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
11
votes
3 answers
Is it possible to insert extra operation in fold expression?
In C++17, fold expression is available, so to print arguments, we could use
#define EOL '\n'
template
void output_argus(Args&&... args)
{
(cout << ... << args) << EOL;
}
int main()
{
output_argus(1, "test",…
r0n9
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11
votes
2 answers
Checking if variadic template parameters are unique using fold expressions
Given a variadic template parameter pack, I want to check if all types given to it are unique using an inline constexpr bool and fold expressions. I trie something like this:
template
inline static constexpr bool is_unique = (... &&…
Joald
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11
votes
1 answer
C++17 fold expression in cout
I am learning the new c++17 fold expression and I saw this code from c++17 fold expression. I would like to know why this code work :
template
void printer(Args&&... args) {
(std::cout << ... << args) << '\n';
}
but not this…
Gabrielle de Grimouard
- 1,917
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10
votes
2 answers
Fold expressions in MSVC
I have the following function that computes the mean value:
template
auto mean_of(const Ts... values)
{
return (... + values) / static_cast(sizeof...(Ts));
}
With VS 2017 15.6.0 Preview 3 the following code
std::cout <<…
Evg
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10
votes
2 answers
On what base fold expression of a parameter pack consisting of a single element is transformed into unparenthesized expression
Consider an example:
#include
template
decltype (auto) foo(Ts... ts) {
return (ts->x + ...);
}
struct X {
int x;
};
int main() {
X x1{1};
…
W.F.
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10
votes
2 answers
Can you use a subexpression within fold expressions?
Is the following a legal fold expression?
template
bool in_range(std::index_sequence) {
return ((Ix < N) && ...);
}
It compiles with clang but not gcc
Ryan Burn
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10
votes
1 answer
Why doesn't a left fold expression invert the output of a right fold expression?
I'm taking a look at C++17 fold expressions and I'm wondering why does the following program outputs
4 5 6
4 5 6
for both of the for_each calls
template
void for_each1(F fun, T&&... args)
{
(fun…
Marco A.
- 43,032
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9
votes
3 answers
Expand parameter pack with index using a fold expression
I've got a template function taking a parameter pack. I want to expand it into calls to a second function while also supplying the index of the item in the pack. I probably can work out how to do it with recursion but I would like to try do it…
Joe
- 5,394
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9
votes
3 answers
How to implement the generalized form of std::same_as (i.e. for more than two type parameters) that is agnostic to parameter order?
Background
We know that the concept std::same_as is agnostic to order (in other words, symmetric): std::same_as is equivalent to std::same_as (related question). In this question, I would like to implement something more general:…
ph3rin
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9
votes
1 answer
How to Insert spaces in (cout << ... << args) using fold expressions?
Given
template
void print(Types&& ...args) {
(cout << ... << args);
}
// ....
print(1, 2, 3, 4); // prints 1234
How to add spaces so we get 1 2 3 4?
Update:
Correct answer:
((std::cout << args << ' ') , ...);
Amin Roosta
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9
votes
3 answers
Retrieve value out of "cascading ifs" fold expression
Let's assume that I want to create my own lambda-based switch with the following syntax:
auto s = make_switch(std::pair{0, []{ return 0; }},
std::pair{1, []{ return 50; }},
std::pair{2, []{ return 100;…
Vittorio Romeo
- 90,666
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9
votes
1 answer
Order of Evaluation for Fold Expressions
Fold expressions seem to be a nice way to apply a function to each element of a tuple. However, if the applied function has side effects, the order of function invocations might be an important concern.
Consider:
#include…
ComicSansMS
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9
votes
1 answer
Are fold expressions a C++14 or a C++1z feature?
https://isocpp.org/std/the-standard states that the working draft N4296
contains the C++14 standard plus minor editorial changes.
I noted that N4296 §5.1.3 contains a specification of "Fold expressions". I thought this to be a feature which did…
jotik
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8
votes
1 answer
Why is this C++ fold expression valid?
On cppreference, I saw that there are four types of fold expressions, unary right, unary left, binary right, and binary left. What is the type of this fold expression here? I'm having a hard time understanding why it is valid.
template
happy_sisyphus
- 1,693
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8
votes
1 answer
Create a string with delimeters from fold expression
I've got the following function:
template
void Foo(Args && ... args)
{
std::stringstream stream;
(stream << ... << args);
// Do somethign with it
}
It can perfectly concat params into a string, but as expected it gives…
original.roland
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