C++17 is another big milestone in the history of C++, it comes with a lot of new features. In C++17, the fold expressions feature is a powerful feature that allows us to fold a parameter pack over a binary operator. Folding Expressions are very useful with variadic templates, and this feature makes template arguments more readable and concise. There are 4 different types of usage and in this post we will give syntax and simple examples about each of them.
Table of Contents
What are the fold expressions that comes in C++ 17 features?
The fold expressions are established after the C++17 standard, they are used to fold (reduce) a parameter pack (fold_pack) over a binary operator (fold_op). The opening and closing parentheses are required in a fold expression.
In a folding expression there are 4 parameters,
fold_packis an expression that has parameter pack and no operatorfold_opis a binary operator, one of the + – * / % ^ & | ~ = < > << >> += -= *= /= %= ^= &= |= <<= >>= == != <= >= && || , .* ->* operatorsfold_initis an initial expression at the beginning or at the end...is an ellipses symbol that used for arguments
There are 4 different syntax in usage, now let’s see them in examples,
Is there a simple example about unary right fold expression?
Unary right fold expression syntax (since C++17),
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( fold_pack fold_op ... ) |
here is an unary right fold expression example,
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template <typename ... Args> bool URF(Args ... args) { return (args && ...); // Unary Right Fold } |
Is there a simple example about unary left fold expression?
Unary left fold expression syntax (since C++17).
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( ... fold_op fold_pack ) |
here is an unary left fold expression example,
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template <typename ... Args> bool ULF(Args ... args) { return (... && args); // Unary Left Fold } |
Is there a simple example about binary right fold expression?
Binary right fold expression syntax (since C++17).
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( fold_pack fold_op ... fold_op fold_init ) |
here is a binary right fold expression example,
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template<typename... Args> int BRF(Args&&... args) { return (args + ... + 100); // Binary right fold addition } |
Is there a simple example about binary left fold expression?
Binary left fold expression syntax (since C++17).
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( fold_init fold_op ... fold_op fold_pack ) |
here is a binary left fold expression example,
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template<typename... Args> int BLF(Args&&... args) { return (1 * ... * args); // Binary left fold multiplication } |
Is there a full example about fold expressions in C++ 17?
Here is a full example about fold expressions in C++17 that has 4 different types in usage.
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#include <iostream> template <typename ... Args> bool URF(Args ... args) { return (args && ...); // Unary Right Fold } template <typename ... Args> bool ULF(Args ... args) { return (... && args); // Unary Left Fold } template<typename... Args> int BRF(Args&&... args) { return (args + ...+ 100); // Binary Right Fold Addition } template<typename... Args> int BLF(Args&&... args) { return (1 *...* args); // Binary left Fold Multiplication } int main() { bool urf = URF(true, true, true, false); std::cout << "Unary Right Fold: " << std::boolalpha << urf << std::endl; bool ulf = ULF(true, true, true, false); std::cout << "Unary Left Fold: " << std::boolalpha << ulf << std::endl; auto brf = BRF(1, 2, 3, 4, 5); std::cout << "Binary Right Fold: " << brf << std::endl; auto blf = BLF(1, 2, 3, 4, 5); std::cout << "Binary Left Fold: " << blf << std::endl; system("pause"); return 0; } |
and the output will be as follows:
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Unary Right Fold: false Unary Left Fold: false Binary Right Fold: 115 Binary Left Fold: 120 |
For more details, please see this https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4295.html
and this https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0036r0.pdf
If you want to know more about about Variadic Templates, here are examples,
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