Fold Expressions
Get to know the types of fold expressions added in C++17 and their benefits in this lesson.
We'll cover the following...
Implementing fold expressions
A fold expression is an expression involving a parameter pack that folds (or reduces) the elements of the parameter pack over a binary operator. To understand how this works, we’ll look at several examples. Earlier in this section, we implemented a variable function template called sum that returned the sum of all its supplied arguments.
For convenience, we’ll show it again here:
template<typename T>T sum(T a){return a;}template<typename T, typename... Args>T sum(T a, Args... args){return a + sum(args...);}
With fold expressions, this implementation that requires two overloads can be reduced to the following form:
template<typename... T>int sum(T... args){return (... + args);}
There’s no need for overloaded functions anymore. The expression (... + args) represents the fold expression, which upon evaluation becomes ((((arg0 + arg1) + arg2) + ... ) + argN). The enclosing parentheses are part of the fold expression. We can use this new implementation, just as we would use the initial one, as follows:
int main() {std::cout << sum(1) << '\n';std::cout << sum(1,2) << '\n';std::cout << sum(1,2,3,4,5) << '\n';}
Type of folds
There are four different types of folds, which are listed as follows:
Types of Folds
Fold | Syntax | Expansion |
Unary right fold |
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Unary left fold |
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Binary right fold |
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Binary left fold |
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