Initialize class' template (aggregate type) member with aggregate initializer but without extra parenthesis
up vote
4
down vote
favorite
2
Having this code:
struct Vec3
int x;
int y;
int z;
;
template <typename T>
class myProperty
public:
myProperty(const T& initValue) : m_valueinitValue
private:
T m_value;
;
When creating myProperty type object:
myProperty<int> ip1;
myProperty<Vec3> vp11, 2, 3;
// myProperty<Vec3> vp21, 2, 3; ERROR: myProperty doesn't have a matching constructor.
Is there an elegant way of making vp2 initialization work? Specializing myProperty for Vec3 is an overkill.
c++ class templates constructor c++14
asked Nov 10 at 10:55
nVxx
244212
add a comment |
up vote
4
down vote
favorite
2
Having this code:
struct Vec3
int x;
int y;
int z;
;
template <typename T>
class myProperty
public:
myProperty(const T& initValue) : m_valueinitValue
private:
T m_value;
;
When creating myProperty type object:
myProperty<int> ip1;
myProperty<Vec3> vp11, 2, 3;
// myProperty<Vec3> vp21, 2, 3; ERROR: myProperty doesn't have a matching constructor.
Is there an elegant way of making vp2 initialization work? Specializing myProperty for Vec3 is an overkill.
c++ class templates constructor c++14
asked Nov 10 at 10:55
nVxx
244212
1
Is makingmyPropertyitself an aggregate an option? (I.e. remove the constructor and makem_valuepublic.) Otherwise the only solution is see is a templated constructor, that forwards its arguments to theTconstructor:template <typename ...P> myProperty(P &&... p) : m_value(std::forward<P>(p)...).
– HolyBlackCat
Nov 10 at 11:01
@HolyBlackCat, that won't compile. To make it work, you'll have to put extra parentheses, likem_value(std::forward<P>(p)...), that wayvp2case will compile butvp1not. And makingmyPropertyan agregate is not an option.
– nVxx
Nov 10 at 11:35
Oops. It should be: m_valuestd::forward<P>(p)..., otherwisemyProperty<int> ip1;doesn't compile. "vp2 case will compile but vp1 not" Does it mean you want bothvp1andvp2to compile, not onlyvp2?
– HolyBlackCat
Nov 10 at 11:38
@HolyBlackCat, actually no, I don't want vp1 to compile :) My bad, didn't test your suggestion thoroughly, assumed something likevp4myVec3Objetwon't compile also, but it does, so this looks good :) Would accept the answer if you post it.
– nVxx
Nov 10 at 11:52
1
Turns out it breaks copy-construction (because it's a better match thanmyProperty(const myProperty &)if the parameter is non-const). I'll post an answer if I figure out how to fix that in a neat way.
– HolyBlackCat
Nov 10 at 12:07
add a comment |
up vote
4
down vote
favorite
2
up vote
4
down vote
favorite
2
2
Having this code:
struct Vec3
int x;
int y;
int z;
;
template <typename T>
class myProperty
public:
myProperty(const T& initValue) : m_valueinitValue
private:
T m_value;
;
When creating myProperty type object:
myProperty<int> ip1;
myProperty<Vec3> vp11, 2, 3;
// myProperty<Vec3> vp21, 2, 3; ERROR: myProperty doesn't have a matching constructor.
Is there an elegant way of making vp2 initialization work? Specializing myProperty for Vec3 is an overkill.
c++ class templates constructor c++14
asked Nov 10 at 10:55
nVxx
244212
Having this code:
struct Vec3
int x;
int y;
int z;
;
template <typename T>
class myProperty
public:
myProperty(const T& initValue) : m_valueinitValue
private:
T m_value;
;
When creating myProperty type object:
myProperty<int> ip1;
myProperty<Vec3> vp11, 2, 3;
// myProperty<Vec3> vp21, 2, 3; ERROR: myProperty doesn't have a matching constructor.
Is there an elegant way of making vp2 initialization work? Specializing myProperty for Vec3 is an overkill.
c++ class templates constructor c++14
c++ class templates constructor c++14
asked Nov 10 at 10:55
nVxx
244212
asked Nov 10 at 10:55
nVxx
244212
asked Nov 10 at 10:55
nVxx
244212
asked Nov 10 at 10:55
nVxx
244212
asked Nov 10 at 10:55
nVxx
244212
244212
1
Is makingmyPropertyitself an aggregate an option? (I.e. remove the constructor and makem_valuepublic.) Otherwise the only solution is see is a templated constructor, that forwards its arguments to theTconstructor:template <typename ...P> myProperty(P &&... p) : m_value(std::forward<P>(p)...).
– HolyBlackCat
Nov 10 at 11:01
@HolyBlackCat, that won't compile. To make it work, you'll have to put extra parentheses, likem_value(std::forward<P>(p)...), that wayvp2case will compile butvp1not. And makingmyPropertyan agregate is not an option.
– nVxx
Nov 10 at 11:35
Oops. It should be: m_valuestd::forward<P>(p)..., otherwisemyProperty<int> ip1;doesn't compile. "vp2 case will compile but vp1 not" Does it mean you want bothvp1andvp2to compile, not onlyvp2?
– HolyBlackCat
Nov 10 at 11:38
@HolyBlackCat, actually no, I don't want vp1 to compile :) My bad, didn't test your suggestion thoroughly, assumed something likevp4myVec3Objetwon't compile also, but it does, so this looks good :) Would accept the answer if you post it.
