Related
Let's assume this class hierarchy below.
class BaseClass {
public:
int x;
}
class SubClass1 : public BaseClass {
public:
double y;
}
class SubClass2 : public BaseClass {
public:
float z;
}
...
I want to make a heterogeneous container of these classes. Since the subclasses are derived from the base class I can make something like this:
std::vector<BaseClass*> container1;
But since C++17 I can also use std::variant like this:
std::vector<std::variant<SubClass1, SubClass2, ...>> container2;
What are the advantages/disadvantages of using one or the other? I am interested in the performance too.
Take into consideration that I am going to sort the container by x, and I also need to be able to find out the exact type of the elements. I am going to
Fill the container,
Sort it by x,
Iterate through all the elements, find out the type, use it accordingly,
Clear the container, then the cycle starts over again.
std::variant<A,B,C> holds one of a closed set of types. You can check whether it holds a given type with std::holds_alternative, or use std::visit to pass a visitor object with an overloaded operator(). There is likely no dynamic memory allocation, however, it is hard to extend: the class with the std::variant and any visitor classes will need to know the list of possible types.
On the other hand, BaseClass* holds an unbounded set of derived class types. You ought to be holding std::unique_ptr<BaseClass> or std::shared_ptr<BaseClass> to avoid the potential for memory leaks. To determine whether an instance of a specific type is stored, you must use dynamic_cast or a virtual function. This option requires dynamic memory allocation, but if all processing is via virtual functions, then the code that holds the container does not need to know the full list of types that could be stored.
A problem with std::variant is that you need to specify a list of allowed types; if you add a future derived class you would have to add it to the type list. If you need a more dynamic implementation, you can look at std::any; I believe it can serve the purpose.
I also need to be able to find out the exact type of the elements.
For type recognition you can create a instanceof-like template as seen in C++ equivalent of instanceof. It is also said that the need to use such a mechanism sometimes reveals poor code design.
The performance issue is not something that can be detected ahead of time, because it depends on the usage: it's a matter of testing different implementations and see witch one is faster.
Take into consideration that, I am going to sort the container by x
In this case you declare the variable public so sorting is no problem at all; you may want to consider declaring the variable protected or implementing a sorting mechanism in the base class.
What are the advantages/disadvantages of using one or the other?
The same as advantages/disadvantages of using pointers for runtime type resolution and templates for compile time type resolution. There are many things that you might compare. For example:
with pointers you might have memory violations if you misuse them
runtime resolution has additional overhead (but also depends how would you use this classes exactly, if it is virtual function call, or just common member field access)
but
pointers have fixed size, and are probably smaller than the object of your class will be, so it might be better if you plan to copy your container often
I am interested in the performance too.
Then just measure the performance of your application and then decide. It is not a good practice to speculate which approach might be faster, because it strongly depends on the use case.
Take into consideration that, I am going to sort the container by x
and I also need to be able to find out the exact type of the elements.
In both cases you can find out the type. dynamic_cast in case of pointers, holds_alternative in case of std::variant. With std::variant all possible types must be explicitly specified. Accessing member field x will be almost the same in both cases (with the pointer it is pointer dereference + member access, with variant it is get + member access).
Sending data over a TCP connection was mentioned in the comments. In this case, it would probably make the most sense to use virtual dispatch.
class BaseClass {
public:
int x;
virtual void sendTo(Socket socket) const {
socket.send(x);
}
};
class SubClass1 final : public BaseClass {
public:
double y;
void sendTo(Socket socket) const override {
BaseClass::sendTo(socket);
socket.send(y);
}
};
class SubClass2 final : public BaseClass {
public:
float z;
void sendTo(Socket socket) const override {
BaseClass::sendTo(socket);
socket.send(z);
}
};
Then you can store pointers to the base class in a container, and manipulate the objects through the base class.
std::vector<std::unique_ptr<BaseClass>> container;
// fill the container
auto a = std::make_unique<SubClass1>();
a->x = 5;
a->y = 17.0;
container.push_back(a);
auto b = std::make_unique<SubClass2>();
b->x = 1;
b->z = 14.5;
container.push_back(b);
// sort by x
std::sort(container.begin(), container.end(), [](auto &lhs, auto &rhs) {
return lhs->x < rhs->x;
});
// send the data over the connection
for (auto &ptr : container) {
ptr->sendTo(socket);
}
It's not the same. std::variant is like a union with type safety. No more than one member can be visible at the same time.
