C fold expressions example
Cfold expressions is a feature introduced in C 17 to simplify recursive operations in variadic parameter templates. 1. Left fold (args ...) sum from left to right, such as sum(1,2,3,4,5) returns 15; 2. Logical and (args && ...) determine whether all parameters are true, and empty packets return true; 3. Use (std::cout
Cfold expressions is a feature introduced by C 17, which is mainly used to concisely perform recursive operations on parameter packages in variadic templates. It is often used in scenarios such as summing, logical judgment, printing, etc., and combined with ... and operators, it can greatly simplify the code.
The following is a few typical examples to illustrate the usage of fold expressions.
✅ 1.Summarize the parameter package (left fold)
#include <iostream>
template<typename... Args>
auto sum(Args... args) {
return (args ...); // Left fold: (((ab) c) ...)
}
int main() {
std::cout << sum(1, 2, 3, 4, 5) << std::endl; // Output 15
return 0;
}Here
(args ...)will add all parameters from left to right.
✅ 2. Check that all boolean values are true (and judge)
template<typename... Args>
bool all_true(Args... args) {
return (args && ...); // Left fold, equivalent to: args1 && args2 && ...
}
int main() {
std::cout << std::boolalpha;
std::cout << all_true(true, true, false) << std::endl; // Output false
std::cout << all_true(true, true, true) << std::endl; // Output true
return 0;
}If the parameter package is empty,
&&collapse will returntrue(becausetrueis the unit element of logical AND).
✅ 3. Print all parameters (using comma operator)
#include <iostream>
template<typename... Args>
void print(Args... args) {
(std::cout << ... << args) << '\n'; // Right fold: args1 << (args2 << (args3 << cout))
}
int main() {
print("Hello", " ", "world", "!", 2024); // Output: Hello world!2024
return 0;
}Use commas
,operators to achieve output one by one. Note that here is right fold(... << args)or(args << ...), but here we use(std::cout << ... << args), which is the binary fold that the package expands in the middle of the operator .
✅ 4. Supports folding of default values (with initial value)
You can also provide an initial value to the folder expression:
template<typename... Args>
auto sum_with_init(Args... args) {
return (args ... 10); // Add 10 at the end, which is equivalent to: (((ab) c) ... 10)
}
int main() {
std::cout << sum_with_init(1, 2, 3) << std::endl; // 1 2 3 10 = 16
return 0;
}Note:
(args ... 10)is right folded, equivalent toargs1 (args2 (args3 10)), but the addition satisfies the law of binding, and the result is the same.
✅ 5. Processing of empty parameter packages
template<typename... Args>
bool all(Args... args) {
return (args && ...); // Return true when empty parameter package
}
template<typename... Args>
bool any(Args... args) {
return (args || ...); // Return false when empty parameter package
}
int main() {
std::cout << all() << std::endl; // true
std::cout << any() << std::endl; // false
return 0;
}
&&Fold empty package →true||Fold empty package →falseCollapse of empty packages will fail to compile (no default value)
? Summary: The syntax form of folded expressions
| type | grammar | illustrate |
|---|---|---|
| One dollar left fold | (pack op ...)
|
For example (args ...)
|
| One dollar right fold | (... op pack)
|
For example (... args)
|
| Binary folding | (pack op ... op init) or (init op ... op pack)
|
With initial value |
Basically these common uses. fold expressions make variadic template programming concise and intuitive, avoiding the complex writing of recursive templates. Not complicated but it is easy to ignore details, such as empty package behavior and combination direction.
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