Question

I am experiencing unexpected performance degradation when using vectors with classes.

I have a csv_File class that reads in a csv file and stores the contents in a 2D vector. There’s a member function that allows access, e.g.

    csv_File file("file.csv");
    file.access(2,2);

To access the 2,2 element.

Then, I have another class csv_Array that stores multiple csv_File objects in a vector, e.g private member vector
There’s a member function that allows access, i.e. it returns a csv_File object, for example:

    csv_Array file_array(5); //store 5 csv_File objects
    file_array.grab(0).access(2,2); 

In the second line, grab returns a csv_File object (in this case, the first one) and access is a member function of the csv_File object.

However, I have noticed that the call
csv_Array.grab(0).access(2,2);
is much slower than it should be (it should be just 3 vector::at calls).

Is there some sort of hidden copy going on that is making this very slow?

EDIT: Here are some of the relevant function prototypes:

    //Access in csv_File
    std::string access(int row, int column);

    //grab in csv_Array and csv_Analysis (mentioned below)
    csv_File grab(int index);

ADDITIONAL DETAILS (if necessary):
The application of this code is to load a bunch of csv files into memory before passing it to a friend class that will do some calculations with the data. Schematically, I have the following:

1) csv_Array has private member vector storage;
2) csv_Analysis is a class that is a friend of csv_Array
3) csv_Analysis accesses vector storage, which is in csv_Array
4) This access is done by passing csv_Analysis a reference to storage in csv_Array (so no copy hopefully….), e.g
public:
csv_Analysis(csv_Array &csv_block);

Thus, the call given above[file_array.grab(0).access(2,2); ]
actually has one additional class “level” in between and is more like

    csv_Analysis analysis_Object(file_array); 
    analysis_Object.grab(0).access(2,2); 

where grab acts in the same way, and is also defined as a member function of csv_Analysis class.

Answer 1

You probably want to make grab return a reference rather than a copy:

csv_File& grab(int index);