218 questions
6
votes
2
answers
168
views
Function templates and class templates with non-type template parameters differ when instantiated with an empty parameter pack of types
I have the following program with a function template and a class template, both of which have a non-type template parameter. If I try to pass an empty parameter pack of types to the class template, ...
- 701
5
votes
2
answers
116
views
How can I make a tuple of types that can be accessed by index without pack indexing?
Using C++26's pack indexing, you can create a simple tuple of types like so:
template <typename... Ts>
struct type_tuple {
template <unsigned Index>
using get = Ts...[Index];
};
...
0
votes
2
answers
81
views
How to pass parameter pack by reference and pointer?
How to pass parameters to a function by reference and through a pointer?
I have an example in which I try to call one of two write functions of the CFoo class. The example compiles if I specify two &...
- 5
3
votes
0
answers
75
views
Why does constrained template function declaration with template parameter pack not compile when constraint has extra template parameter before pack?
I tried to write a generalized version of std::same_as that needs to work with any positive number of type arguments.
I wrote the following concept, keeping the first template parameter separate from ...
- 5,650
5
votes
1
answer
103
views
No concept subsumption with template parameter pack?
In C++20, concept subsumption refers to a concept being a superset of another concept. For example, in the following example, Fooable subsumes BetterFooable:
template <typename T>
concept ...
- 5,650
6
votes
3
answers
205
views
Parameter pack to initialize member std::array
I have a class Color which is a thin wrapper around std::array. I would like to be able to use it in 2 ways:
Color<5> color{1,2,3,4,5} should produce color.values = [1,2,3,4,5]
Color<5> ...
- 580
4
votes
2
answers
84
views
Partial template specialization for when all template parameters are the same type
Is it possible in C++14 (so no constraints, require or fold expressions, but there is SFINAE) to have a partial template specialization of a class template
template <typename ...T>
class Generic ...
- 73
3
votes
0
answers
74
views
Unpack the first N-1 elements of a parameter pack [duplicate]
template <typename... Args>
auto foo(Args&&... args)
{
// N = sizeof...(args)
// need args[0], args[1] ..., args[N-2] as tuple and/or forward them
}
I have C++20 at my ...
- 3,733
1
vote
2
answers
133
views
How to interleave C++ template parameter packs
I have this c++ template function
template <int N, int B, auto... Args>
void somefunc() {
// do work...
}
, which I can call like this:
somefunc<1, true, 2, false>();
I want to ...
- 323
3
votes
1
answer
46
views
How does a Swift parameter pack solve the stated motivational problem?
The Swift evolution parameter packs proposal begins with a Motivation section that cites the following bit of familar ugliness:
func < (lhs: (), rhs: ()) -> Bool
func < <A, B>(lhs: (A, ...
- 534k
0
votes
1
answer
35
views
Use parameter pack to initialize map of templated
For a class that looks like this:
class A
{
public:
template <int X> void foo(double param);
const std::map<int, std::function<void(double)>> foo_map;
};
Where foo_map ...
- 103
1
vote
1
answer
60
views
Const-qualifier lost on variadic template parameter when trying to pretty-print function signature [duplicate]
I wish to use template programming with type traits to pretty-print the signature of a function. While the gist of what I came up with works, the const-qualifier on the function arguments is dropped. ...
1
vote
1
answer
51
views
deduced conflicting types for parameter 'T' (<const int &> vs. <int>)
I have a class Base and MyFunc templates with parameter packs.
I have a class Derived, which specifies a const T& type in the parameter pack.
template <typename... T>
class Base {};
...
- 13
1
vote
2
answers
93
views
How to pairwise iterate over a type list and value list
I've got a typelist like
template <typename ...Types> struct type_list {};
and am looking for a way to iterate over pairs of elements from that type list and some value list from within a ...
- 792
1
vote
1
answer
38
views
expand and fold parameter pack in specific way
#include <string>
#include <string_view>
template<typename... Args>
std::string_view concatenateBuffer(std::string &buffer, Args &&... args){
static_assert((std::...
- 10.5k
0
votes
2
answers
174
views
Packing, unpacking and storing parameter pack in a tuple
I have a function pointer and parameters, and I'd like to save these and potentially modify them and call the function with them.
I've seen parts of this answered however I'm unsure how a complete ...
- 49
3
votes
2
answers
212
views
Swift parameter packs and Sendable
The following code produces a warning about a non-sendable type:
func pack<each U: Sendable>(arg: repeat each U, function: @escaping @Sendable (repeat each U) -> Void) {
Task { @MainActor ...
