How to make a list of class in C++ - c++

Question

Part of the task I have is to create a class, which contains two lists of two other classes and create "The big 4"(constructor, copy constructor, operator=, destructor)
Here's what I did:
using namespace std;
class A{...};
class B{...};
class C{
list<A> a;
list<B> b;
public:
C();
~C();
C(const C& c);
void operator=(const C& c);
};
C::C(){
//How to allocate memory for a and b?
}
C::~C(){
//How to free the memory?
}
C::C(const C& c){
a=c.a;
b=c.b;
}
void operator=(const C& c){
if(&c==this) return;
// how do I delete a and b?
a=c.a;
b=c.b;
}
Could you clear out the things that I don't understand(as comments in the code). And also give advice if I haven't done anything correctly?

Answer 1

Since you're using values (std::list values), there is nothing to do. Your constructor will call the std::list constructors automatically, which allocate any resources needed. Your destructor will call the std::list destructor, which frees resources which it acquired.
You would need some extra work if you either hold pointers to lists (i.e. std::list<A> *a;) or lists of pointers (std::list<A*> a;') - or both (std::list<A*> *a;).

Answer 2

//How to allocate memory for a and b?
You don't need to allocate memory by yourself. because you are allocating your member variables on stack. which is automatically set up when someone instanciate's your class.
//How to free the memory?
You don't need to free memory by yourself. for the same reason. your stack variables will be automatically deallocated when scop ends.
// how do I delete a and b?
Why do you need to delete a and b ? you are copying the data from other object to this.

Answer 3

if you use std::list, your class A or B should:
1 can be allocate on stack and heap
2 can not be a singleton object.

Answer 4

Your variables are using the auto storage class they do not need to be explicitly deleted.
Your assignment operator
void operator=(const& C rhs);
shouldn't return void. // as this is not what the built in types do and prevents assignment chaining.
e.g.
int x, y, z;
x = y = z = 0;
It looks like your trying to implement move semantics with your comment
// how do I delete a and b?
your assignment operator should copy the state or you should document that it uses non-standard behavior. e.g. move semantics, boost::noncopyable Et cetera.

Answer 5

With the advent of the move-constructor, it is now the Big Six. Study and understand all the ins and outs here, and you will have a masters degree in class boiler plates.
#include <list>
class A {};
class B {};
class C{
std::list<A> a;
std::list<B> b;
public:
typedef std::list<A>::size_type size_type;
explicit C(size_type sz =0);
virtual ~C();
C(const C& c);
// Specialize external swap. Necessary for assignment operator below,
// and for ADL (argument-dependant lookup).
friend void swap(C& first, C& second);
// Assignment-operator. Note that the argument "other" is passed by value.
// This is the copy-and-swap idiom (best practice).
C& operator=(C other); // NOTE WELL. Passed by value
// move-constructor - construct-and-swap idiom (best practice)
C(C&& other);
};
C::C(size_type sz) : a(sz), b(sz) {}
C::~C(){}
C::C(const C& c) :a(c.a), b(c.b){}
void swap(C& first, C& second) {
// enable ADL (best practice)
using std::swap;
swap(first.a, second.a);
swap(first.b, second.b);
}
// Assignment-operator. Note that the argument "other" is passed by value.
// This is the copy-and-swap idiom (best practice).
C& C::operator=(C other) {
swap(*this, other); // Uses specialized swap above.
return *this;
}
// move-constructor - construct-and-swap idiom (best practice)
C::C(C&& other): a(0) , b(0) {
swap(*this, other);
}