C++ Notes to Self: Ranged Based for Loop

‘The range based for loop is added in C++ 11 standard and is a more compact form of its traditional equivalent.’ [tutorialspoint.com]

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Range based for loop allows for the iteration over arrays and containers without the need for an index variable. It can be used on any object that behaves like a range type such as C++ containers or initializer list.

A few subtleties about the range-based for loop:

• Instead of declaring the type of the indexer, we can declare it as auto.
• On each iteration, a copy is created and given to the indexer. To avoid this overhead, we declare the indexer variable as a reference.
• To enforce read-only semantics within the loop, declare the indexer as a const.

What are the differences between C-style and range-based for loops?

• The for-loop uses index-based iteration. The range-based loop does not.
• The for-loop requires a condition that will terminate the loop. The range-based loop supplies its own end condition.
• We need to manually increment or decrement the index variable in the for-loop. We don’t need to do this in the range-based loop, thereby avoiding any mistakes.
• Because we manually adjust the index variable, the for-loop gives finer control over the iteration. With range-based loop, the iteration doesn’t deviate at all.

Underneath the hood, the range-based loop relies on std::begin() and std::end() that are defined for all C++ containers, arrays and initializer lists. (See code below.) A user-defined class that behaves as a range type and works with iterators needs to implement the features these C++ containers have.

#include <iostream>

using namespace std;

int main(int argc, char **argv) {
    int arr[]{1,2,3,4};

    for(int x: arr) {
        cout << x <<  " ";
    }
    cout << endl;

    for(const int &y: arr) { // To avoid copy semantics use reference variable
        cout << y << " ";
    }
    cout << endl;

    for(const auto &z: { 111, 222, 333, 444}) { 
        cout << z << " ";
    }
    cout << endl;

    int mlist[] {10, 20, 30, 40};
    const int *mstart = std::begin(mlist);
    const int *mend   = std::end(mlist);
    while(mstart != mend) {
        cout << *mstart++ << " ";
    }
    cout << endl;

    return 0;
}