Resizing a dynamic array of objects in c++ - arrays

Question

Hi I am working on a project where I access info from a file and then put it into an array of objects then manipulate the data in the object from a options from a menu. The problem I am currently having is that one of the options in the menu is to add a new element to the object. The project states that I must use an array of objects so I can't just use a vector the class that I'm putting the array into to resize it uses a temporary dynamic array for the object then deleting the original array.
Here's what the class looks like
class Info
{
private:
string name;
double money;
public:
Info(){
name="";
money=0;
}
void Setname(string n){
name=n;
}
void Setmoney(double m){
money=m;
}
string GetName()const{
return name;
}
double GetMoney()const{
return money;
}
};
now that was just a sample of the class the actual class has equations with it to alter the money variable but for the purpose of this question this is all that is needed. Now here is the function where I am having the problem
void Addinfo(Info in [], int & size){
string newname;
double newmoney;
cout<<"What name are you going to use?"<<endl;
cin>>newname;
cout<<"Now How much money do you have currently"<<endl;
cin>>newmoney;
Info *temp= new Info[size+1];
for(int index=0; index<size;index++){
temp[index].Setname(in[index].GetName());
temp[index].Setmoney(in[index].GetMoney());
}
delete []in;
temp[size].Setname(newname);
temp[size].Setmoney(newmoney);
in=temp;
size=size+1;
}
Now when I run the program everything runs fine until I try using this function in which the data in the arrays gets corrupts. Am I supposed to make the in Info variable a new dynamic array that can hold the can hold all the info then use another for loop to put the variables into the new dynamic array or I am supposed to do something else. Also remember that I must use arrays for this. Also when deleting a dynamic array am I supposed to make the former array equal to zero after deleting or is that something else?

Answer 1

When you have a function with a type valueName[] array parameter then you just pass the address of that array parameter to the function. The calling function has the ownership of that array. Besides the function signature you always have to consider a contract between the caller and the called function that defines the ownership of the data passed by pointer.
Your function AddInfo gets an array passed by pointer and the calling function expects that the data is available after the function call. So the function violates the contract when you delete []in.
Your function uses a parameter in as (local) variable when you assign a new value with in=temp;. That's legal. But you can't expect that the changed local variable has any effect to the caller. With the current function signature is possible to call the function in this way:
Info infos[5];
Addinfo(&info[3], 2);
Obviously it makes no sense to modify &info[3]. When your contract shall allow adding some data to the array you need a signature that allows changing a pointer. One example would be:
void Addinfo(Info*& in, int& size, string newname, double newmoney)
{
Info *temp= new Info[size+1];
for(int index=0; index<size;index++){
temp[index].Setname(in[index].GetName());
temp[index].Setmoney(in[index].GetMoney());
}
temp[size].Setname(newname);
temp[size].Setmoney(newmoney);
delete []in;
in = temp;
size=size+1;
}
void Addinfo(Info*& in, int& size)
{
string newname;
double newmoney;
// input data
cout<<"What name are you going to use?"<<endl;
cin>>newname;
cout<<"Now How much money do you have currently"<<endl;
cin>>newmoney;
// TODO: data validation.
// add data to array
Addinfo(in, size, newname, newmoney);
}
I have factored out the change of the array from the input. This allows a more simple testing of that function.