1
vote
1
answer
87
views
How to print the line number of the caller function from a template function?
I am implementing a log function which logs along with line number of the code. The snippet of the code as follows:
#include <iostream>
using namespace std;
char m_buffer[500];
template<...
- 311
0
votes
0
answers
20
views
Inconsistency between g++ and clang++ on template parameter pack deduction
I found the following minimal example when experimenting metaprogramming:
#include <iostream>
#include <utility>
#include <tuple>
template <size_t ...Is>
void print(const std::...
- 33
0
votes
1
answer
42
views
Creating a std::array of templated functions results based on the types of a tuples elements
I want to create an array where each element is (at runtime) calculated based on the type of each element of a Tuple. In practice I want to assign a runtime id to each type that exists in a tuple by ...
- 17
1
vote
2
answers
89
views
Transforming Parameter pack in template without wrapping struct
I have the following code, which works:
template <typename ...Types>
struct Wrapper
{
template <void (*F)(typename Types::ParameterType...)>
static void toNativeFunction(Stuff ...
1
vote
0
answers
47
views
Proper usage of Comma fold in parameter pack C++
I have a class with a variadic argument templated constructor and I am trying to separate out the parameter pack one by one and pass it to member set function. I tried std::move and std::forward as ...
- 361
0
votes
0
answers
38
views
C++: Prevent method from expanding too early when passed as argument
I attempt to write a basic logger that should pass any message to the shell and a log file (for now, both write to std::out) using std::vformat. For that, i created a wrapper log_message():
template &...
- 154
1
vote
1
answer
485
views
std::print() and template parameter pack
Would be possible to print a template parameter pack using std::print?
something like:
void func(auto... args){
std::print("???", args...);
}
EDIT: if it wasn't clear: "???" ...
- 409
1
vote
0
answers
176
views
Swift store closure with parameter packs
I would like to create a closure type that is always called on the main thread using the new parameter packs introduced in Swift 5.9. E.g.
protocol Repository {
func loadData(completion: ...
- 32
1
vote
0
answers
115
views
Pass a List to a Function to act as Multiple Arguments in C++ [duplicate]
In Python, we can unpack a list to pass the elements of the list as arguments to a function. I am looking for a similar feature in C++.
I have a list of unknown size and a function which has number of ...
- 59
1
vote
1
answer
349
views
Variadic Generics vs. Value and Type Parameter Packs in Swift
There are two evolution proposals:
Variadic Generics and Value and Type Parameter Packs.
So... what is the difference? They feel conceptually very similar.
The second one, with super heavy syntax, is ...
- 1,488
0
votes
1
answer
96
views
correct order in template parameter pack expansion
I want to fill a tuple<Ts...> from a vector of variants. (NB: This question follows up on Creating a tuple from a folding expression return values , see 2nd comment to the answer )
Expanding the ...
- 477
0
votes
0
answers
76
views
ExpressibleBy#Literal with parameter pack "Cannot convert value of type '#ExpressibleType' to expected argument type '#Type'"
I'm trying to create a generic function to apply some image filters.
The applyFilters(_:) function uses Parameter Pack of ImageFilter.
Calling applyFilters(_:) with static functions that take a type ...
- 1
7
votes
1
answer
399
views
std::type_identity to support several variadic argument lists
std::type_identity can be used to provide non-deducible context. So, I wondered if it will work on a deduced variadic list. But different compilers give different results.
https://godbolt.org/z/...
- 3,560
2
votes
2
answers
68
views
How to create a stack ND C-array from param pack of length N?
For the parameter pack size_t ...len, e.g. [len={3,5,2}, I wish to create
double x[3][5][2];
or an equivalent type that I can reference to it (cf Requirement below).
Question
Is there a short way of ...
- 93
4
votes
3
answers
204
views
How to generate parameter packs via meta-programming?
Suppose
template<T,typename... Types> void bar(Types..., int,Types...,float,T);
I am trying to write a template struct GimmePack<double,k> so that
bar<char, GimmePack<double,3>::...
- 93
0
votes
1
answer
125
views
How to deduce parameter-pack from brace-enclosed initializer list? [duplicate]
This is a convenience question.
I have a valid implementation of FooImp< Aux<int...>, Aux<int...> > in the namespace FooDom when both int... parameter packs are of same length.
My ...
- 93
0
votes
2
answers
91
views
Accessing index in parameter pack during template substitution
I have a template function in C++17, where the N first arguments need to be integral type, and this should be checked during template substitution. (The template function should not exists for ...
