Range-based Loops

As mentioned earlier, range-based loops were added in C++11. A range loop will visit each element in a collection, setting the range variable to each value in the collection, in turn. These loops are very similar to the simplified for loop added in Java 5, and the for in loops in Python.

Let's look at the three variations of range-based loops which are offered in C++. We'll start with value iteration, which is the closest to the version used in Java.

Here's a short example, which prints each character in a string on a line by itself:

string snake{"Ouroboros"};
for (char c : snake)
{
    cout << c << endl;
}

On each loop cycle, the variable c is initialized with a copy of the next character in the string snake. When all of the characters have been processed, the loop stops. Thus, you can read this loop as saying "for each character in snake, do something".

With value iteration, changes to the variable c have no effect on the string snake. If you want to change the string itself, then use reference iteration. Here's a second example which does that:

string str{"one two three"};
for (char& c : str)
{
    if (c == ' ') { c == '_'; }
}
cout << str << endl;

Finally, if the items you are iterating over are very large, and you want to make sure you don't change them, then you'd use const-ref iteration like this (made up) example.

Album photos(get_phone_photos());
for (const Image& img : photos)
{
    if (is_cute_cat(img))
    {
        display(img);
    }
}

Because each picture in your photos library might be very large, you wouldn't want to copy them with value iteration. Similarly, since you wouldn't want the CuteCats app to have the ability to modify (or perhaps erase) your cute cat photos, the loop should access each variable as a const Image&.