I have a vector 3D class
class Vector3D{
public: float x; float y; float z;
//some functions, e.g. operator+ - * /
//some 3D-specific function
};
and a vector N-D class.
template<int constSize> class VecFloatFix{
float database[constSize];
//some functions, e.g. operator+ - * /
};
I noticed that there is code-duplication between two classes, so I think I should make Vector3D derived from VecFloatFix<3> :-
class Vector3D : public VecFloatFix<3>{
//some 3D-specific function
};
Everything seems to be good, except that there are a lot of user code access Vector3D::x,y,z directly.
Is it possible to make Vector3D derived from VecFloatFix<3> while not break user's code?
My best guess is around :-
template<int constSize> class VecFloatFix{
union{
float database[constSize];
float x,y,z; ????? sound like a hack
}
//some functions, e.g. operator+ - * /
};
Edit: Hardcoding x,y,z into VecFloatFix is unsustainable.
If I have a new class Vector2D that derived from VecFloatFix<2>, Vector2D::z will compile fine (dangerous).
Here is a version that only exposes x, y, z components for vectors of size 3. Obviously other sizes may also be specialized.
template<int constSize> struct VecFloatStorage
{
float database[constSize];
};
template<> struct VecFloatStorage<3>
{
union
{
float database[3];
struct { float x, y, z; };
};
};
template<int constSize> class VecFloatFix : public VecFloatStorage<constSize>
{
public:
// Methods go here.
};
I don't know if the standard guarantees struct { float x, y, z; } to have the same memory layout as float data[3], however in practice I am pretty certain that assumption holds.
The GLM library is using a similar trick, except they don't have an array member at all, instead providing an indexing operator that returns (&this->x)[idx].
This is by no ways guaranteed to work as it uses implementation-defined and possibly undefined behaviour. A sensible implementation will probably behave as expected though.
template<int constSize>
class VecFloatFix{
public:
union {
float database[constSize];
struct {
int x, y, z;
};
};
};
This also leaves database public. Don't see a way around this, but no big deal since you provide operator[] anyway.
This assumes constSize >= 3. If you need smaller sizes, this is doable through a bit more hackery. All vectors will have x y and z members but only 3D and above will have them all usable. The 2D vector will have only x and y usable (any use of z is likely to result in an error) and the 1D vector will have just x. Note I refuse to take responsibility for any of the following.
template<int constSize>
class VecFloatFix{
public:
union {
float database[constSize];
struct {
float x;
};
struct {
spacer<constSize, 1> sp1;
typename spacer<constSize, 1>::type y;
};
struct {
spacer<constSize, 2> sp2;
typename spacer<constSize, 2>::type z;
};
};
};
where spacer is defined this way:
template <int N, int M, bool enable>
struct filler;
template <int N, int M>
struct filler<N, M, true>
{
float _f[M];
typedef float type;
};
template <int N, int M>
struct filler<N, M, false>
{
struct nothing {};
typedef nothing type;
};
template <int N, int M>
struct spacer
{
filler<N, M, (N>M)> _f;
typedef typename filler<N, M, (N>M)>::type type;
};
Test drive:
VecFloatFix<4> vec4;
VecFloatFix<3> vec3;
VecFloatFix<2> vec2;
VecFloatFix<1> vec1;
`smoke test`
vec3.database[0] = 42;
vec2.database[1] = 99;
std::cout << vec3.x << std::endl;
std::cout << vec2.y << std::endl;
// make sure `y` aliases `database[1]`
std::cout << & vec2.y << std::endl;
std::cout << & vec2.database[1] << std::endl;
// make sure sizes are as expected
std::cout << sizeof(vec4) << " " << sizeof (vec3) << " " << sizeof(vec2) << " " << sizeof(vec1) << std::endl;
I came across this strange code snippet which compiles fine:
class Car
{
public:
int speed;
};
int main()
{
int Car::*pSpeed = &Car::speed;
return 0;
}
Why does C++ have this pointer to a non-static data member of a class? What is the use of this strange pointer in real code?
It's a "pointer to member" - the following code illustrates its use:
#include <iostream>
using namespace std;
class Car
{
public:
int speed;
};
int main()
{
int Car::*pSpeed = &Car::speed;
Car c1;
c1.speed = 1; // direct access
cout << "speed is " << c1.speed << endl;
c1.*pSpeed = 2; // access via pointer to member
cout << "speed is " << c1.speed << endl;
return 0;
}
As to why you would want to do that, well it gives you another level of indirection that can solve some tricky problems. But to be honest, I've never had to use them in my own code.