// C++ 17
std::variant<int,float,char> x;
x = 5; // now contains int
int i = std::get<int>(v); // i = 5;
std::get<float>(v); // Throws
The other option is based on inheritance. All members are visible depending on which pointer you have.
Your selection will depend on if you want all the variables to be visible and what error reporting you want.
Related: don't use a vector of pointers. Use a vector of shared_ptr.
Unrelated: I'm somewhat not of a supporter of the new union variant. The point of the older C-style union was to be able to access all the members it had at the same memory place.
Classic way to get a reference to class member or its value we use getters like getValue(). Could this be an alternative way? :
class A{
ComplexClass value_;
public:
//No need. ComplexClass const& getValue() const { return value_; }
ComplexClass const& value = value_; /// ???
}
Will this work? How do you like such syntax?
UPD.
This point is to make user code simpler. Personally I better like auto x = a.value than auto x = a.getValue(). Of course this is a deal of taste.
Will this still work nice if:
class A{
public:
ComplexClass const& value = value_;
protected:
ComplexClass value_;
}
I ask because I met some troubles with one compiler.
One of the main reasons to prefer member functions over data members is flexibility and maintainability. If you only ever wrote some code once, perfectly, and it would never be changed or reused, then you could certainly have public data members that users refer to directly.
The interesting question is what happens when your code doesn't meet these criteria, e.g. if it is going to evolve, and if other people start using it. Then once you have a public data member, you are forced to always have that data member. Details of your class layout are now part of your public contract. Simple refactorings like moving common parts into nested member objects are no longer private and break existing users.
Your proposed reference data member adds almost no benefit over a public data member (except for in very trivial cases), and unlike member functions, non-static data members affect the class layout. Class layout is another thing you will probably want to keep stable once you have users, so that old, compiled libraries can continue to be linked against new user code. Member functions are much easier to evolve while keeping the data layout unchanged.
There's a nice example in the standard library where such a mistake was made: std::pair<T1, T2> is specified to contain public data members first and second. That means that all user specializations must adhere to the same specification, and cannot easily employ things like base layout optimizations. Had first and second been specified as member functions, such optimizations could be applied trivially.
After seeing this question a few minutes ago, I wondered why the language designers allow it as it allows indirect modification of private data. As an example
class TestClass {
private:
int cc;
public:
TestClass(int i) : cc(i) {};
};
TestClass cc(5);
int* pp = (int*)&cc;
*pp = 70; // private member has been modified
I tested the above code and indeed the private data has been modified. Is there any explanation of why this is allowed to happen or this just an oversight in the language? It seems to directly undermine the use of private data members.
Because, as Bjarne puts it, C++ is designed to protect against Murphy, not Machiavelli.
In other words, it's supposed to protect you from accidents -- but if you go to any work at all to subvert it (such as using a cast) it's not even going to attempt to stop you.
When I think of it, I have a somewhat different analogy in mind: it's like the lock on a bathroom door. It gives you a warning that you probably don't want to walk in there right now, but it's trivial to unlock the door from the outside if you decide to.
Edit: as to the question #Xeo discusses, about why the standard says "have the same access control" instead of "have all public access control", the answer is long and a little tortuous.
Let's step back to the beginning and consider a struct like:
struct X {
int a;
int b;
};
C always had a few rules for a struct like this. One is that in an instance of the struct, the address of the struct itself has to equal the address of a, so you can cast a pointer to the struct to a pointer to int, and access a with well defined results. Another is that the members have to be arranged in the same order in memory as they are defined in the struct (though the compiler is free to insert padding between them).
For C++, there was an intent to maintain that, especially for existing C structs. At the same time, there was an apparent intent that if the compiler wanted to enforce private (and protected) at run-time, it should be easy to do that (reasonably efficiently).
Therefore, given something like:
struct Y {
int a;
int b;
private:
int c;
int d;
public:
int e;
// code to use `c` and `d` goes here.
