Questions tagged [deduction-guide]
30 questions
2
votes
2 answers
Can a class template have more than one user defined deduction guide for the same constructor?
Is it valid to have more than one user defined deduction guide for the same constructor in a class template?
For example:
template
class A {
T t;
public:
A(T t): t(std::move(t)) { /* ... */ }
};
template
requires (…
Amir Kirsh
- 12,564
- 41
- 74
2
votes
2 answers
Is it possible to get a template function to deduce the type of a template argument using either conversion or deduction guide?
Is it possible to get the compiler to deduce the type for the template function call, using either the type conversion or the deduction guide?
And, if not, why not?
#include
template
class Wrapper
{
public:
Wrapper(T&&…
Michel
- 338
- 1
- 13
2
votes
1 answer
Can both a 'constexpr'-marked variable and a variable of static-storage duration allowed to be stored through a class type with a deduction guide?
Consider the following piece of code:
template
struct wrap {
T thing;
constexpr wrap(T thing) : thing(thing) {}
};
template
wrap(const T&) -> wrap;
template
void fun();
struct X {
int a;
};
int…
lekner
- 21
- 1
2
votes
1 answer
Variadic template error: 'In instantiation of' (gcc 9.2)
I'm learning about variadic template on c++17 on the Jason Turner's youtube channel then I copy his example code (shown below). In his video he uses the site godbolt.org with the gcc 7.
#include
template
struct Merged : B…
TheArchitect
- 1,160
- 4
- 15
- 26
1
vote
1 answer
Constrain an input type in deduction guide
I want my class object's ctor to accept a functor that is able to be converted to a std::packaged_task whereas R is auto deduced from the functor's return value. Here's what I've got so far:
Demo
#include
#include
#include…
glades
- 3,778
- 1
- 12
- 34
1
vote
1 answer
Deduction guide based on number of parameters passed to constructor
Here's something I'm trying that doesn't seem to work: I want to toggle a compile time switch based on how a class object is instantiated. If there's just one constructor argument, the LengthOpt should equal to false, otherwise to true (my…
glades
- 3,778
- 1
- 12
- 34
1
vote
0 answers
Type is deducted successfully when is wrapped in method, but not when used in direct alias
Code where type deduction is wrapped method
struct InterfaceOverriderFactory
{
template
decltype(auto) operator()(Type) const noexcept
{
return typename…
Nufun
- 58
- 4
1
vote
0 answers
Template deduction with some user-supplied arguments
Suppose there is a class with two template parameters
template
class C {
C(B::X x) {}
};
is it possible to have a deduction guide where only one of the parameters is inferred and the other is…
user835943
- 171
- 9
1
vote
1 answer
Deduction Guide for a template template parameter
I have a set of structure's class as such:
template
struct Foo {
T x_;
T y_;
constexpr Foo(T x, T y) : x_{x}, y_{y} {}
};
template class Func>
class Bar {
private:
Foo foo_;
…
Francis Cugler
- 7,788
- 2
- 28
- 59
0
votes
1 answer
Is it possible to use fold expression in class template deduction guide?
I did a simple test as follow:
#include
template struct MyArray {
template MyArray(First, Rest...);
int dimension[N];
T m_array[D];
};
template…
Nima Ghorab
- 309
- 1
- 3
- 13
0
votes
1 answer
Deduction guide for brace initializer list
There is a implementation quite similar to std::initializer_list used in an environment where standart C++ library is not available:
template
class initializer_list {
public:
using value_type = T;
using reference = const T &;
…
Martin Kopecký
- 912
- 5
- 16
0
votes
1 answer
Template candidate ignored despite a user provided deduction guide
I have a struct sequence similar to std::integer_sequence, but it can hold values of multiple types as non-type template parameters (sort of like a compile-time tuple). It is implicitly constructible from std::integer_sequence by using a constexpr…
janekb04
- 4,304
- 2
- 20
- 51
0
votes
1 answer
Deduced conflicting types without template
I'm trying to create a type that can store either an int, a double, or an uint, like so:
struct Value
{
/*...*/
Value& operator=(const int value) { /*...*/ }
Value& operator=(const double value) { /*...*/ }
Value& operator=(const…
moechofe
- 101
- 6
0
votes
1 answer
Why does the class deduction guide fail when using a typedef?
In a piece of the code I currently write I make use of a class deduction guide. You can find the excerpt of the code stripped down to a simple (but meaningless example) below:
I've got a class User, which derives its first template parameter from…
mutableVoid
- 1,284
- 2
- 11
- 29
0
votes
1 answer
How to automaticly deduce template parameter for a function parameter of type `basic_string_view` or the like?
The following is not working:
template >
void f(std::basic_string_view sv) {
}
int main(){
std::basic_string_view sv = "no";//ok
std::basic_string_view svw = L"shit";//ok
…
darune
- 10,480
- 2
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- 62