Fold Expressions (C++17)

One-Liner

Folds a parameter pack from a variadic template into a single expression using a specified operator, eliminating the need to manually write recursive base cases.

Header

None required (language feature)

Core API Quick Reference

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 supports 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

• Compile-time pure computations (such as condition checks in static_assert) have zero runtime overhead, making them highly suitable
• Replacing recursive template instantiation can reduce compile-time memory usage and compilation time
• Avoid using complex fold expressions in frequently called hot paths to prevent code bloat from increasing Flash usage
• When using comma folds to expand multiple statements, we must confirm that the overhead of each statement is within an acceptable range

Compiler Support

GCC	Clang	MSVC
6.0	3.6	19.1

See Also

• cppreference: Fold expressions

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