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I new to programming. In stack overflow i couldn't see difference between : & :: is mentioned. Could anyone can explain in detail it helps to beginner learners like me. Thank you.
So you would use :: when you're defining/using methods from a class, so like for example
class foo{
public:
int bar;
int hi(int x);
int func(); // static member function
Foo(int num): bar(num) {}; // use of a colon, initialization list
};
int foo::hi(int x){
//define the function
}
Also if you have static member functions, you can just call those whenever through using foo::func(). You can find more about static member functions online.
The single colon is for member initialization list (you can look this topic up online) where you can initialization member variables in the construction of your class.
You can also find single colon used in polymorphism, when you derive a class from a base class. You can find more information about c++ polymorphism online.
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I'm trying to do a bit of refactoring and I am curious about how would you approach this problem.
Basically I'm trying to create an initialization function for each class. There are classes that inherit from some others, and i would like to use parent initialization function if possible. How would you address this?
I would like to use these structs with memcpy and maybe using also them with the keywords align and __attribute__((packed)); and they must be usable with extern "C". I would exclude then constructors and destructors.
An example to explain:
struct A
{
int a;
};
void initialize(A& a)
{
a = 0;
}
struct B : A
{
int b;
};
void initialize(B& b)
{
initialize(b); // here I want void initialize(A& a), not recursion
b = 0;
};
Maybe I have to do some kind of cast? Ideally I'm looking a solution that does not create overhead.
Use a static_cast.
In your code, the initialize(b) call will recurse infinitely, because b is better matched as B& than as A& (the argument of the function you want to call), thus the overload resolution picks the same function and recurs.
You specified that you want to initialise the A part of the b object. Why not tell that to the compiler? Tell it that you want to call initialise in it as though it was an A, like so:
initialize(static_cast<A&>(b));
As for your concern that you mentioned in the comment - no copies are being made here. If I used static_cast<A>, however, a temporary object would be created, but that's not the case. I am not casting b to an object of a type A. I am casting it to a reference of a type A, which will result in creation of temporary reference. Since A& matches with A& better than with B&, the first function will be chosen, thus avoiding the recursion.
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i need to get clear about, the best way to call the super class constructor/method explicitly.
I tried with the following both way to call superclass constructor:
Myclass::Myclass(int a, int b):X(a),Y(b)
{
// do something
}
and
Myclass::Myclass(int a, int b)
{
X = a;
Y = b;
}
So my question over here is:
Which is the best way to call super class constructor/method explicitly?
And what are the benefits will get in both way?
what is the best practice and why?
There is any performance issue lies with both way?
Regarding my question i found this link:
What are the rules for calling the superclass constructor? but still i have little more doubt what i asked above.
if there is any online tutorial, blog or video also u can mention over here, it will great help full for me. Thank in advance.....
The only correct way to call superclass's constructor is from the initialization list:
Myclass::Myclass(int a, int b)
:X(a),Y(b)
{}
The other way in fact calls different constructors:
Myclass::Myclass(int a, int b)
// implicit :X(),Y()
{
// These two don't call constructors but actually declare variables
X(a);
Y(b);
}
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I'm creating class that has a templated object (Item<T>) as a member, basically like this:
class myClass
{
int other_int;
public:
int member_function();
vector<Item<T>> vec;
};
Currently, I have Item<string>, but I need to be able to use it with non string objects. Is there a way to do this without templating myClass (which would obviously be a lot of work for a complicated class)?
If your class will only use Item< string>, you may try:
class myClass
{
int other_int;
public:
int member_function();
vector<Item<string>> vec;
};
But if you want any other type of Item in the vector, the answer is No, there is no magic solutions.
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In a method of an object of type class A, I handle an object of class B that has a public method .join(*A).
I want my object of type A calling this someObjectOfTypeB.join(*A) method, to use a pointer to itself as the parameter.
void A::someMethod()
{
B b();
b.join(I want to a to use a pointer to itself as a parameter);
}
A a();
a.someMethod();
Upon further investigation, this was not the problem as I led myself to believe; and is indeed the correct way of doing what I wanted to do.
Try using this:
void A::someMethod()
{
B b;
b.join(this);
}
As #AndrewLazarus and #JonathanWakely commented, use B b; instead of B b(). The later declares a function b without parameters which returns B, and that is not what you want.
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I am working on OOPs and using C++. I have one class accessing object of other class/struct.
struct data
{
int a;
int b;
string str;
} sd;
class format
{
int x;
void show()
{
cout << data.a << endl;
}
};
which one is best to use here class or struct?
First of all, it's struct, not strut.
Second, you cannot access member a like you do, data.a, but rather sd.a, because you need to access it on an instance, not on the name of the struct.
For the detailed differences between class and struct see this SO question and its two best rated answers.
I use this convention:
A struct only have members that it make sense to manipulate directly
A class may have complicated rules for assigning members
This somewhat fits well with the default accessibility rules. But as said before in this thread, the choice depends on convention.
that depends on your requirement the only difference in struct and class is in struct all members are public by default and private in case of class