- 9,042
7
votes
1
answer
183
views
Is TemplatePack<>...[I] allowed in C++26
Pack indexing was introduced in C++26 and I was hoping for this feature to have a significant impact on metaprogramming stuff, especially for indexing packs which would otherwise require workarounds.
...
- 2,045
0
votes
2
answers
98
views
Is it possible to define a method overload for each element of a template parameter pack?
Suppose I have a class template with a type template parameter pack, like
template<typename... types>
class Foo {};
Is there any way I could define methods in Foo, say void bar(T x); with ...
- 4,553
1
vote
1
answer
80
views
Parameter pack in overload resolution
Consider the following code:
template<class T>
void f(const T& a, const T&b) { std::cout << "#1"; }
template<class ...T>
void f(T&& ...a) { std::cout <&...
- 97
-1
votes
2
answers
239
views
error C3856: 'RecursiveType': symbol is not a class template [duplicate]
I am trying to create type with recursive template like std::tuple, unfortunately it does not compile, however I did it seemingly same way as in tuple source code. I am compiling with Visual Studio ...
- 1,125
3
votes
2
answers
80
views
Is there a less verbose way to initialize a tuple of heterogenous tuples (or similarly templated types)
I'm wanting to construct a tuple of hetergenous tuples but this seemingly requires an explicit constructor call for each member:
auto foo = std::tuple{
std::tuple{1, "foo", 3},
std::...
- 133
2
votes
2
answers
455
views
How to create a variadic ZipSequence type with Swift's parameter packs?
According to SE-0398, we can now create types with a variable number of generic type parameters. The document uses ZipSequence as an example:
In the generic parameter list of a generic type, the each ...
- 266k
2
votes
1
answer
81
views
Resolve Template Parameter Pack Overload Ambiguity
Minimum Example
I have the following struct with a few specializations:
template<size_t ...Tpar>
struct Base{};
struct X{};
struct Y{};
template<typename T, size_t ...Tpar>
struct Spline:...
- 171
0
votes
1
answer
50
views
How do I best implement a Derived with members depending on a Template Parameter Pack Specialization?
Goal
I want to implement a struct Derived<size_t ...Tpar>, satisfying certain requirements. I have a functioning solution code, but it has certain disadvantages. I seek a better way of doing it.
...
- 171
0
votes
1
answer
160
views
using c++20 concepts to find nth element of parameter pack [duplicate]
I am looking for a non-recursive, modern method (which compiles, at least with gcc and clang) for finding the 𝑛th element of a variadic parameter pack. Indeed, it would seem such a solution, using c++...
- 1
2
votes
1
answer
1k
views
Is it possible to iterate over the arguments of a Swift parameter pack?
Parameter packs are a cool new feature of Swift 5.9, but surprisingly there doesn't seem to be a lot of discussion about them, and I hadn't seen this question answered on StackOverflow.
- 16.5k
2
votes
1
answer
814
views
Swift 5.9: Parameter Packs getting Pack reference 'each V' can only appear in pack expansion
Hi Im trying to use the new variadic generic feature of swift 5.9.
So I have a code that Im trying to refactor it:
func resolve<T>() -> T {
// Resolve method
}
func autoResolve<T>(_ ...
- 3,323
0
votes
1
answer
157
views
Simplify creation of compile-time binary tree type
In the context of creating a binary tree type for a larger C++ project, I need to create a compile time type representing the nodes in each level.
Does anyone know, using the newest standards, if ...
- 394
2
votes
1
answer
1k
views
How to use Swift parameter packs in a View
I want to easily provide mutable bindings to my views for SwiftUI previews (so that the preview is interactive unlike when you pass a .constant(...) binding). I followed the BindingProvider approach ...
- 4,106
0
votes
2
answers
82
views
definition dependent template deduction failure
Within a larger project I got issues when trying to use templates to define multiple classes.
After some tinkering I found out, that my code was working when I changed the order of the template ...
- 3
1
vote
1
answer
107
views
Use parameter pack in template parameters list
I want to write an abstract Pipeline class that gets some functions with a specific signature as template parameters and runs them depending on the parse stage for input data. this is the code that I ...
-4
votes
2
answers
84
views
Why I can't use parameter pack in the constructor
Here is my code in hpp:
template <size_t... Bits>
class TelemetryCompactDataStrategy : public TelemetryCommonDataStrategy
{
static_assert((Bits + ...) <= 32, "Bit count must not ...
- 1