Edit: I can't think off-hand of a convincing use for pointers to member data. Pointer to member functions can be used in pluggable architectures, but once again producing an example in a small space defeats me. The following is my best (untested) try - an Apply function that would do some pre &post processing before applying a user-selected member function to an object:
void Apply( SomeClass * c, void (SomeClass::*func)() ) {
// do hefty pre-call processing
(c->*func)(); // call user specified function
// do hefty post-call processing
}
The parentheses around c->*func are necessary because the ->* operator has lower precedence than the function call operator.
This is the simplest example I can think of that conveys the rare cases where this feature is pertinent:
#include <iostream>
class bowl {
public:
int apples;
int oranges;
};
int count_fruit(bowl * begin, bowl * end, int bowl::*fruit)
{
int count = 0;
for (bowl * iterator = begin; iterator != end; ++ iterator)
count += iterator->*fruit;
return count;
}
int main()
{
bowl bowls[2] = {
{ 1, 2 },
{ 3, 5 }
};
std::cout << "I have " << count_fruit(bowls, bowls + 2, & bowl::apples) << " apples\n";
std::cout << "I have " << count_fruit(bowls, bowls + 2, & bowl::oranges) << " oranges\n";
return 0;
}
The thing to note here is the pointer passed in to count_fruit. This saves you having to write separate count_apples and count_oranges functions.
Another application are intrusive lists. The element type can tell the list what its next/prev pointers are. So the list does not use hard-coded names but can still use existing pointers:
// say this is some existing structure. And we want to use
// a list. We can tell it that the next pointer
// is apple::next.
struct apple {
int data;
apple * next;
};
// simple example of a minimal intrusive list. Could specify the
// member pointer as template argument too, if we wanted:
// template<typename E, E *E::*next_ptr>
template<typename E>
struct List {
List(E *E::*next_ptr):head(0), next_ptr(next_ptr) { }
void add(E &e) {
// access its next pointer by the member pointer
e.*next_ptr = head;
head = &e;
}
E * head;
E *E::*next_ptr;
};
int main() {
List<apple> lst(&apple::next);
apple a;
lst.add(a);
}
Here's a real-world example I am working on right now, from signal processing / control systems:
Suppose you have some structure that represents the data you are collecting:
struct Sample {
time_t time;
double value1;
double value2;
double value3;
};
Now suppose that you stuff them into a vector:
std::vector<Sample> samples;
... fill the vector ...
Now suppose that you want to calculate some function (say the mean) of one of the variables over a range of samples, and you want to factor this mean calculation into a function. The pointer-to-member makes it easy:
double Mean(std::vector<Sample>::const_iterator begin,
std::vector<Sample>::const_iterator end,
double Sample::* var)
{
float mean = 0;
int samples = 0;
for(; begin != end; begin++) {
const Sample& s = *begin;
mean += s.*var;
samples++;
}
mean /= samples;
return mean;
}
...
double mean = Mean(samples.begin(), samples.end(), &Sample::value2);
Note Edited 2016/08/05 for a more concise template-function approach
And, of course, you can template it to compute a mean for any forward-iterator and any value type that supports addition with itself and division by size_t:
template<typename Titer, typename S>
S mean(Titer begin, const Titer& end, S std::iterator_traits<Titer>::value_type::* var) {
using T = typename std::iterator_traits<Titer>::value_type;
S sum = 0;
size_t samples = 0;
for( ; begin != end ; ++begin ) {
const T& s = *begin;
sum += s.*var;
samples++;
}
return sum / samples;
}
struct Sample {
double x;
}
std::vector<Sample> samples { {1.0}, {2.0}, {3.0} };
double m = mean(samples.begin(), samples.end(), &Sample::x);
EDIT - The above code has performance implications
You should note, as I soon discovered, that the code above has some serious performance implications. The summary is that if you're calculating a summary statistic on a time series, or calculating an FFT etc, then you should store the values for each variable contiguously in memory. Otherwise, iterating over the series will cause a cache miss for every value retrieved.
