Fold Expressions (C++17)

In a Nutshell

Fold a variadic template's pack of arguments into a single expression with a given operator, sparing you the trouble of hand-writing recursive termination conditions.

Header File

No header required (language feature)

Core API Cheat Sheet

Operation	Signature	Description
Unary right fold	(pack op ...)	Expands to E1 op (... op (EN-1 op EN))
Unary left fold	(... op pack)	Expands to (((E1 op E2) op ...) op EN)
Binary right fold	(pack op ... op init)	Right fold with an initial value
Binary left fold	(init op ... op pack)	Left fold with an initial value
Empty-pack fold (&&)	(... && args)	Result is true when the pack is empty
Empty-pack fold (||)	(... || args)	Result is false when the pack is empty
Empty-pack fold (,)	(expr, ...)	Result is void() when the pack is empty

op can be any of the 32 binary operators: + - * / % ^ & | = < > << >> += -= *= /= %= ^= &= |= <<= >>= == != <= >= && || , .* ->*

Minimal Example

#include <iostream>
// Standard: C++17

template<typename... Args>
void print(Args&&... args) {
    (std::cout << ... << args) << '\n';
}

template<typename... Args>
bool all(Args... args) {
    return (... && args);
}

int main() {
    print(1, " + ", 2, " = ", 3);
    std::cout << all(true, true, false) << '\n';
}

Embedded Applicability: Medium

• Pure compile-time computation (such as condition checks in static_assert) has zero runtime overhead, making it highly suitable
• Replaces recursive template instantiation, which can reduce compile-time memory usage and compile time
• Avoid complex fold expressions on frequently called hot paths, to prevent code bloat from increasing Flash usage
• When a comma fold expands multiple statements, confirm that the cost of each statement stays within acceptable bounds

Compiler Support

GCC	Clang	MSVC
6.0	3.6	19.1

See Also

• cppreference: Fold expressions

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