I am not sure if I can describe the problem, but I will try my best. Here is the situation:
If I can want a class1 has a pointer as a member variable pointing to another class, class2. Meanwhile, I want class2 also has a pointer as a member variable pointing to class1. Is that possible?
class Class1
{
private:
Class2* classptr;
... ...
public:
... ...
};
class Class2
{
private:
Class1* classptr;
... ...
public:
... ...
};
It appears to me that none of Class1 and Class2 has been recognized as an identifier. I guess none of Class1 and Class2 is created. Correct me if I am wrong.
You need a forward declaration. Either:
class Class1
{
private:
class Class2* classptr;
... ...
public:
... ...
};
or:
class Class2;
class Class1
{
private:
Class2* classptr;
... ...
public:
... ...
};
Related
I have a struct obj in my base class. I don't know how to access the data members of the struct through Derv1 class (Derived from the base class). I tried making both Base and Derv1 a friend of struct - it still tells me ' data member is private' (its private in Base only).
example :
struct A{
public :
int total;
//some stuff
};
class MyClass{ // [Base] class
private:
A var1;
};
class Derv1{
private:
//some stuff
public void display_var1(Derv1 x){
return x.var1.total;
} // trying to return the value of A.total
};
I hope this makes sense so that you can help me out ..
Thank You kindly,
First, you have to make sure that Derv derives from MyClass.
class Derv1 : public MyClass { ... };
and then, you will need to figure out the best way to display the variable.
My suggestion:
Create a virtual member function in the base class.
Override the function in the derived class.
Make sure to call the base class implementation in the derived class implementatin.
class MyClass { // [Base] class
public:
virtual void display() const
{
// Display var1 anyway you wish to.
}
private:
A var1;
};
class Derv1 : public MyClass {
public:
virtual void display() const
{
// Call the base class implementation first
MyClass::display():
// Display anything else that corresponds to this class
}
private:
//some stuff
};
i think you must extend your Derv1 class into the Base class:
class Derv1: public MyClass{
to inherit the members of the base class
I have some code like that:
class Class1 {
// some code here
Class2 inverse() {
// some code here
}
};
class Class2 : public Class1 {
// some code here
};
And I got an error that Class2 is unknown in function inverse. Can I declare that class and how?
You can forward declare Class2 before Class1. And then you'll have to separate your inverse function declaration from definition. So you should define its after Class2 is defined:
class Class2;
class Class1 {
// some code here
Class2 inverse();
};
class Class2: public Class1 {
// some code here
};
Class2 Class1::inverse()
{
return Class2();
}
But, honestly, it is bad design.
Class2 is not defined when you reach Class2 inverse() so even forward declaring would not completely solve that issue. However you can do two things. Either return a pointer or a reference to Class2
class Class2;
^^^^^^^^^^^^
class Class1 {
Class2& inverse() {
// ^^^ or possibly Class2*
//
}
};
class Class2: public Class1 {
}
Or defined inverse after Class2 have been completely defined
class Class2;
class Class1 {
Class2 inverse();
};
class Class2: public Class1 {
//
}
Class2 Class1::inverse() {
//
}
I would go with the second option if possible.
I have a base class and the derived class. I need to access the protected member of the base class in the derived class. However, Eclipse does not allow me to access the data member as if it were a member of a derived class without caring that it was inherited. How do I do that?
class BaseClass {
protected:
static int a;
int b;
}
class DerivedClass: public BaseClass {
void SomeMethod {
a=10; // cannot resolve symbol
b=10; // cannot resolve symbol
BaseClass::a=10; //does not complain
BaseClass::b=10; //does not complain
}
}
I couldn't completely understand your question, but fixing the syntax errors, the following should work:
class BaseClass {
protected:
static int a;
int b;
}; // <-- Missing semicolon
int BaseClass::a = 0; // Define static member
class DerivedClass: public BaseClass {
void SomeMethod() { // <-- Missing ()
a=10;
b=10;
}
};// <-- Missing semicolon
For C++ learning purposes, I have the files class1.h, class1.cpp, class2.h and class2.cpp. I would like to instantiate an object named class1Obj inside class2. Where and how do I instantiate this object? Do I instantiate classObj inside the class2 constructor?
In the past I have created a pointer to a class, which worked well for that time, but I think a pointer is not the route I should take this time because the classObj will only be used inside class2.
class class1
{
//...
};
class class2
{
class1 member;
//...
};
In class2 ctor, you can initialize member in the constructor initialization list.
class2::class2(...)
: member(...)
{
//...
}
Well how did you create a pointer in the past? Presumably, you did something like this:
class class2
{
public:
class2()
{
class1Pointer = new class1();
}
// Destructor, copy constructor/assignment, etc...
private:
class1* class1Pointer;
};
Now you want to do exactly the same but this time you don't want a pointer to class1, you want a class1 itself:
class class2
{
public:
class2() {}
// Destructor, copy constructor/assignment, etc...
private:
class1 class1Obj;
};
The object will be default initialized when your class2 object is created. If your class1 constructor should take some arguments, use an initialization list:
class class2
{
public:
class2() : class1Obj(1, 2, 3) {}
// Destructor, copy constructor/assignment, etc...
private:
class1 class1Obj;
};
Instantiate a class inside a class :
#include <iostream>
using namespace std;
class Foo
{
public:
Foo(int i)
{
}
};
class Bar
{
Foo i; //<--- instantiate a class inside a class ----
public:
Bar() : i(1) //<--- instantiate a class inside a class ----
{
}
};
int main(void)
{
Bar b;
cout<<" \nPress any key to continue\n";
cin.ignore();
cin.get();
return 0;
}
It depends on your Class1. If its constructor accepts some parameters, then you must initialize it explicitly in Class2 constructor or in initialization list.
Class2 {
public:
class2() {
//Here m_class1Obj will be instantiated
m_class1Obj = Class1(/*some params*/);
}
private:
Class1 m_class1Obj;
};
Or
Class2 {
public:
class2() : m_class1Obj() {}
private:
Class1 m_class1Obj;
};
Ok, I have a class:
class class1
{
public:
class2 object2;
int a;
};
where:
class class2
{
public:
void function2();
};
Basically, I need function2 in object2 to be able to access "a." How would I go about doing this? Thanks.
class class2
{
public:
void Function2(class1& c1)
{
c1.a;
}
}
simple.