Consider the performance of this code:
struct Sample {
float w, x, y, z;
};
std::vector<Sample> series = ...;
float sum = 0;
int samples = 0;
for(auto it = series.begin(); it != series.end(); it++) {
sum += *it.x;
samples++;
}
float mean = sum / samples;
On many architectures, one instance of Sample will fill a cache line. So on each iteration of the loop, one sample will be pulled from memory into the cache. 4 bytes from the cache line will be used and the rest thrown away, and the next iteration will result in another cache miss, memory access and so on.
Much better to do this:
struct Samples {
std::vector<float> w, x, y, z;
};
Samples series = ...;
float sum = 0;
float samples = 0;
for(auto it = series.x.begin(); it != series.x.end(); it++) {
sum += *it;
samples++;
}
float mean = sum / samples;
Now when the first x value is loaded from memory, the next three will also be loaded into the cache (supposing suitable alignment), meaning you don't need any values loaded for the next three iterations.
The above algorithm can be improved somewhat further through the use of SIMD instructions on eg SSE2 architectures. However, these work much better if the values are all contiguous in memory and you can use a single instruction to load four samples together (more in later SSE versions).
YMMV - design your data structures to suit your algorithm.
You can later access this member, on any instance:
int main()
{
int Car::*pSpeed = &Car::speed;
Car myCar;
Car yourCar;
int mySpeed = myCar.*pSpeed;
int yourSpeed = yourCar.*pSpeed;
assert(mySpeed > yourSpeed); // ;-)
return 0;
}
Note that you do need an instance to call it on, so it does not work like a delegate.
It is used rarely, I've needed it maybe once or twice in all my years.
Normally using an interface (i.e. a pure base class in C++) is the better design choice.
IBM has some more documentation on how to use this. Briefly, you're using the pointer as an offset into the class. You can't use these pointers apart from the class they refer to, so:
int Car::*pSpeed = &Car::speed;
Car mycar;
mycar.*pSpeed = 65;
It seems a little obscure, but one possible application is if you're trying to write code for deserializing generic data into many different object types, and your code needs to handle object types that it knows absolutely nothing about (for example, your code is in a library, and the objects into which you deserialize were created by a user of your library). The member pointers give you a generic, semi-legible way of referring to the individual data member offsets, without having to resort to typeless void * tricks the way you might for C structs.
It makes it possible to bind member variables and functions in the uniform manner. The following is example with your Car class. More common usage would be binding std::pair::first and ::second when using in STL algorithms and Boost on a map.
#include <list>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
class Car {
public:
Car(int s): speed(s) {}
void drive() {
std::cout << "Driving at " << speed << " km/h" << std::endl;
}
int speed;
};
int main() {
using namespace std;
using namespace boost::lambda;
list<Car> l;
l.push_back(Car(10));
l.push_back(Car(140));
l.push_back(Car(130));
l.push_back(Car(60));
// Speeding cars
list<Car> s;
// Binding a value to a member variable.
// Find all cars with speed over 60 km/h.
remove_copy_if(l.begin(), l.end(),
back_inserter(s),
bind(&Car::speed, _1) <= 60);
// Binding a value to a member function.
// Call a function on each car.
for_each(s.begin(), s.end(), bind(&Car::drive, _1));
return 0;
}
You can use an array of pointer to (homogeneous) member data to enable a dual, named-member (i.e. x.data) and array-subscript (i.e. x[idx]) interface.
#include <cassert>
#include <cstddef>
struct vector3 {
float x;
float y;
float z;
float& operator[](std::size_t idx) {
static float vector3::*component[3] = {
&vector3::x, &vector3::y, &vector3::z
};
return this->*component[idx];
}
};
int main()
{
vector3 v = { 0.0f, 1.0f, 2.0f };
assert(&v[0] == &v.x);
assert(&v[1] == &v.y);
assert(&v[2] == &v.z);
for (std::size_t i = 0; i < 3; ++i) {
v[i] += 1.0f;
}
assert(v.x == 1.0f);
assert(v.y == 2.0f);
assert(v.z == 3.0f);
return 0;
}
One way I've used it is if I have two implementations of how to do something in a class and I want to choose one at run-time without having to continually go through an if statement i.e.
class Algorithm
{
public:
Algorithm() : m_impFn( &Algorithm::implementationA ) {}
void frequentlyCalled()
{
// Avoid if ( using A ) else if ( using B ) type of thing
(this->*m_impFn)();
}
private:
void implementationA() { /*...*/ }
void implementationB() { /*...*/ }
typedef void ( Algorithm::*IMP_FN ) ();
IMP_FN m_impFn;
};
Obviously this is only practically useful if you feel the code is being hammered enough that the if statement is slowing things done eg. deep in the guts of some intensive algorithm somewhere. I still think it's more elegant than the if statement even in situations where it has no practical use but that's just my opnion.
