I've encountered this code example and I remembered I've seen it before and I didn't know what it is for and what it does? I've searched on the internet but without luck.
Code:
class C
{
int x; // a non-static variable, implicitly private
public:
C() : x(0) {} // default constructor
// a static member function, which uses a non-static variable perfectly well
static int Incr(C& instance) { return ++(instance.x); }
} g_c;
int main(void)
{
C c2;
return C::Incr(g_c) + C::Incr(c2);
}
What does g_c after the last class bracket means?
} g_c;
Here g_c is declared to be an object of the class type C.
Such construct enables you to create object(s) of unnamed type as:
class //Nameless class!
{
//data members
}obj1, obj2;
In this example, obj1 and obj2 are declared to be objects of a class type which has not been given any name — the class is nameless! In such situation, you cannot declare objects in a conventional sense (i.e Type obj1, obj2; sense). So this construct helps you do that.
You can even derive from other named classes while being nameless (and declaring the objects of the nameless class):
class : public A, public B //Nameless class is deriving from A and B
{
//data members
}obj1, obj2;
In short, this construct ensures that the user wouldn't be able to create more objects than intended, unless some evil programmer uses/misuses/abuses C++11 (or template) as:
decltype(obj1) obj3; //hehe!
Hope that helps!
It's shorthand for:
class C
{
....
};
C g_c;
You declare variables using the format type variable_name;. For example:
A x;
Where A may be the name of a class.
But instead of using a pre-existing class type, you can also define the class at the same time as you declare a variable of the new class's type:
class { ... } x;
or define the class and give it a name:
class A { ... } x;
In C++ it is common to just define the class and give it a name, but leave off the variable:
class A { ... };
but you don't have to leave off the variable.
That's just a way of creating objects of that type of Class. Structs mostly use them to initialize new variables.
Related
I've encountered this code example and I remembered I've seen it before and I didn't know what it is for and what it does? I've searched on the internet but without luck.
Code:
class C
{
int x; // a non-static variable, implicitly private
public:
C() : x(0) {} // default constructor
// a static member function, which uses a non-static variable perfectly well
static int Incr(C& instance) { return ++(instance.x); }
} g_c;
int main(void)
{
C c2;
return C::Incr(g_c) + C::Incr(c2);
}
What does g_c after the last class bracket means?
} g_c;
Here g_c is declared to be an object of the class type C.
Such construct enables you to create object(s) of unnamed type as:
class //Nameless class!
{
//data members
}obj1, obj2;
In this example, obj1 and obj2 are declared to be objects of a class type which has not been given any name — the class is nameless! In such situation, you cannot declare objects in a conventional sense (i.e Type obj1, obj2; sense). So this construct helps you do that.
You can even derive from other named classes while being nameless (and declaring the objects of the nameless class):
class : public A, public B //Nameless class is deriving from A and B
{
//data members
}obj1, obj2;
In short, this construct ensures that the user wouldn't be able to create more objects than intended, unless some evil programmer uses/misuses/abuses C++11 (or template) as:
decltype(obj1) obj3; //hehe!
Hope that helps!
It's shorthand for:
class C
{
....
};
C g_c;
You declare variables using the format type variable_name;. For example:
A x;
Where A may be the name of a class.
But instead of using a pre-existing class type, you can also define the class at the same time as you declare a variable of the new class's type:
class { ... } x;
or define the class and give it a name:
class A { ... } x;
In C++ it is common to just define the class and give it a name, but leave off the variable:
class A { ... };
but you don't have to leave off the variable.
That's just a way of creating objects of that type of Class. Structs mostly use them to initialize new variables.
class A
{
public:
A a;
int b;
CString c;
}
a type A inside A class
So if i want to define a Type of class name in the same class, what should i do .
Thanks
What about using a smart pointer to the object A, after you have defined the A object in the main program?
I don't think you can make a recursive definition: it would end up with an infinite loop on calling the class' constructor.
Well, just new in classes and trying to create a simple program, that has 2 classes. Class A and Class B. Well I'm trying to initialize my variable in constructor in class A, and then make some action and pass it to class B, where I'm also can make some action. So class A is a base class. However, when I compile the program I got two mistake -
error: ‘i’ was not declared in this scope
For class A and class B. So I have two question 1) why constructor doesn't declare variables (according to the books constructor called first)? 2) what ways I can transfer the variable from class A to use in class B?
#include <iostream>
#include <cstdio>
using namespace std;
class A {
public:
A(){
int i =1;
}
~A(){}
int foo () {
int p = i+1;
i++;
return p;
}
};
class B : public A {
public:
int showme() {
return i;
}
};
int main() {
A j;
B k;
cout<< k.showme()<<endl;
cout<< j.foo()<<endl;
return 0;
}
First off, get a good book:
The Definitive C++ Book Guide and List
and familiarize yourself with the basics. Your question indicates there is a complete mess right now in your understanding of C++.
Now on to the actual Q's.
