Recursive Compound structure
I have a class that looks something like this:
template<class KeyType, class... Types>
class BasicCompound
{
public:
using mapped_type = std::variant
<
ValueWrapper<BasicCompound>
, ValueWrapper<Types>...
>;
using key_type = KeyType;
// Accessors for retreiving and modifying content
// ...
private:
std::map<key_type, mapped_type> m_content;
};
ValueWrapper decides to put the content either inline or in a std::unique_ptr. Would it be possible with a similar interface, possibly through some kind of proxy, to make the recursiveness optional? By optional I mean that the user should not automatically get the possibility to store the BasicCompound inside itself, but rather specify it in the list of types.
What I have thought of:
- A
usingdirective does not work. A new type cannot be defined in it self, and a predeclaration of a following typedef is not allowed. - Adding a
boolto the list of types, and usestd::conditional_tformapped_type. However, if the user wants to store anX<BasicCompound>, this approach fails. - Inject
mapped_typefrom outside. Then I cannot hide the use of theValueWrapperthing.
Using inheritance over a
typdeflike
struct MyCompound : BasicCompound<std::string, MyCompound, int> {};
This works but then the structure is not strictly recursive, as
MyCompoundnow is a different type fromBasicCompound. Maybe a CRTP-like approach could solve that problem, but then the inner compound type must be treated differently than the other types.
c++ recursive-datastructures
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
add a comment |
I have a class that looks something like this:
template<class KeyType, class... Types>
class BasicCompound
{
public:
using mapped_type = std::variant
<
ValueWrapper<BasicCompound>
, ValueWrapper<Types>...
>;
using key_type = KeyType;
// Accessors for retreiving and modifying content
// ...
private:
std::map<key_type, mapped_type> m_content;
};
ValueWrapper decides to put the content either inline or in a std::unique_ptr. Would it be possible with a similar interface, possibly through some kind of proxy, to make the recursiveness optional? By optional I mean that the user should not automatically get the possibility to store the BasicCompound inside itself, but rather specify it in the list of types.
What I have thought of:
- A
usingdirective does not work. A new type cannot be defined in it self, and a predeclaration of a following typedef is not allowed. - Adding a
boolto the list of types, and usestd::conditional_tformapped_type. However, if the user wants to store anX<BasicCompound>, this approach fails. - Inject
mapped_typefrom outside. Then I cannot hide the use of theValueWrapperthing.
Using inheritance over a
typdeflike
struct MyCompound : BasicCompound<std::string, MyCompound, int> {};
This works but then the structure is not strictly recursive, as
MyCompoundnow is a different type fromBasicCompound. Maybe a CRTP-like approach could solve that problem, but then the inner compound type must be treated differently than the other types.
c++ recursive-datastructures
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
add a comment |
I have a class that looks something like this:
template<class KeyType, class... Types>
class BasicCompound
{
public:
using mapped_type = std::variant
<
ValueWrapper<BasicCompound>
, ValueWrapper<Types>...
>;
using key_type = KeyType;
// Accessors for retreiving and modifying content
// ...
private:
std::map<key_type, mapped_type> m_content;
};
ValueWrapper decides to put the content either inline or in a std::unique_ptr. Would it be possible with a similar interface, possibly through some kind of proxy, to make the recursiveness optional? By optional I mean that the user should not automatically get the possibility to store the BasicCompound inside itself, but rather specify it in the list of types.
What I have thought of:
- A
usingdirective does not work. A new type cannot be defined in it self, and a predeclaration of a following typedef is not allowed. - Adding a
boolto the list of types, and usestd::conditional_tformapped_type. However, if the user wants to store anX<BasicCompound>, this approach fails. - Inject
mapped_typefrom outside. Then I cannot hide the use of theValueWrapperthing.
Using inheritance over a
typdeflike
struct MyCompound : BasicCompound<std::string, MyCompound, int> {};
This works but then the structure is not strictly recursive, as
MyCompoundnow is a different type fromBasicCompound. Maybe a CRTP-like approach could solve that problem, but then the inner compound type must be treated differently than the other types.
c++ recursive-datastructures
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
I have a class that looks something like this:
template<class KeyType, class... Types>
class BasicCompound
{
public:
using mapped_type = std::variant
<
ValueWrapper<BasicCompound>
, ValueWrapper<Types>...
>;
using key_type = KeyType;
// Accessors for retreiving and modifying content
// ...
private:
std::map<key_type, mapped_type> m_content;
};
ValueWrapper decides to put the content either inline or in a std::unique_ptr. Would it be possible with a similar interface, possibly through some kind of proxy, to make the recursiveness optional? By optional I mean that the user should not automatically get the possibility to store the BasicCompound inside itself, but rather specify it in the list of types.
What I have thought of:
- A
usingdirective does not work. A new type cannot be defined in it self, and a predeclaration of a following typedef is not allowed. - Adding a
boolto the list of types, and usestd::conditional_tformapped_type. However, if the user wants to store anX<BasicCompound>, this approach fails. - Inject
mapped_typefrom outside. Then I cannot hide the use of theValueWrapperthing.