};
The compiler should be required to maintain the same rules as C with respect to Y.a and Y.b. At the same time, if it's going to enforce access at run time, it may want to move all the public variables together in memory, so the layout would be more like:
struct Z {
int a;
int b;
int e;
private:
int c;
int d;
// code to use `c` and `d` goes here.
};
Then, when it's enforcing things at run-time, it can basically do something like if (offset > 3 * sizeof(int)) access_violation();
To my knowledge nobody's ever done this, and I'm not sure the rest of the standard really allows it, but there does seem to have been at least the half-formed germ of an idea along that line.
To enforce both of those, the C++98 said Y::a and Y::b had to be in that order in memory, and Y::a had to be at the beginning of the struct (i.e., C-like rules). But, because of the intervening access specifiers, Y::c and Y::e no longer had to be in order relative to each other. In other words, all the consecutive variables defined without an access specifier between them were grouped together, the compiler was free to rearrange those groups (but still had to keep the first one at the beginning).
That was fine until some jerk (i.e., me) pointed out that the way the rules were written had another little problem. If I wrote code like:
struct A {
int a;
public:
int b;
public:
int c;
public:
int d;
};
...you ended up with a little bit of self contradition. On one hand, this was still officially a POD struct, so the C-like rules were supposed to apply -- but since you had (admittedly meaningless) access specifiers between the members, it also gave the compiler permission to rearrange the members, thus breaking the C-like rules they intended.
To cure that, they re-worded the standard a little so it would talk about the members all having the same access, rather than about whether or not there was an access specifier between them. Yes, they could have just decreed that the rules would only apply to public members, but it would appear that nobody saw anything to be gained from that. Given that this was modifying an existing standard with lots of code that had been in use for quite a while, the opted for the smallest change they could make that would still cure the problem.
Because of backwards-compatability with C, where you can do the same thing.
For all people wondering, here's why this is not UB and is actually allowed by the standard:
First, TestClass is a standard-layout class (§9 [class] p7):
A standard-layout class is a class that:
has no non-static data members of type non-standard-layout class (or array of such types) or reference, // OK: non-static data member is of type 'int'
has no virtual functions (10.3) and no virtual base classes (10.1), // OK
has the same access control (Clause 11) for all non-static data members, // OK, all non-static data members (1) are 'private'
has no non-standard-layout base classes, // OK, no base classes
either has no non-static data members in the most derived class and at most one base class with non-static data members, or has no base classes with non-static data members, and // OK, no base classes again
has no base classes of the same type as the first non-static data member. // OK, no base classes again
And with that, you can are allowed to reinterpret_cast the class to the type of its first member (§9.2 [class.mem] p20):
A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa.
In your case, the C-style (int*) cast resolves to a reinterpret_cast (§5.4 [expr.cast] p4).
A good reason is to allow compatibility with C but extra access safety on the C++ layer.
Consider:
struct S {
#ifdef __cplusplus
private:
#endif // __cplusplus
int i, j;
#ifdef __cplusplus
public:
int get_i() const { return i; }
int get_j() const { return j; }
#endif // __cplusplus
};
By requiring that the C-visible S and the C++-visible S be layout-compatible, S can be used across the language boundary with the C++ side having greater access safety. The reinterpret_cast access safety subversion is an unfortunate but necessary corollary.
As an aside, the restriction on having all members with the same access control is because the implementation is permitted to rearrange members relative to members with different access control. Presumably some implementations put members with the same access control together, for the sake of tidiness; it could also be used to reduce padding, although I don't know of any compiler that does that.
The whole purpose of reinterpret_cast (and a C style cast is even more powerful than a reinterpret_cast) is to provide an escape path around safety measures.
The compiler would have given you an error if you had tried int *pp = &cc.cc, the compiler would have told you that you cannot access a private member.
In your code you are reinterpreting the address of cc as a pointer to an int. You wrote it the C style way, the C++ style way would have been int* pp = reinterpret_cast<int*>(&cc);. The reinterpret_cast always is a warning that you are doing a cast between two pointers that are not related. In such a case you must make sure that you are doing right. You must know the underlying memory (layout). The compiler does not prevent you from doing so, because this if often needed.
When doing the cast you throw away all knowledge about the class. From now on the compiler only sees an int pointer. Of course you can access the memory the pointer points to. In your case, on your platform the compiler happened to put cc in the first n bytes of a TestClass object, so a TestClass pointer also points to the cc member.
