fold expressions, since C++17, are used to reduce the parameter packs of variadic templates over a binary operator.
Questions tagged [fold-expression]
144 questions
8
votes
2 answers
How to use if constexpr in template fold expressions?
I would like to write a sum function with variable number of argument with the condition that it should ignore argument that are not std::is_arithmetic
I have figured out a recursive version that works
auto old_sum(){
return…
motam79
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8
votes
1 answer
Unpacking variadic tuples in c++17
Is there anything better in c++17 (maybe C++2a) than the classic C++14 way to unpack variadic tuple with std::index_sequence?
Anything better than this:
template
class MultiIterator
{
public:
MultiIterator(I const& ...i)
…
PiotrNycz
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8
votes
2 answers
Expanding parameter pack into lambda with fold expression - gcc vs clang
Considering the following code snippet:
template
void post(TF){ }
template
struct funcs : TFs...
{
funcs(TFs... fs) : TFs{fs}... { }
void call()
{
(post([&]{ static_cast(*this)(); }),…
Vittorio Romeo
- 90,666
- 33
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- 416
7
votes
2 answers
Fold expression for a parameter pack with comma operator: How to add additional parameters when expanding the pack?
I want to design a compile-time-string class CTString that can e.g. be constructed from a parameter pack of string literals. This works using a comma-fold-expression (for this toy example, I tried to avoid the use of any system headers to make it…
eigenleif
- 95
- 6
7
votes
3 answers
What is a good alternative to this C++17 fold expression in C++14?
Here is a nice, succinct fold expression based lambda in C++17:
#include
using ::std::uint64_t;
constexpr auto sumsquares = [](auto... n) { return ((n * n) + ...); };
// I want this to work.
uint64_t foo(uint64_t x, uint64_t y, uint64_t…
Omnifarious
- 54,333
- 19
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7
votes
3 answers
generating calls to lambdas with arbitrary number of parameters
The following definition has proven to be very useful for me:
template
void apply_on_each_args(Func f, Args... args)
{
(f(args), ...);
}
Basically, the arguments pack folded on comma operator, allows to define several…
lrleon
- 2,610
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7
votes
1 answer
How to make `short-circuit evaluation` also available in `fold expressions`?
#include
#define FORWARD(arg)\
std::forward(arg)
template
constexpr bool AndL(Args&&... args)
{
return (... && FORWARD(args));
}
template
constexpr bool AndR(Args&&... args)
{
…
xmllmx
- 39,765
- 26
- 162
- 323
6
votes
4 answers
How to correctly unfold a pack in order to create N element array
I was expecting this code to create 10 elements vector each constructed as A{1, 2, 3.0}, but in reality the output is 1
#include
#include
struct A {
int a{0};
int b{0};
double c{0.0};
};
template
Dmitry
- 1,065
- 7
- 15
6
votes
2 answers
Copy-assigning to a tuple from an iterator range in C++17
I've been hoping to use new C++17 features like fold expressions and std::apply() to simplify some C++11 code I have that uses tools like std::index_sequence and std::initializer_list for some operations on tuples. One task in particular is giving…
jwimberley
- 1,696
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- 24
6
votes
1 answer
Using fold expression with std::apply on two tuples
I just started learning C++17 fold expressions. I understand that it is possible to apply a fold expression to a tuple, like in the following example (inspired by replies to this question):
#include
#include
int main() {
…
pmjobin
- 813
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- 15
6
votes
5 answers
Print spaces between each element using a fold expression
I am using a fold expression to print elements in a variadic pack, but how do I get a space in between each element?
Currently the output is "1 234", the desired output is "1 2 3 4"
template >
struct…
Lim Ta Sheng
- 371
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6
votes
3 answers
C++ unary right fold vs unary left fold with comma operator
Why comma separated unary left fold produce the same result as the right one?
Live code
template
void right_fold(Args... args){
((std::cout << args),...);
}
template
void left_fold(Args... args){
…
tower120
- 5,007
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- 88
6
votes
1 answer
Howto use a fold expression with a specific type?
I've a function to check if a std::string contains a substring. I pass the strings around as std::string_view, such that no copy takes place.
bool containsSubstr(std::string_view str, std::string_view substr)
{
return str.find(substr) !=…
dani
- 3,677
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6
votes
2 answers
C++17 Limit type of fold expression for initialization of a template-class
I basically try to write my own game engine for practice and personal use (I know, it's a nearly impossible task, but as I said, it's mostly for learning new things).
Currently, I'm working on my math library (mainly vectors and matrices) and I came…
Plebshot
- 210
- 1
- 9
6
votes
1 answer
Implementing operator less for arrays using fold expressions
I am playing around with fold expression in c++17 with the latest clang++. I tried to implement the less operator for an array using this which I want to use for fixed size strings.
Here is where I got to. Is there a better way to do this,…
zrb
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