Using inheritance over a
typdeflike
struct MyCompound : BasicCompound<std::string, MyCompound, int> {};
This works but then the structure is not strictly recursive, as
MyCompoundnow is a different type fromBasicCompound. Maybe a CRTP-like approach could solve that problem, but then the inner compound type must be treated differently than the other types.
c++ recursive-datastructures
c++ recursive-datastructures
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
edited Nov 22 '18 at 20:02
user877329
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
asked Nov 22 '18 at 19:47
user877329user877329
2,79632658
2,79632658
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
You can't have use a specialization of a class template as one of the template arguments of that specialization; there's no way to write it, and its name would be infinitely long. You can, however, use a wrapper to hold the recursive type:
template<class> class MaybeRecursive;
template<class T>
struct Wrapper {
using type=T;
};
struct MRWrapper {
using type=MaybeRecursive<MRWrapper>;
};
template<class T>
struct MaybeRecursive {
using type=typename T::type;
type *child;
};
void f() {
int x;
MaybeRecursive<Wrapper<int>> nonrec={&x};
MRWrapper::type rec={&rec};
}
MRWrapper can be made a class template to provide additional template arguments to MaybeRecursive.
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
@user877329: You can't writeX<X<...>,Y>where the...makes the template argument equivalent to the outer type. Nor can you writeX<A,Y>whereAis an alias forX<A,Y>. This is (implementation guidance for) the closest you can get.
– Davis Herring
Nov 23 '18 at 17:18
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
You can't have use a specialization of a class template as one of the template arguments of that specialization; there's no way to write it, and its name would be infinitely long. You can, however, use a wrapper to hold the recursive type:
template<class> class MaybeRecursive;
template<class T>
struct Wrapper {
using type=T;
};
struct MRWrapper {
using type=MaybeRecursive<MRWrapper>;
};
template<class T>
struct MaybeRecursive {
using type=typename T::type;
type *child;
};
void f() {
int x;
MaybeRecursive<Wrapper<int>> nonrec={&x};
MRWrapper::type rec={&rec};
}
MRWrapper can be made a class template to provide additional template arguments to MaybeRecursive.
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
@user877329: You can't writeX<X<...>,Y>where the...makes the template argument equivalent to the outer type. Nor can you writeX<A,Y>whereAis an alias forX<A,Y>. This is (implementation guidance for) the closest you can get.
– Davis Herring
Nov 23 '18 at 17:18
add a comment |
You can't have use a specialization of a class template as one of the template arguments of that specialization; there's no way to write it, and its name would be infinitely long. You can, however, use a wrapper to hold the recursive type:
template<class> class MaybeRecursive;
template<class T>
struct Wrapper {
using type=T;
};
struct MRWrapper {
using type=MaybeRecursive<MRWrapper>;
};
template<class T>
struct MaybeRecursive {
using type=typename T::type;
type *child;
};
void f() {
int x;
MaybeRecursive<Wrapper<int>> nonrec={&x};
MRWrapper::type rec={&rec};
}
MRWrapper can be made a class template to provide additional template arguments to MaybeRecursive.
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
@user877329: You can't writeX<X<...>,Y>where the...makes the template argument equivalent to the outer type. Nor can you writeX<A,Y>whereAis an alias forX<A,Y>. This is (implementation guidance for) the closest you can get.
– Davis Herring
Nov 23 '18 at 17:18
add a comment |
You can't have use a specialization of a class template as one of the template arguments of that specialization; there's no way to write it, and its name would be infinitely long. You can, however, use a wrapper to hold the recursive type:
template<class> class MaybeRecursive;
template<class T>
struct Wrapper {
using type=T;
};
struct MRWrapper {
using type=MaybeRecursive<MRWrapper>;
};
template<class T>
struct MaybeRecursive {
using type=typename T::type;
type *child;
};
void f() {
int x;
MaybeRecursive<Wrapper<int>> nonrec={&x};
MRWrapper::type rec={&rec};
}
MRWrapper can be made a class template to provide additional template arguments to MaybeRecursive.
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
You can't have use a specialization of a class template as one of the template arguments of that specialization; there's no way to write it, and its name would be infinitely long. You can, however, use a wrapper to hold the recursive type:
template<class> class MaybeRecursive;
template<class T>
struct Wrapper {
using type=T;
};
struct MRWrapper {
using type=MaybeRecursive<MRWrapper>;
};
template<class T>
struct MaybeRecursive {
using type=typename T::type;
type *child;
};
void f() {
int x;
MaybeRecursive<Wrapper<int>> nonrec={&x};
MRWrapper::type rec={&rec};
}
MRWrapper can be made a class template to provide additional template arguments to MaybeRecursive.
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
answered Nov 22 '18 at 21:10
Davis HerringDavis Herring
8,4801736
8,4801736
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
@user877329: You can't writeX<X<...>,Y>where the...makes the template argument equivalent to the outer type. Nor can you writeX<A,Y>whereAis an alias forX<A,Y>. This is (implementation guidance for) the closest you can get.
– Davis Herring
Nov 23 '18 at 17:18
add a comment |
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
@user877329: You can't writeX<X<...>,Y>where the...makes the template argument equivalent to the outer type. Nor can you writeX<A,Y>whereAis an alias forX<A,Y>. This is (implementation guidance for) the closest you can get.
– Davis Herring
Nov 23 '18 at 17:18
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
I guess that this is very similar to my option (3) in terms of usage: I cannot use the content directly, but must go through the wrapper type.
– user877329
Nov 23 '18 at 17:04
@user877329: You can't write
X<X<...>,Y> where the ... makes the template argument equivalent to the outer type. Nor can you write X<A,Y> where A is an alias for X<A,Y>. This is (implementation guidance for) the closest you can get.– Davis Herring
Nov 23 '18 at 17:18
@user877329: You can't write
X<X<...>,Y> where the ... makes the template argument equivalent to the outer type. Nor can you write X<A,Y> where A is an alias for X<A,Y>. This is (implementation guidance for) the closest you can get.– Davis Herring
Nov 23 '18 at 17:18
add a comment |
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