Pointers to classes are not real pointers; a class is a logical construct and has no physical existence in memory, however, when you construct a pointer to a member of a class it gives an offset into an object of the member's class where the member can be found; This gives an important conclusion: Since static members are not associated with any object so a pointer to a member CANNOT point to a static member(data or functions) whatsoever
Consider the following:
class x {
public:
int val;
x(int i) { val = i;}
int get_val() { return val; }
int d_val(int i) {return i+i; }
};
int main() {
int (x::* data) = &x::val; //pointer to data member
int (x::* func)(int) = &x::d_val; //pointer to function member
x ob1(1), ob2(2);
cout <<ob1.*data;
cout <<ob2.*data;
cout <<(ob1.*func)(ob1.*data);
cout <<(ob2.*func)(ob2.*data);
return 0;
}
Source: The Complete Reference C++ - Herbert Schildt 4th Edition
Here is an example where pointer to data members could be useful:
#include <iostream>
#include <list>
#include <string>
template <typename Container, typename T, typename DataPtr>
typename Container::value_type searchByDataMember (const Container& container, const T& t, DataPtr ptr) {
for (const typename Container::value_type& x : container) {
if (x->*ptr == t)
return x;
}
return typename Container::value_type{};
}
struct Object {
int ID, value;
std::string name;
Object (int i, int v, const std::string& n) : ID(i), value(v), name(n) {}
};
std::list<Object*> objects { new Object(5,6,"Sam"), new Object(11,7,"Mark"), new Object(9,12,"Rob"),
new Object(2,11,"Tom"), new Object(15,16,"John") };
int main() {
const Object* object = searchByDataMember (objects, 11, &Object::value);
std::cout << object->name << '\n'; // Tom
}
Suppose you have a structure. Inside of that structure are
* some sort of name
* two variables of the same type but with different meaning
struct foo {
std::string a;
std::string b;
};
Okay, now let's say you have a bunch of foos in a container:
// key: some sort of name, value: a foo instance
std::map<std::string, foo> container;
Okay, now suppose you load the data from separate sources, but the data is presented in the same fashion (eg, you need the same parsing method).
You could do something like this:
void readDataFromText(std::istream & input, std::map<std::string, foo> & container, std::string foo::*storage) {
std::string line, name, value;
// while lines are successfully retrieved
while (std::getline(input, line)) {
std::stringstream linestr(line);
if ( line.empty() ) {
continue;
}
// retrieve name and value
linestr >> name >> value;
// store value into correct storage, whichever one is correct
container[name].*storage = value;
}
}
std::map<std::string, foo> readValues() {
std::map<std::string, foo> foos;
std::ifstream a("input-a");
readDataFromText(a, foos, &foo::a);
std::ifstream b("input-b");
readDataFromText(b, foos, &foo::b);
return foos;
}
At this point, calling readValues() will return a container with a unison of "input-a" and "input-b"; all keys will be present, and foos with have either a or b or both.
Just to add some use cases for #anon's & #Oktalist's answer, here's a great reading material about pointer-to-member-function and pointer-to-member-data.
