I'm trying to learn C and figuring out how to access a private member variable that is an array of objects. My objective is trying to print out the data that's in the array of objects. Suppose that my header looks like this.

using namespace std;

const unsigned MAX_RESULTS = 10;

class StudentRecords{
public:
  StudentRecords();

  //bunch of other getters and setters here

  Result GetResults() const; //my lame attempt at trying to access the obj-array private var

private:
  Result results[MAX_RESULTS]; // array of Result of MAX_RESULTS number of elements
  // other private variables
};

ostream& operator <<( ostream& os, const StudentRecords& R);

In the above, there is supposed to be a private array of Result objects called results, which has a size of MAX_RESULTS, which is supposed to be 10 here. Now, using my overloaded operator << the idea is to print the contents of Result to 'file' so as to speak. Since it's an array, I want to print out all the results in the array using a for loop.

Result StudentRecords::GetResults() const
{
    return results[MAX_RESULTS];
}

ostream & operator <<( ostream & os, const StudentRecords& R )
{
  for(unsigned i = 0; i < SomeNumber; i  )
  {
      os << R.GetResults()[i] << '\n'; //this won't work of course see error
  }
  return os;
}

There will be an error stating:

error: no match for 'operator[]' (operand types are 'Result' and 'unsigned int')|

I already overloaded the << operator in my Result class in order to print out the values in that class. The problem is that I don't know how to iterate through the results array. From what I've googled I understand that you can use some kind of pointer function for example here: C : Setters and Getters for Arrays

When I try to write the function like this:

Result* GetResults() const;

I will get an error stating:

error: cannot convert 'const Result' to 'Result*' in return|

Leaving out the * allows the code to compile, but predictably I get a bunch of garbage values from my array. So assuming that my class has an array of objects, and those objects have their own variables, how do I print out the values from my array of objects? I appreciate the help.

CodePudding user response：

Result StudentRecords::GetResults() const
{
    return results[MAX_RESULTS];
}

This is a common newbie misunderstanding. Because you declared the array as results[MAX_RESULTS] you think that results[MAX_RESULTS] somehow means the whole array. But it doesn't, when you are using an array [] is used to access individual elements of the array. And of course there is no element at index MAX_RESULTS, that's past the end of the array.

Bottom line is that the syntax for declarations and the syntax for expressions is not the same.

You could do something like this

const Result* StudentRecords::GetResults() const
{
    return results;
}

This returns a pointer to the array, not the array itself (which is actually impossible to do in C ). But that should be close enough for now.

CodePudding user response：

As pointed out by various people, there are various things to consider when tackling this issue.

1. The getter for the object array can take an index number (good old int i) to target the element inside the array. e.g. R.GetResult(i)
2. A declaration is not an expression! In my getter I used the expression "return results[MAX_RESULTS];" where MAX_RESULTS is a constant number... but this ends up meaning to return the array at the MAX_RESULTS-th element, clearly not intended.
3. One thing for the future that I am going to try is to create a pointer function to the array member variable.
4. It was suggested to quickly learn about vectors, since the declaration of "Result results[MAX_RESULTS];" is, to put it in the colorful words of a commentator, "an unholy mix of C and C that makes things more complicated than they need be further down the road."