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
2
votes
1 answer
How to use fold expression to instantiate each type in a variadic template function?
I am trying to create an "object manager" that will create and store instances of a template class. The class takes one template argument and inherits from a base class to allow instances with different template args to be stored in the same…
yah_nosh
- 155
- 1
- 8
2
votes
1 answer
Does the operand to a fold expression need to be parenthesized?
For the following program:
#include
auto f(auto ...args)
{
(std::cout << 1 << ... << args);
}
int main()
{
f(0, 0, 0);
}
gcc prints 1000, but clang gives an error:
error: expression not permitted as operand of fold expression
…
cigien
- 57,834
- 11
- 73
- 112
2
votes
3 answers
C++17 fold syntax to test vector composition
Standard C++17 implementation of vector any, all:
template
bool contains(const C& c, const T& value) {
return std::find(c.begin(), c.end(), value) != c.end();
}
template
bool any(const C& c, T&&... value) {
…
Charles Pehlivanian
- 2,083
- 17
- 25
2
votes
2 answers
c++ fold expression with user defined class
I'd like make a log function with multiple parameters, and I also want to call the log function with my class.
Here is my code (compiler : Visual studio 2019 or x86-64 gcc9.2)
Question 1> I cannot understand log function.
Is it possible to use fold…
Eric Kim
- 55
- 5
2
votes
1 answer
Fold expressions and parameter pack: difference betwen Args&& and Args inside static_assert
This code is from
https://en.cppreference.com/w/cpp/language/fold
template
void push_back_vec(std::vector& v, Args&&... args){
static_assert((std::is_constructible_v && ...));
…
Nick
- 9,962
- 4
- 42
- 80
2
votes
2 answers
Fold expression with strings and their size
I have C++14 code like this.
I am updating to C++17. Is there a way this to be rewritten as fold expression?
namespace cat_impl_{
inline size_t catSize(std::string_view const first) {
return first.size();
}
template
Nick
- 9,962
- 4
- 42
- 80
2
votes
4 answers
Fold expressions and cout
I have the following code that works, but I am confused about how it works.
template
void print(Args&&... args) {
(std::cout << ... << std::forward(args)) << '\n';
}
int main()
{
print(1,2.0,"3");
}
output:
123
My…
NoSenseEtAl
- 28,205
- 28
- 128
- 277
2
votes
1 answer
fold expression in assignment
I am trying to use a fold expression to simplify some code. In following code, I am trying to insert elements into an array, but the fold expression does not compile
struct test {
std::string cmd[20];
test() {
int i = 0;
auto insert =…
unknown.prince
- 710
- 6
- 19
2
votes
1 answer
How to detect availability of a parameter of given type in variadic funtion arguments list and act after all parameters have been processed
I have the following operator<<() overload for my class with C++17 folds:
template
ostream& operator <<(Args&&... args)
{
//Currently:
return (m_osCout << ... << args);
//What I need:
IF ANY OF THE parameters in…
mami
- 109
- 1
- 7
2
votes
2 answers
Limit allowed COM Interfaces by using template function with limited variadic parameters (std::is_same)
I have wrapper classes contaning COM pointers (or smart pointers) to different interfaces.
INTEND:
Some COM classes can be obtained from various other COM interfaces, and I want to make a template constructor with variadic types, which would allow…
Sergey Kolesnik
- 3,009
- 1
- 8
- 28
2
votes
2 answers
c++17 fold expression print function that can support 2,0000 element efficient
Hello how can make it print it more efficient with large element like 20,000 element ?
i took very long to compile ,
i try to follow this
static const array of values in Variadic Template C++
but i can't make it work with fold expression, any…
user3770234
- 55
- 6
2
votes
1 answer
Create recursive function argument list from one value and a size
My intent is to create a function argument list of size n so I can pass it to a helper that uses a fold expression to recursively multiply the values together.
I'm a little stuck on how to make the argument list to pass to the helper.
Is there a way…
Casey
- 10,297
- 11
- 59
- 88
2
votes
0 answers
Are there guarantees on compile complexity of parameter pack expansion using fold expressions
I can imagine a straightforward efficient implementation of fold expression expansion when all types of parameter pack are the same (simplified c++11 example):
#include
#include
template
struct bool_pack {…
W.F.
- 13,888
- 2
- 34
- 81
1
vote
1 answer
Uninitialized variable using IIFE and fold expression in C++
I was playing around with IIFE's and parameter packs in MSVC C++20 when I stumbled across a strange error. The following code is a minimal reproducible example.
#include
#include
template
struct Foo {
…
BubLblckZ
- 434
- 4
- 14
1
vote
1 answer
Accessing indiviual alias inside another alias that is a parameter pack with a fold expression
Given this types:
template
struct base_unit {
using ratio = r;
using symbol = s;
};
template
struct derived_unit {
using units = std::tuple;
};
If, for example, I have a type…
Alex Vergara
- 1,766
- 1
- 10
- 29