1) C-tor does not declare a class member variable, it can only declare a local variable since a c-tor is also a function. You need to declare the member variable explicitly like so:
class A {
int i;
public:
A(int _i): i(_i) { }
};
2) The term "transfer" is incorrect per se. In this case, the two classes are in an inheritance hierarchy, so the variable i is inherited by class B and if you declare it as protected, then it will be accessible in the way you do it. The function B::showme() is defined correctly. Fix the first part and the example is going to work.
3) Do not confuse classes and objects. When there is an object of class A declared, it is in no way related to another object of class A or B. j and k share their own private instances of int i (provided that you fix (a)) and if you want to have k's instance of i to be equal to that of j, you can e.g. implement a copy constructor or a pair of getter/setter functions.
Suppose I have class B which gets a value 'v' in the constructor from another class A. How can I read this value from class C?
Class C will be instantiated on demand, but A has created B and passed the 'v' already. 'v' will change in every instantiation. I have tried to make 'v' static in Class B. Would it work? I could not implement it properly.
Class A {
public:
int* v;
B b1;
A(int* var) : v(var), b1(var) {};
}
How to access the same version of 'v' from a C class?
I can define B and C however I like in order to achieve the goal. But I cannot change A for that purpose.
You need a (public) static member
class A { //let's stick with your naming convention!
public:
static int a;
}
A::a = 4;
However allowing people to change A::a means that your program will probably end up relying on a global unencapsulated state... which is usually a sign of a bad design.
If you member was const however you are really relating a constant to your class, which is not so bad.
class A {
public:
static const int a = 4;
}
std::cout << "A:a is always " << A::a << std::endl;
EDIT BASED ON UPDATED QUESTION
If you require help with building a class I would recommend that you use a factory of some kind. If I understand your requirement you want to be able to inject a value into every class A and class B instance. This value "v" is based on a Class C.
So...
class C {
private:
// up to you where you implement the getUniqueNumberForNow function
// (global free function for example)
static const int v = getUniqueNumberForNow();
public:
static A createA(){
return A(v);
}
static B createB(){
return B(v);
}
}
The getUniqueNumberForNow() function just gets whatever your value should be. It will be then stored in class C and can be used during the creation of A an\or B. Now just make A and B's CTORs private and make C a friend of both and you will have only one way to create an A or B, and it will always use the correct value.
See NeilMonday's link in the comments below for info on friend classes.
Last thing is if you want to have the value change for every instantiation of A you can just do this in the factory:
static A createA(){
return A(getUniqueNumberForNow());
}
However if that is really what you want then just do this:
class A {
public:
A() : val (getUniqueNumberForNow()), b(B(val)){}
}
You cannot access a 'v' which has never passed to the class. Instead you can make a static copy of it as a member in your class A. It will update every time A is instantiated like this:
Class A {
public:
int* v;
static int* staticv;
...// Constructor etc
}
in your .cc code of A:
int* A::staticv;
...
A::staticv=this->v;
now any class can access to this value by:
A::staticv;
Is the this pointer ever required? I guess you'd need it if you were functionally passing around the instance of the class pointed to by this. But in terms of setting/retrieving/calling/whatever members, is this always optional?
I've tagged this C++ because that's the language I'm specifically wondering about, but if someone can confirm that the construct is the same for Java and other OO languages that use a this pointer, it'd be appreciated.
There's three cases I can think of:
When you simply want to pass a pointer to the current class:
class B;
struct A {
B* parent_;
A(B* parent) : parent_(parent) {}
};
struct B {
A* a;
B() : a(new A(this)) {}
};
In a constructor or member function, where a member is shadowed by an argument:
struct A {
int a;
void set_a(int a) { this->a = a; }
};
Here, the member variable "a" is shadowed by the argument "a", so this-> is used to access the member instead of the argument.
(example above edited to be a function rather than constructor, since you wouldn't normally assign this way in a constructor)
When accessing a public/protected member variable or function in a base class of a template class
template <class T>
struct A {
int a;
};
template <class T>
struct B : public A<T> {
int f() { return this->a; }
}
here, a alone would not be a dependent name, so the compiler would expect to find a declaration of it in B or a base of B that does not depend on T. Adding this-> makes the lookup dependent on the type of this, and since this is a dependent type, the lookup of a is deferred until f() is instantiated.
One could write return A::a instead of return this->a, but in the presence of multiple bases, whether direct or indirect, using this-> is more flexible. This alternative syntax is also limited to member variables and non-virtual functions - if it is used with a virtual function, it the function will be called directly instead of doing virtual function call.
You need it when you have a local variable that has the exact same name as the member variable. In this case, the local variable is said to shadow the member variable. To get to the member variable in this situation, you must use this.
Some people consider it good practice to explicitly mention that the variable you are modifying is a member variable by using this all the time, but this is not always the case.
sometimes "this" is required, maybe when you are passing your object to another function.
look at this c# code( for opening a modal form with this parent)
Form1 f = new Form();
f.ShowDialog(this);