Welcome to Programming and Data Structures

You may have noticed that the previous page described table.at(0) as returning a reference to the first row of the table. The actual prototype for .at() looks something like:

T& vector<T>::at(size_t index);

Recall that T is a placeholder for the type of data in the vector. If it is a vector of int, the return type would be int&. This means the function does not return a new integer, it returns a reference to an integer that already exists. Returning a reference from ordinary functions doesn’t usually make sense. All the local variables in a function are destroyed when the function returns. So if we return a reference to one of those local variables, it will be a reference to a variable that no longer exists. This is not the case with member functions for things like a vector or a string. The contents of the vector or string will continue to exist after at() has done its work.

Returning the reference is what allows the caller to use .at() to modify data. But we need to be careful to use the reference and not make a copy of the data it refers to. For example, if we do this:

int x = table.at(0).at(1);
x = 5; // no change to table

We are copying the value found by table.at(0).at(1) into x. Modifying x would have no effect on the table. To store the result of table.at(0).at(1) and use it as a reference we would need to store the result into an int&. Doing so allows us to use that variable as an alias for the element in the table:

int x = table.at(0).at(1);
x = 5; // no change to table
int& row0col1 = table.at(0).at(1);
row0col1 = 3; // changes table

If we wanted to store a reference to an entire row, it would be as a reference to a vector of integers:

vector<int>& row0 = table.at(0);
cout << row0.size();  // prints 5

To avoid having to worry about making reference variables, we will favor always specifying the row or element we want to talk about, even if it means saying something like table.at(0).at(1) multiple times in a row. But if you choose to try to make a variable that stores “the current row” or “the element at 0, 1”, make sure to store it as a reference unless your goal is to copy the data.