This is because you are manipulating the memory where your class is located in memory. In your case it just happen to store the private member at this memory location so you change it. It is not a very good idea to do because you do now know how the object will be stored in memory.
I have a situation similar to this one
struct Child
{
u16 x, y;
// other fields
};
struct Father
{
struct Child child1;
struct Child child2;
// other fields
};
Father tilemap[WIDTH][HEIGHT];
Now I just realized I would like to save four bytes for x,y which are set always to the same values for both children of the same father.
All around my code I pass around many Father* and many Child* while recovering coordinates with father->child1->x or child1->x respectively. I would like to safely move the coordinates at Father level but I'm unsure about some facts.
Will the order of declared fields be respected versus any optimization or possible implementation of gcc/g++? Can I be confident that &father == &father.child1?
The real issue here is that I pass Child* without knowing if it's a child1 or child2 field so I cannot directly know the offset to recover address of father (and coordinates consequently).. I was wondering to use a bit at Child level to distinguish them but will I be easily able to recover address of father then?
Any suggestion would be appreciated, thanks
EDIT: just as a further info, I'm using C++ as my main language but these structs don't contain ANY strange methods, just fields and empty constructor.
The general rules about field layout in C are:
The address of the first member is the same as the address of the struct itself. That is, the offsetof of the member field is 0.
The addresses of the members always increase in declaration order. That is, the offsetof of the n-th field is lower than that of the (n+1)-th member.
In C++, of course, that is only true if it is a standard layout type, that is roughly, a class or struct with no public/private/protected mixed members, no virtual functions and no members inherited from other classes.
Disclaimer: Partial answer. C++ only
Will the order of declared fields be respected versus any optimization
or possible implementation of gcc/g++?
The order of the members in the memory layout will not be tampered with by the compiler. It's the same order you declared the members in.
Can I be confident that &father == &father.child1?
In this particular case, yes. But it does not follow from the mere fact that child1 is the first member of father that &father == &father.child1?. This is true only if father is a POD, which in this case it is.
The pertinent section of the C standard says this (emphasis mine):
Within a structure object, the non-bit-field members and the units in which bit-fields
reside have addresses that increase in the order in which they are declared. A pointer to a
structure object, suitably converted, points to its initial member (or if that member is a
bit-field, then to the unit in which it resides), and vice versa. There may be unnamed
padding within a structure object, but not at its beginning.
The C++ standard makes the same promise:
A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its
initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa. [ Note:
There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning,
as necessary to achieve appropriate alignment. —end note ]
So when you ask:
Can I be confident that &father == &father.child1?
The answer is yes.
Try the following
struct Child
{
int isChild1;
u16 x, y;
// other fields
};
...
Father *father_p;
if (*child_p).isChild1
father_p = child_p;
else
father_p = child_p - sizeof(struct Child);
(*father_p).x = ... // whatever you want to do with coordinates
You should be careful not to pass a Child which isn't incorporated in a corresponding Father, in which case you'll get a bogus address in father_p and (maybe) corrupt your memory.
Instead of relying on memory layout, have you considered using something along the lines of the Flyweight pattern?
Instead of storing two Childs in Father, you can store two stripped-down BasicChilds (that don't have x,y data) and generate full-fledged Childs on-the-fly as needed:
struct BasicChild
{
float foo;
float bar;
void printPosition(int x, int y) {std::cout << x << "," << y << "\n";}
};
struct Child
{
Child(BasicChild basicChild, int x, int y)
: basicChild_(basicChild), x_(x), y_(y){}
float foo() {return basicChild_.foo;}
float bar() {return basicChild_.bar;}
void printPosition() {basicChild_.printPosition(x_, y_);}
private:
int x_, y_;
BasicChild basicChild_;
};
struct Father
{
Child child1() {return Child(child1_, x_, y_);}
Child child2() {return Child(child2_, x_, y_);}
private:
int x_, y_;
BasicChild child1_;
BasicChild child2_;
};
If you decide to rely on any compiler specific behaviour then the best you can do is to add static asserts for any assumptions you are relying on. This way, if anything changes about the layout due to compiler upgrade, compile options, or pragmas elsewhere in code, you will be told at compile time straight away, and exactly what the problem is. This then would serve as a basis for porting to other platforms too if that is a requirement.
