Mixing
What's the syntax to partially specialize a parmeter pack argument for void type?
6
I can't find a way to get this to work. Is it even possible? I don't see why it wouldn't be.
template <auto id, typename FirstField, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
template <auto id>
struct FieldTypeById<id, void> {
using Type = void;
};
int main()
{
using t1 = FieldTypeById<0, int>::Type;
using t2 = FieldTypeById<1>::Type;
return 0;
}
https://godbolt.org/z/AggnDq
c++ templates c++17 template-specialization
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
add a comment |
6
I can't find a way to get this to work. Is it even possible? I don't see why it wouldn't be.
template <auto id, typename FirstField, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
template <auto id>
struct FieldTypeById<id, void> {
using Type = void;
};
int main()
{
using t1 = FieldTypeById<0, int>::Type;
using t2 = FieldTypeById<1>::Type;
return 0;
}
https://godbolt.org/z/AggnDq
c++ templates c++17 template-specialization
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
add a comment |
6
6
6
1
I can't find a way to get this to work. Is it even possible? I don't see why it wouldn't be.
template <auto id, typename FirstField, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
template <auto id>
struct FieldTypeById<id, void> {
using Type = void;
};
int main()
{
using t1 = FieldTypeById<0, int>::Type;
using t2 = FieldTypeById<1>::Type;
return 0;
}
https://godbolt.org/z/AggnDq
c++ templates c++17 template-specialization
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
I can't find a way to get this to work. Is it even possible? I don't see why it wouldn't be.
template <auto id, typename FirstField, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
template <auto id>
struct FieldTypeById<id, void> {
using Type = void;
};
int main()
{
using t1 = FieldTypeById<0, int>::Type;
using t2 = FieldTypeById<1>::Type;
return 0;
}
https://godbolt.org/z/AggnDq
c++ templates c++17 template-specialization
c++ templates c++17 template-specialization
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
asked Nov 22 '18 at 8:02
Violet GiraffeViolet Giraffe
14.6k28135246
14.6k28135246
add a comment |
add a comment |
1 Answer
1
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8
The problem in your example isn't the specialization, it's fine. The problem is that FieldTypeById<1> cannot deduce the type FirstField is. You can amend that by simply adding a default to the primary template:
template <auto id, typename FirstField = void, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
Now all arguments are given explicitly, taken from defaults, or deduced (as an empty pack). After all the arguments are known, the specialization for those arguments can be used.
See it live
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
Hmmm. If the "main" template hasFirstField = void, meaning that it can be directly matched withFieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with= voidandstd::conditionalinside it, but still, I wonder if it can be done via a specialization.
– Violet Giraffe
Nov 22 '18 at 8:08
2
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
1
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
|
show 3 more comments
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
8
The problem in your example isn't the specialization, it's fine. The problem is that FieldTypeById<1> cannot deduce the type FirstField is. You can amend that by simply adding a default to the primary template:
template <auto id, typename FirstField = void, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
Now all arguments are given explicitly, taken from defaults, or deduced (as an empty pack). After all the arguments are known, the specialization for those arguments can be used.
See it live
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
Hmmm. If the "main" template hasFirstField = void, meaning that it can be directly matched withFieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with= voidandstd::conditionalinside it, but still, I wonder if it can be done via a specialization.
– Violet Giraffe
Nov 22 '18 at 8:08
2
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
1
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
|
show 3 more comments
8
The problem in your example isn't the specialization, it's fine. The problem is that FieldTypeById<1> cannot deduce the type FirstField is. You can amend that by simply adding a default to the primary template:
template <auto id, typename FirstField = void, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
Now all arguments are given explicitly, taken from defaults, or deduced (as an empty pack). After all the arguments are known, the specialization for those arguments can be used.
See it live
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
Hmmm. If the "main" template hasFirstField = void, meaning that it can be directly matched withFieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with= voidandstd::conditionalinside it, but still, I wonder if it can be done via a specialization.
– Violet Giraffe
Nov 22 '18 at 8:08
2
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
1
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
|
show 3 more comments
8
8
8
The problem in your example isn't the specialization, it's fine. The problem is that FieldTypeById<1> cannot deduce the type FirstField is. You can amend that by simply adding a default to the primary template:
template <auto id, typename FirstField = void, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
Now all arguments are given explicitly, taken from defaults, or deduced (as an empty pack). After all the arguments are known, the specialization for those arguments can be used.
See it live
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
The problem in your example isn't the specialization, it's fine. The problem is that FieldTypeById<1> cannot deduce the type FirstField is. You can amend that by simply adding a default to the primary template:
template <auto id, typename FirstField = void, typename... OtherFields>
struct FieldTypeById {
using Type = int;
};
Now all arguments are given explicitly, taken from defaults, or deduced (as an empty pack). After all the arguments are known, the specialization for those arguments can be used.
See it live
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
edited Nov 22 '18 at 8:13
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
answered Nov 22 '18 at 8:04
StoryTellerStoryTeller
99.8k12201271
99.8k12201271
Hmmm. If the "main" template hasFirstField = void, meaning that it can be directly matched withFieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with= voidandstd::conditionalinside it, but still, I wonder if it can be done via a specialization.
– Violet Giraffe
Nov 22 '18 at 8:08
2
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
1
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
|
show 3 more comments
Hmmm. If the "main" template hasFirstField = void, meaning that it can be directly matched withFieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with= voidandstd::conditionalinside it, but still, I wonder if it can be done via a specialization.
– Violet Giraffe
Nov 22 '18 at 8:08
2
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
1
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
Hmmm. If the "main" template has
FirstField = void, meaning that it can be directly matched with FieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with = void and std::conditional inside it, but still, I wonder if it can be done via a specialization.– Violet Giraffe
Nov 22 '18 at 8:08
Hmmm. If the "main" template has
FirstField = void, meaning that it can be directly matched with FieldTypeById<1>, is the specialization ever going to be instantiated? I can now see that I can achieve the same goal using just one template with = void and std::conditional inside it, but still, I wonder if it can be done via a specialization.– Violet Giraffe
Nov 22 '18 at 8:08
2
2
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
@VioletGiraffe - The specialization is instantiated when the arguments to the template match it. But default arguments can only appear on the primary template, because that's where the pattern matching starts (to explain it colloquially).
– StoryTeller
Nov 22 '18 at 8:10
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
Do I understand correctly that in order to specialize a template, one must first make sure that the desired specialized instantiation can be matched to the primary template?
– Violet Giraffe
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
@StoryTeller : is that due to fact that when we pass argument list in the template there is no type deduction (unlike function template) ? I thought about that !!
– Blood-HaZaRd
Nov 22 '18 at 8:19
1
1
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
Thank you, that's an important thing to learn that I somehow missed during my many years of using C++!
– Violet Giraffe
Nov 22 '18 at 8:22
|
show 3 more comments
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