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
3
votes
2 answers
lambda fold expression get index
Is there a simple way to add the current index of the arg
As in include it in the lambda call so I don't need to use index++
template
void Function(Args... args) {
std::size_t size = sizeof...(Args);
std::size_t index =…
docasok733
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- 6
3
votes
1 answer
How to expand into initializer list with fold expressions?
I would like to insert as many zeros into a vector as there are arguments to a variadic templated function (i.e. number of arguments zeros into a vector). I am trying to use fold expressions to achieve this and it works when using…
j00hi
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3
votes
1 answer
Apply function to tuple of vectors to get tuple of elements (variadic templates)
I have a container class which takes a list of vector or array objects (and stores a tuple of references to the original list of objects). Now, when calling the container via operator()(size_t idx) I want the function to return a tuple that contains…
Phil-ZXX
- 2,359
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3
votes
3 answers
Fold expression vs compile recursion
In c++17 we have fold expression which can greatly simplify code that could otherwise be implemented using compiler-recursion and SFINAE or overloading.
For example, in the following code
#include
#include
template
francesco
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3
votes
1 answer
Fold expression within assertion compiles on some machines, but not on others
I have the following code that is (for demonstration purposes) supposed to assert that all arguments evaluate to true using C++17 fold expressions.
#include
template
void fn(Ts... ts)
{
assert(ts && ...);
}
int…
Post Self
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3
votes
0 answers
declaration of void variable shadows template parameter in fold expression
Is the following g++'s behaviour a bug?
#include
#include
template< std::size_t ...i >
constexpr
bool f(int k, std::index_sequence< i... >)
{
int j = (std::size_t((void(i), 1)) + ...) * k;
return 0 <…
Tomilov Anatoliy
- 15,657
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3
votes
0 answers
A C++17 parameter pack compiling error
I've got a program from internet like below, to show the ability of "parameter pack" in C++17
#include
#include
#include
#include
#include
#include
template
void…
Troskyvs
- 7,537
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3
votes
1 answer
GCC unpacks (I < ...) fold expression the wrong way
I've opened a bug to GCC, but I'd like to know if I'm right in my expectations.
Consider this proposal and the following fold expression:
(args < ...)
It should be equivalent to:
((args$0 < args$1) < ...) < args$n
Consider the following…
skypjack
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2
votes
1 answer
How to create an array of N floats values with fold expression?
Suppose the following function
template
constexpr std::array make_ones()
{
std::array ret{};
for (size_t k = 0; k != N; ++k)
{
ret[k] = 1.0f;
}
return ret;
}
Is it possible to write that…
user877329
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2
votes
0 answers
Which C++ fold idiom is preferred, short circuit comma or nested lambda?
First option technically has less cognitive load because you don't need to be concerned with || or , operators. It is unfortunate that two nested lambdas are required to first get Is, and then to fold on the inner lambda. See cppreference fold…
okovko
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2
votes
2 answers
Can you achieve fn(x1 ^ fn(x0)) with a fold expression?
Is it possible to achieve the following using a fold expression?
template
auto foo(Args... args)
{
//calling foo(x0, x1, x2) should be exactly equivalent to
//calling fn(x2 ^ fn(x1 ^ fn(x0)))
}
Chris_F
- 4,991
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2
votes
2 answers
use template fold expressions instead of recursive instantions of a function template?
Given a variadic parameter pack, I would like to call a function with each element and its index in the pack.
I have a simple example below which uses recursive instantiations of a function template to achieve this. (on godbolt)
#include…
Steve Lorimer
- 27,059
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2
votes
1 answer
Template with fold expression creates unwanted parameter copy
The following prototype is intended to make synchronized print:
#include
#include
#include
#include
#include // for OutputDebugString
std::mutex sync_mutex;
template
void…
Alex F
- 42,307
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2
votes
2 answers
Short circuiting in fold expressions
This is a self triggered question based on a self-answer I gave here.
This seems a pretty convincing explanation of why short-circuiting of logical operators is available in fold expressions, and of the fact that wrapping a fold expression in a…
Enlico
- 23,259
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2
votes
1 answer
Conditionally remove function calls in fold expression
I know that I can use SFINAE to disable generation of templated functions based on a condition, but that doesn't really work in this case. I want to initialize an array at compile-time that should contain values that matches a condition. Something…
Ted Klein Bergman
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