In general, structs and classes in C++ are identical, except that data is public in a struct by default. The other difference is that the struct keyword cannot be used as the type in a template, although a struct can be used as the parameter.

There is a more thorough discussion here: C++ - struct vs. class

Answer 8

technically, struct{} and class{} are the same.

they differ on semantic level, with different member visibility.

struct{...} is equivalent to class{public:...}

And, it is also legal to declare a class using struct keyword. (add member functions, access specifier to struct{})

Generally, using struct for Plain-Old-Data (POD) type, class for Object-Oriented type to improve readability.

typedef struct{} should only be used to hide implementation detail(eg: supply a close-source library to users)

From my opinion, in your case, using struct is better, because Point's member need to be modified directly by other code.