https://www.dre.vanderbilt.edu/~schmidt/PDF/C++-ptmf4.pdf
with pointer to member, we can write generic code like this
template<typename T, typename U>
struct alpha{
T U::*p_some_member;
};
struct beta{
int foo;
};
int main()
{
beta b{};
alpha<int, beta> a{&beta::foo};
b.*(a.p_some_member) = 4;
return 0;
}
I love the * and & operators:
struct X
{
int a {0};
int *ptr {NULL};
int &fa() { return a; }
int *&fptr() { return ptr; }
};
int main(void)
{
X x;
int X::*p1 = &X::a; // pointer-to-member 'int X::a'. Type of p1 = 'int X::*'
x.*p1 = 10;
int *X::*p2 = &X::ptr; // pointer-to-member-pointer 'int *X::ptr'. Type of p2 = 'int *X::*'
x.*p2 = nullptr;
X *xx;
xx->*p2 = nullptr;
int& (X::*p3)() = X::fa; // pointer-to-member-function 'X::fa'. Type of p3 = 'int &(X::*)()'
(x.*p3)() = 20;
(xx->*p3)() = 30;
int *&(X::*p4)() = X::fptr; // pointer-to-member-function 'X::fptr'. Type of p4 = 'int *&(X::*)()'
(x.*p4)() = nullptr;
(xx->*p4)() = nullptr;
}
Indeed all is true as long as the members are public, or static
I think you'd only want to do this if the member data was pretty large (e.g., an object of another pretty hefty class), and you have some external routine which only works on references to objects of that class. You don't want to copy the member object, so this lets you pass it around.
A realworld example of a pointer-to-member could be a more narrow aliasing constructor for std::shared_ptr:
template <typename T>
template <typename U>
shared_ptr<T>::shared_ptr(const shared_ptr<U>, T U::*member);
What that constructor would be good for
assume you have a struct foo:
struct foo {
int ival;
float fval;
};
If you have given a shared_ptr to a foo, you could then retrieve shared_ptr's to its members ival or fval using that constructor:
auto foo_shared = std::make_shared<foo>();
auto ival_shared = std::shared_ptr<int>(foo_shared, &foo::ival);
This would be useful if want to pass the pointer foo_shared->ival to some function which expects a shared_ptr
https://en.cppreference.com/w/cpp/memory/shared_ptr/shared_ptr
Pointer to members are C++'s type safe equivalent for C's offsetof(), which is defined in stddef.h: Both return the information, where a certain field is located within a class or struct. While offsetof() may be used with certain simple enough classes also in C++, it fails miserably for the general case, especially with virtual base classes. So pointer to members were added to the standard. They also provide easier syntax to reference an actual field:
struct C { int a; int b; } c;
int C::* intptr = &C::a; // or &C::b, depending on the field wanted
c.*intptr += 1;
is much easier than:
struct C { int a; int b; } c;
int intoffset = offsetof(struct C, a);
* (int *) (((char *) (void *) &c) + intoffset) += 1;
As to why one wants to use offsetof() (or pointer to members), there are good answers elsewhere on stackoverflow. One example is here: How does the C offsetof macro work?
I have a class that includes several members of type double.
Suppose I need to make a function that re-orders a vector of class objects based on the values of one of the members on the class. So:
class myClass{
...
public:
double x, y, z;
...
}
void SpecialSort_x(std::vector<myClass>& vec) {
// re-order stuff according to values of vec[i].x
...
}
But now, I want to be able to do the same re-ordering, but according to values of the other members of the class (y and z in the code above).
Instead of making two more functions that are identical to the first one, except with all references to x changed to y or z, I would like to make a single polymorphic function that can re-order the vector according to any of the members of myClass.
What is the best way to do this?
You can use std::sort, combined with a lambda and a pointer to member thus:
#include <vector>
#include <algorithm>
class MyClass
{
public:
double x, y, z;
};
typedef double MyClass::* Field;
void specialSort(std::vector<MyClass>& vec, Field field)
{
std::sort(vec.begin(), vec.end(), [field](const MyClass & a, const MyClass & b) -> bool
{
return a.*field < b.*field;
});
}
int main()
{
std::vector<MyClass> vec;
Field member = &MyClass::x;
specialSort(vec, member);
return 0;
}
And you could also templatise the sort using:
template<class T>
void specialSort(std::vector<T>& vec, double T::* field)
{
std::sort(vec.begin(), vec.end(), [field](const T& a, const T& b) -> bool
{
return a.*field < b.*field;
});
}
I agree with everyone suggesting alternate approaches given the problem description here.
However, if you ever really have the need to access a class member chosen at runtime, you can use a pointer-to-member type. There is usually a more elegant way to accomplish the effect you want, though.