Examples in this Q: What does static_assert do, and what would you use it for?
Or does it?
Should an object-oriented design use a language construct that exposes member data by default, if there is an equally useful construct that properly hides data members?
EDIT: One of the responders mentioned that if there's no invariant one can use a struct. That's an interesting observation: a struct is a data structure, i.e. it contains related data. If the data members in a struct are related isn't there's always an invariant?
In C++, structs and classes are identical except for the default public/privateness of their members. (This default is easily, and usually, overridden.)
However, most programmers think of a struct as a "data object" and a class as an "interactive object". That's not a bad thing; and in fact should be taken advantage of. If something is just an inanimate lump of data (even maybe if it has a couple of inspector methods), use a struct for it; it'll save a bit of effort when a programmer is trying to see what it's for.
Don't be a hiding zealot. If your get/set methods do nothing but simply copy verbatim the value onto/from a hidden, private field, you've gained nothing over a public member and only complicate unnecessarily your class (and, depending on the intelligence of the compiler, slow its usage a bit).
There's a case for not allowing direct access when your setter methods do some validation, copy the data somewhere else, process it a bit before storing it, etc. Same in the case of getters that actually calculate the value they return from multiple internal sources, and hide the way it's derived (I believe Bertrand Meyer speaks a bit about this in his book)
Or if allowing the users of your class to directly change such a value would have unintended side effects or breaks an assumption some of your member classes have about the values. On those situations, by all means, do hide your values.
For instance, for a simple "Point" class, that only holds a couple coordinates and colour, and methods to "Plot" it and "Hide" it on screen, I would see no point in not allowing the user to directly set the values for its fields.
In C# for example I use structs for some simple better-left-as-values data types:
public struct Point
{
int X;
int Y;
}
and for any P/Invoke to libraries where the arguments are structs you'll have to use them for certain.
Do they belong in the general design of an application? Of course they do, use a struct when it makes sense to do so. Just like you'd use a enum with bit flags when it makes sense to do so instead of resorting to some complicated string parsing for storing combined values.
In C++, the difference between a struct and a class is the default visibility of its contents (i.e. public for a struct, and private for a class). I guess this difference was to keep C compatibility.
But semantically, I guess this is subject to interpretation.
An example of struct
In a struct, everything is public (by default), meaning the user can modify each data value as desired, and still the struct remains a valid object. Example of struct:
struct CPoint
{
int x ;
int y ;
CPoint() : x(0), y(0) {}
int getDistanceFromOrigin() const
{
return std::sqrt(x * x + y * y) ;
}
} ;
inline CPoint operator + (const CPoint & lhs, const CPoint & rhs)
{
CPoint r(lhs) ;
r.x += rhs.x ;
r.y += rhs.y ;
return r ;
}
You can change the x value of a CPoint, and it still remains a valid CPoint.
Note that, unlike some believe, a C++ struct can (and should) have constructors, methods and non-member functions attached to its interface, as shown above.
An example of class
In a class, everything is private (by default), meaning the user can modify the data only through a well defined interface, because the class must keep its internals valid. Example of class:
class CString
{
public :
CString(const char * p) { /* etc. */ } ;
CString(const CString & p) { /* etc. */ } ;
const char * getString() const { return this->m_pString ; }
size_t getSize() const { return this->m_iSize ; }
void copy { /* code for string copy */ }
void concat { /* code for string concatenation */ }
private :
size_t m_iSize ;
char * m_pString ;
} ;
inline CString operator + (const CString & lhs, const CString & rhs)
{
CString r(lhs) ;
r.concat(rhs) ;
return r ;
}
You see that when you call concat, both the pointer could need reallocation (to increase its size), and the size of the string must be updated automatically. You can't let the user modify the string by hand, and forget updating the size.
So, the class must protect its internal, and be sure everything will be correctly updated when needed.
Conclusion
For me, the difference between a struct and a class is the dependencies between the aggregated data.
If each and every piece of data is independent from all the others, then perhaps you should consider a struct (i.e., a class with public data member).
If not, or if in doubt, use a class.
Now, of course, in C#, the struct and class are two different type of objects (i.e. value types for structs, and referenced types for classes). But this is out of this topic, I guess.
