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
0
votes
0 answers
Compile error: function parameter 'args' with unknown value cannot be used in a constant expression
I have this piece of code:
template
struct Base {
constexpr std::uint8_t getValue() noexcept {
return static_cast(this)->getValueImpl();
}
};
class Derived : public Base {
friend struct Base;
…
in_text
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- 3
0
votes
1 answer
template function specialization using variadic arguments
I have a class that takes a variable number of arguments (including no arguments) but when I try to pass a struct as argument for its constructor, I get a compile time error:
error: converting to 'const ioPin' from initializer list would use…
Juan_David
- 126
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0
votes
3 answers
C++ 17 Expand a Fold Expression (as pairs of values)
I am writing some code to experiment with fold expressions.
I don't think my approach to solving this problem is the best possible approach.
I am trying to write a function which "does something" with pairs of values obtained from expanding a…
FreelanceConsultant
- 13,167
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2 answers
C++17 can variadic template class define new type and variadic data member?
#include
template
struct Clazz {
void doSomething(const std::shared_ptr& p0,
const std::shared_ptr& p1) {}
T0 t0;
T1 t1;
};
template
xishvai
- 3
- 2
0
votes
2 answers
Why does the fold expression not apply to for loop?
Below is from an authoritative C++ proposal:
template
constexpr void tuple::swap(tuple& other)
{
for...(constexpr size_t N : view::iota(0uz, sizeof...(TYPES)))
{
swap(get(*this), get(other));
…
xmllmx
- 39,765
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0
votes
1 answer
C++20: generate type from given variadic types
Lets say, I have a variadic type list which can grow
e.g.
#define MY_TYPES void, float, int, vector, .....
I am looking for way to generate these type definition at compile time in c++17/20 in generic way, e.g.
#define to_tuple(MY_TYPES)…
Joe Smith
- 3
- 1
0
votes
1 answer
Lisp like C++ function calling with std::tuple and fold expressions
The most basic thing I can think about Lisp is how to call functions. To those who doesn't know how to do that in Lisp, is something like that:
(fun1 a b (fun2 c d))
; c and d are parameters to the function fun2,
; and a, b and the result of fun2…
Gustavo Chierici
- 3
- 1
0
votes
1 answer
non default constructible parameters within lambdas in fold expressions
I'm trying to see if it's possible to use a parameter that is not default constructible within a lambda as part of a fold expression. Following code works if T is default constructible.
template
struct X
{
void foo()
{
…
user3882729
- 1,339
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- 11
0
votes
1 answer
Create template pack from set of traits
Is it possible (and if so, how) to generate a template pack from a indexed set of type traits so they can be used to instantiate a variant or a tuple?
#include
template
struct IntToType;
template<>
struct IntToType<0>
{
using…
user877329
- 6,717
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0
votes
1 answer
Initializer list, parameter pack expansion, fold expressions and order of evaluation
I have the following code to address a n-dimensional tensor class (offset is a std::vector of std::size_t):
template
double Tensor::at(int first, Ts... others) {
int i = 0;
std::size_t index =…
ptrchv
- 55
- 5
0
votes
1 answer
Variadic template function rvalue parameters silently moved C++ to the function
The following code compiles and prints: move move. I would prefer that it didn't compile since merge takes rvalue references and I don't move t1 and t2 to it.
class A {
public:
A() = default;
A(const A& other) { std::cout << "copy "; };
…
Rudolf Lovrenčić
- 147
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- 9
0
votes
1 answer
Using SFINAE to ensure the Variadic Parameter Pack Arguments are Derived of a Specific Type
I currently have a class hierachy with the following structure:
#include
-
#include
Francis Cugler
- 7,788
- 2
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- 59
0
votes
1 answer
C++ how to iterate through variadic template types and add them to tuple?
This one is rather complex, so I haven't been able to solve it myself.
Here's the relevant code, I'll explain more in depth after.
#include
#include
#include
#include
#include
struct Prop
{
…
Wesley Barlow
- 11
- 1
0
votes
1 answer
c++ macro expansion(__VA_ARGS__ item name and value)
Is there any trick to achieve the function of the pseudo-code below?
THX.
template
void fake_fold((T1 t1, T2 t2)...) {
(std::make_pair(t1, t2)), ...; // std::pair, T1 type is always…
breaker00
- 167
- 7
0
votes
0 answers
Expression Template Operators with Variadic Templates
Using variadic templates we can write a function add, which takes any number of input parameters, and then returns a single Expression (to be lazily evaluated). When called for a specific index i, this expression sums all elements at i for all the…
Phil-ZXX
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