Problems with default value of pointer to pointer in constructor - c++

Question

When we are using a "double" pointer to class, what are we writing in the constructor with arguments? Are we using one pointer for the allocated memory ?
Here is the code. It doesn't compile and I don't understand why. Thanks for the help.
#include <iostream>
#include <cmath>
#include <cstring>
using namespace std;
class Article{
private:
char title[100];
char author[50];
char *content;
bool publish;
public:
Article(char *title="", char *author="", char *content="", bool publish=0){
strcpy(this->title, title);
strcpy(this->author, author);
this->content=new char[strlen(content)+1];
strcpy(this->content, content);
this->publish=publish;
}
void show(){
cout<<title<<endl;
cout<<author<<endl;
cout<<content<endl;
}
~Article(){
delete [] content;
}
};
class Newspaper{
private:
char name[100];
Article **p;
int articles;
Article first;
public:
Newspaper(char *name="", Article **p=Article(), int articles=0, Article first=Article()){
strcpy(this->name, name);
}
};
int main() {
char title[100], author[50], content[100];
int n;
cin >> n;
char name[100];
cin.getline(name, 100);
cin.getline(name, 100);
Article first("VAZNO","OOP","Vezba:OOP",true);
Newspaper v(name,first);
Article **s = new Article*[n];
for(int i = 0; i < n; ++i) {
cin.getline(title, 100);
cin.getline(author, 50);
cin.getline(content, 100);
v.addArticle(Article(title, author, content, true)); //se koristi copy konstruktor
}
v.showFirst();
v.showLongest();
cout << v.totalofauthor(author) << endl;
for(int i = 0; i < n; ++i) {
delete s[i];
}
delete [] s;
return 0;
}

Answer 1

Solution to your question:
From your code it seems that a Newspaper uses a pointer to a pointer (what you call a double pointer) for keeping track of the Articles:
Newspaper(char *name, Article **p , // This would be ok
int articles = 0, Article first = Article())
but that you've trouble to define a default argument for it:
Newspaper(char *name="", Article **p=Article(), // oops doesn't compile
int articles = 0, Article first = Article())
The reason is that a "double" pointer is not the same as the object itself. If you want a default argument here, you must provide a double pointer as well.
This code does exactly this:
Newspaper(char *name = "", Article **p = new Article*, // this is ok !
int articles = 0, Article first = Article()){
So this is the solution to your question.
But what's your problem ?
But what's the purpose or providing a dummy pointer, pointing to nowhere as default argument ?
Later you try to create a Newspaper based on an article again:
Newspaper v(name, first); // won't work, because first is not a pointer either
So the problem is not the constructor, but the whole principle. It seems that you really want to create a Newspaper based on an Article, and that you use defaults to cover the case where someone wants to create a Newspaper without Article.
Apparently your design also foresees that Articles may be added dynamically:
v.addArticle(Article(title, author, content, true));
Finally it seems that you really have a problem with pointers: when in the constructor you write, you didn't initialise the poitner to a memory region large enough to hold the name:
Newspaper(char *name = "", ...) {
strcpy(this->name, name); // Ouch !! pointer name is not initalized !
}
So once you'll get your code compiled, your programme won't work ! As this->name is a pointner that is never initialized, your strcpy() will cause memory corruption and undefined behaviour (crash!?).
Recommendations
First, get a book or a tutorial to understand and master pointers. If you do'nt you'll quickly be completely lost in your C++ class.
In the meantime, get rid of your char* and strcpy() and alike, and use std::string instead.
Last, consider the use of std::vector to manage the dynamic Article container. A dynamic array of poitners implemented with Article** would need extra logic, such as maintaining the size, reallocation of memory once the number of article grows, not speaking of the ownership (allocation/deallocation) of the Articles you put in the array.
Based on these recommendations, your Newspaper would look like this:
class Newspaper{
private:
string name;
vector<Article> p; // p.size() will provide the number of articles
public:
Newspaper(string name = "") : name(name) { } // empty newspaper
Newspaper(string name, Article& a) : Newspaper(name) { // combo
addArticle(a);
}
void addArticle(Article &a) {
p.push_back(a); // add and article to the vector
}
... // rest of the code, here
};
And here a little more