Learn the Basics of C++ Fold Expressions with this Comprehensive Tutorial

C++ Fold Expressions 101

C++ introduced fold expressions in version 17. They can be used to simplify code and make it more readable. In this article, we'll explain what they are and how they can be used.

What is a Fold Expression?

A fold expression is a way of combining elements of a parameter pack into a single expression. For example, if you have three integer parameters, you could use a fold expression to add them all together:

int sum = (a + b + c);
    

Fold expressions can also be used to apply an operation to all the elements in a parameter pack, for example by using the following shorthand notation:

int sum = (... + listOfInts);
    

The above code will add all the elements of the listOfInts parameter pack.

Types of Fold Expressions

There are three types of fold expressions:

• Left folds start with the first element in the pack and iterate towards the last element, applying the operation from left-to-right.
• Right folds start with the last element in the pack and iterate towards the first element, applying the operation from right-to-left.
• Unary folds are special cases where the parameter pack has only one element.

Using Fold Expressions

Fold expressions aren't limited to just addition. You can use them with any operator, including comparison operators like greater-than or less-than, as well as logical operators like && (AND) or || (OR). For example, here's a fold expression that checks if all the elements in a parameter pack are greater than 10:

bool result = (... > 10);
    

Note that the order of the elements in the parameter pack matters. If you want to make sure that all the elements are greater than 10, you would use a left fold since it iterates from left to right. Similarly if you want to check if any of the elements are greater than 10, you would use a right fold since it iterates from right to left.

Conclusion

Fold expressions are a powerful tool that can help make your code more succinct and readable. They can save you time, and give you confidence that your program will work correctly.