Technically, a struct is a class with the default visibility of public (a real class has a default visibility of private).
There is more of a distinction in common use.
A struct is normally just a collection of data, to be examined and processed by other code.
A class is normally more of a thing, maintaining some sort of control over its data, and with behavior specified by associated functions.
Typically, classes are more useful, but every so often there's uses for something like a C struct, and it's useful to have a notational difference to show it.
The matter is easy. If the class does have invariants to guarantee, you should never make the members constraining the invariant public.
If your struct is merely an aggregate of different objects, and doesn't have an invariant to hold, you are indeed free and encouraged to put its members public. That's the way std::pair<T, U> in C++ does it.
What's that invariant stuff?
Simple example: Consider you have a Point class whose x and y members must always be >= 0 . You can make an invariant stating
/* x >= 0 && y >= 0 for this classes' objects. */
If you now make those members public, clients could simply change x and y, and your invariant could break easily. If the members, however, are allowed to contain all possible values fitting their own invariants respectively, you could of course just make those members public: You wouldn't add any protection to them anyway.
A struct is essentially a model class but with different syntax.
public struct Point {
int x;
int y;
}
is logically the same as:
public class Point {
private int x;
private int y;
public void setX(int x) { this.x=x; }
public int getX(); { return x; }
public void setY(int y) { this.y=y; }
public int getY(); { return y; }
}
Both are a mutable model that holds pair of integer values called x and y. So I would say that it's a valid object oriented construct.
Yes. It's like a mini-class.
Yes, they do. They have different semantic than classes. A struct is generally considered and treated as a value type, while a class is generally considered and treated as a reference type. The difference is not as much pronunciated in every day programming; however, it is an imprtant difference when it comes to things like marshalling, COM interop and passing instances around.
I use structs regularly - mostly for data received from the network or hardware. They are usually wrapped in a class for use by higher level parts of the program.
My rule of thumb is a struct is always pure data, except for a constructor. Anything else is a class.
Most answers seem to be in favor of a struct as something to be acceptable and useful, as long as it does not have a behavior (i.e. methods). That seems fair enough.
However, you can never be sure that your object does not evolve into something that may need behavior, and hence some control over its data members. If you're lucky enough that you have control over all users of your struct, you can go over all uses of all data members. But what if you don't have access to all users?
A struct, as used in C or C++, and the struct used in C# ( or any .Net language ), are such different animals that they probably should not even have the same name... Just about any generalization about structs in one language can easily be false, or true for a completely unrelated reason, in the other.
If there is a need for invariant, make it a class. Otherwise, struct is OK.
See these similar questions:
When should you use a class vs a struct in C++?
What are the differences between struct and class in C++
plus:
According to Stroustrup in the C++ Programming Language:
Which style you use depends on circumstances and taste. I usually prefer to use struct for classes that have all data public. I think of such classes as "not quite proper types, just data structures."
Formally, in C++ a struct is a class with the visibility of its members set to public by default. By tradition structs are used to group collection of homogeneous data that have no particular reasons for being accessed by specific methods.
The public visibility of its members makes structs preferred to class to implement policy classes and metafunctions.
There's nothing wrong with using structs per se, but if you're finding yourself needing them, you should ask what's wrong with your analysis. Consider, eg, the Point class above: it gives some little improvement in readability, since you can always use
Point foo;
//...
foo.x = bar;
in place of, say, having a two element array as in
#define X 0
#define Y 1
//...
foo[X] = bar;
But classes are meant to hide details behind a contract. If your Point is in some normalized space, the values may range in the half-open interval [0.0..1.0); if it's a screen they may range in [0..1023]. If you use a struct in place of accessors, how will you keep someone from assigning foo.x = 1023 when x should be everywhere < 1.0?
The only reason C++ programmers used structs for Points is that back at the dawn of time --- 20 years ago, when C++ was new --- inlining wasn't handled very well, so that foo.setX(1023) actually took more instructions than foo.x = 1023. That's no longer true.
Structs are fine as long as they're kept small. As you probably know, they are allocated on the stack (not the heap) so you need to watch the size. They can come in handy for small data structures like Point, Size, etc. A class is usually the better choice though, being a reference type and all.