For example:
#include <iostream>
#include <vector>
struct X {
double a;
double b;
double c;
};
void operate_on_member(const X& x, double X::*pm)
{
std::cout << x.*pm << '\n';
}
int main()
{
std::vector<X> xs {
{ 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9 }
};
for (const auto& x : xs)
operate_on_member(x, &X::a);
for (const auto& x : xs)
operate_on_member(x, &X::b);
for (const auto& x : xs)
operate_on_member(x, &X::c);
}
This question already has answers here:
Iterating over a struct in C++
(8 answers)
Closed 1 year ago.
Is it possible in C++ to iterate through a Struct or Class to find all of its members? For example, if I have struct a, and class b:
struct a
{
int a;
int b;
int c;
}
class b
{
public:
int a;
int b;
private:
int c;
}
Would it be possible to loop them to say get a print statement saying "Struct a has int named a, b, c" or "Class b has int named a, b, c"
There are a couple of ways to do this, but you need to use some macros to either define or adapt the struct.
You can use the REFLECTABLE macro given in this answer to define the struct like this:
struct A
{
REFLECTABLE
(
(int) a,
(int) b,
(int) c
)
};
And then you can iterate over the fields and print each value like this:
struct print_visitor
{
template<class FieldData>
void operator()(FieldData f)
{
std::cout << f.name() << "=" << f.get() << std::endl;
}
};
template<class T>
void print_fields(T & x)
{
visit_each(x, print_visitor());
}
A x;
print_fields(x);
Another way is to adapt the struct as a fusion sequence (see the documentation). Here's an example:
struct A
{
int a;
int b;
int c;
};
BOOST_FUSION_ADAPT_STRUCT
(
A,
(int, a)
(int, b)
(int, c)
)
Then you can print the fields as well using this:
struct print_visitor
{
template<class Index, class C>
void operator()(Index, C & c)
{
std::cout << boost::fusion::extension::struct_member_name<C, Index::value>::call()
<< "="
<< boost:::fusion::at<Index>(c)
<< std::endl;
}
};
template<class C>
void print_fields(C & c)
{
typedef boost::mpl::range_c<int,0, boost::fusion::result_of::size<C>::type::value> range;
boost::mpl::for_each<range>(boost::bind<void>(print_visitor(), boost::ref(c), _1));
}
No, it's not possible, because there is no reflection in C++.
If you have members of the same type (as you do in your first specific example) that you want to both (a) have names, and (b) be iterable, then you can combine an array with an enum:
enum names { alice, bob, carl };
struct myStruct;
{
std::array<int, 3> members;
}
Then you can both
myStruct instance;
// iterate through them...
for (auto elem : instance.members)
{
// work with each element in sequence
}
// and call them by name, taking away the need to remember which element is the first, etc.
instance.members[bob] = 100;
Clearly not a general solution, but I've found this useful in my own work.
Provided your member variables are of the same type, you can do something like this that i stole from the GLM library:
class Point
{
Point();// you must re-implement the default constructor if you need one
union
{
struct
{
double x;
double y;
double z;
};
std::array<double, 3> components;
};
};
Admittedly this isn't the most elegant solution from a maintainability standpoint, manually keeping count of the number of variables you have is asking for trouble. However It will work without additional libraries or macros and is applicable in most situations that you'd want this behaviour.
Unions don't support automatically generated default constructors so you'll need to write one that tells the object how to initialise the union.
for (double component : point.components)
{
// do something
}
Assuming that all class members are of the same type, you may employ a C++17 feature called structured binding. Assuming that all members are public this would work:
struct SI
{
int x;
int y;
int z;
};
struct SD
{
double x;
double y;
double z;
};
template<typename T>
void print(const T &val)
{
const auto& [a, b, c] = val;
for (auto elem : {a, b, c})
{
std::cout << elem << " ";
}
std::cout << std::endl;
}
This would work with any struct that has exactly 3 public elements of the same (copyable) type. In case of non-public members the function must be a friend or a member. This approach however cannot be easily extented to arbitrary number of elements.
This is an improved version of QCTDevs answer:
class MyClass
{
union
{
struct Memberstruct
{
double test0;
double test1;
double test2;
} m;
array<double, sizeof( Memberstruct ) / sizeof( double )> memberarray;
};
bool test() { &(m.test1) == &(memberarray[1]); }
};
The requirement is still to have all the same datatypes, and you also need to implement the default Constructor, if needed.
It is improved in that you don't need to manually maintain the size of the array.
A drawback is an altered syntax in comparison to the class without this workaround.