Questions tagged [pack-expansion]
13 questions
26
votes
1 answer
Problem with order of evaluation in parameter pack expansion
I wrote a template that takes an istream& and a function and is supposed to extract all parameters for this function from the istream, call the function with these parameters and return the result. It all works fine, except for the order of…
Lukas-T
- 11,133
- 3
- 20
- 30
8
votes
1 answer
How to create a template with variable number of type parameters?
In our codebase we use std::variant, ...> a lot.
That obviously requires a lot of writing. How to make a template?
template
using VarSP = std::variant>;
Where should T go in the above…
Nurbol Alpysbayev
- 19,522
- 3
- 54
- 89
7
votes
1 answer
Black Magic using Initializer_list and pack expansion
To expand flexible function parameters, there is a method using std::initializer_list. However I couldn't understand it. Can anyone explain this in an understandable way?
template
auto print(T value, Args... args) {
…
Yueyang Pan
- 97
- 4
5
votes
1 answer
MSVC vs Clang/GCC bug during overload resolution of function templates one of which contains a parameter pack
I was using parameter pack when I noticed that one such case(shown below) compiles fine in gcc and clang but not in msvc:
template void func(T a, T b= T{})
{
}
template void func(T a, S... b)
{
}
int main()
{
…
Jason
- 36,170
- 5
- 26
- 60
3
votes
1 answer
How can you have multiple lines or statements inside a C++ pack expansion?
Suppose that I have some simple code like the one below, which just prints all of the values in a tuple and keeps track of the current iteration.
#include
#include
#include
using std::cout;
int main() {
…
Doot
- 555
- 5
- 15
2
votes
1 answer
How to generalize variadic function fn to fn
Assume there is variadic function template foo(const S& s, Args... args).
struct S {};
template
void foo(const S& s, Args... args);
void check_foo()
{
S s0{};
S s1{};
char a0 = 0;
short a1 = 1;
int …
ilynxy
- 151
- 4
1
vote
0 answers
How the pattern on a function template argument pack works?
Hello I have this example about variadic function templates and patterns:
int& neg(int& x){ x = -x; return x;}
void f(int& x){
std::cout << x << ", ";
}
template
void f(T& x, Args&...args){
f(…
Itachi Uchiwa
- 3,044
- 12
- 26
0
votes
1 answer
GCC & Clang vs MSVC Bug while expanding template parameter pack in the same parameter clause for function templates
I came across the following statement in the standard:
If a template-parameter is a type-parameter with an ellipsis prior to its optional identifier or is a parameter-declaration that declares a pack ([dcl.fct]), then the template-parameter is a…
Jason
- 36,170
- 5
- 26
- 60
0
votes
1 answer
Multiple inheritance from a pack expansion
I recently saw this in production code and couldn't quite figure out what it does:
template
struct pool : pool_type... {
//...
};
I've never seen pack expansion happening for parent classes. Does it just inherit every type passed…
glades
- 3,778
- 1
- 12
- 34
0
votes
2 answers
Variadic expansion to access multi-dimensional std::array
Suppose the following multi_array structure:
template
struct multi_array
{
using storage_type = typename storage_type::type;
using value_type = type;
using size_type =…
demiralp
- 45
- 1
- 3
0
votes
3 answers
Trying to require multiple variadic types to be specific types
This is currently in pseudo code as it is an idea that I'm working on before I begin to write it as full code.
I know that I can create a normal variadic function that uses va_arg and va_list such as printf() does, however, I want to avoid using…
Francis Cugler
- 7,788
- 2
- 28
- 59
0
votes
1 answer
Pass param packed args into a std::queue to call with a different function later
I asked a similar question earlier without realizing that that wasn't quite specific enough.
So I have this function that has to take in all the arguments of a print function, with the ... and all, and then put it into a queue that will call the…
Leo Riesenbach
- 3
- 2
0
votes
0 answers
Pass a parameter packed arg list to a queue
What i need to do is have a function that takes in a list of arbitrary arguments, using the parameter pack operator, and put that whole list into a queue from which I can later pass those args into another function.
This kind of thing:
void…
Leo Riesenbach
- 3
- 2