– nVxx
Nov 10 at 11:52
1
Turns out it breaks copy-construction (because it's a better match thanmyProperty(const myProperty &)if the parameter is non-const). I'll post an answer if I figure out how to fix that in a neat way.
– HolyBlackCat
Nov 10 at 12:07
add a comment |
1
Is makingmyPropertyitself an aggregate an option? (I.e. remove the constructor and makem_valuepublic.) Otherwise the only solution is see is a templated constructor, that forwards its arguments to theTconstructor:template <typename ...P> myProperty(P &&... p) : m_value(std::forward<P>(p)...).
– HolyBlackCat
Nov 10 at 11:01
@HolyBlackCat, that won't compile. To make it work, you'll have to put extra parentheses, likem_value(std::forward<P>(p)...), that wayvp2case will compile butvp1not. And makingmyPropertyan agregate is not an option.
– nVxx
Nov 10 at 11:35
Oops. It should be: m_valuestd::forward<P>(p)..., otherwisemyProperty<int> ip1;doesn't compile. "vp2 case will compile but vp1 not" Does it mean you want bothvp1andvp2to compile, not onlyvp2?
– HolyBlackCat
Nov 10 at 11:38
@HolyBlackCat, actually no, I don't want vp1 to compile :) My bad, didn't test your suggestion thoroughly, assumed something likevp4myVec3Objetwon't compile also, but it does, so this looks good :) Would accept the answer if you post it.
– nVxx
Nov 10 at 11:52
1
Turns out it breaks copy-construction (because it's a better match thanmyProperty(const myProperty &)if the parameter is non-const). I'll post an answer if I figure out how to fix that in a neat way.
– HolyBlackCat
Nov 10 at 12:07
1
1
Is making
myProperty itself an aggregate an option? (I.e. remove the constructor and make m_value public.) Otherwise the only solution is see is a templated constructor, that forwards its arguments to the T constructor: template <typename ...P> myProperty(P &&... p) : m_value(std::forward<P>(p)...) .– HolyBlackCat
Nov 10 at 11:01
Is making
myProperty itself an aggregate an option? (I.e. remove the constructor and make m_value public.) Otherwise the only solution is see is a templated constructor, that forwards its arguments to the T constructor: template <typename ...P> myProperty(P &&... p) : m_value(std::forward<P>(p)...) .– HolyBlackCat
Nov 10 at 11:01
@HolyBlackCat, that won't compile. To make it work, you'll have to put extra parentheses, like
m_value(std::forward<P>(p)...), that way vp2 case will compile but vp1 not. And making myProperty an agregate is not an option.– nVxx
Nov 10 at 11:35
@HolyBlackCat, that won't compile. To make it work, you'll have to put extra parentheses, like
m_value(std::forward<P>(p)...), that way vp2 case will compile but vp1 not. And making myProperty an agregate is not an option.– nVxx
Nov 10 at 11:35
Oops. It should be
: m_valuestd::forward<P>(p)..., otherwise myProperty<int> ip1; doesn't compile. "vp2 case will compile but vp1 not" Does it mean you want both vp1 and vp2 to compile, not only vp2?– HolyBlackCat
Nov 10 at 11:38
Oops. It should be
: m_valuestd::forward<P>(p)..., otherwise myProperty<int> ip1; doesn't compile. "vp2 case will compile but vp1 not" Does it mean you want both vp1 and vp2 to compile, not only vp2?– HolyBlackCat
Nov 10 at 11:38
@HolyBlackCat, actually no, I don't want vp1 to compile :) My bad, didn't test your suggestion thoroughly, assumed something like
vp4myVec3Objet won't compile also, but it does, so this looks good :) Would accept the answer if you post it.– nVxx
Nov 10 at 11:52
@HolyBlackCat, actually no, I don't want vp1 to compile :) My bad, didn't test your suggestion thoroughly, assumed something like
vp4myVec3Objet won't compile also, but it does, so this looks good :) Would accept the answer if you post it.– nVxx
Nov 10 at 11:52
1
1
Turns out it breaks copy-construction (because it's a better match than
myProperty(const myProperty &) if the parameter is non-const). I'll post an answer if I figure out how to fix that in a neat way.– HolyBlackCat
Nov 10 at 12:07
Turns out it breaks copy-construction (because it's a better match than
myProperty(const myProperty &) if the parameter is non-const). I'll post an answer if I figure out how to fix that in a neat way.– HolyBlackCat
Nov 10 at 12:07
add a comment |
1 Answer
1
active
oldest
votes
up vote
6
down vote
A simple solution is to use a variadic template constructor:
template <typename ...P> myProperty(P &&... p) : m_valuestd::forward<P>(p)...
It makes myProperty<Vec3> vp21, 2, 3; compile.
Also it stops myProperty<Vec3> vp11, 2, 3; from compiling (which seems to match your intentions).
The problem with this option is that it prevents copy construction from working propertly.
(If the parameter is a non-const myProperty<T> lvalue, then this variadic constructor is a better match than myProperty(const myProperty &).)
This can be solved with SFINAE:
C++17 with <experimental/type_traits>:
#include <experimental/type_traits>
#include <utility>
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<std::experimental::is_detected_v<list_constructible, T, P...>>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
C++14:
#include <type_traits>
#include <utility>
template <typename...> using void_t = void;
template <typename DummyVoid, template <typename...> class A, typename ...B> struct is_detected : std::false_type ;
template <template <typename...> class A, typename ...B> struct is_detected<void_t<A<B...>>, A, B...> : std::true_type ;
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<is_detected<void, list_constructible, T, P...>::value>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
6
down vote
A simple solution is to use a variadic template constructor:
template <typename ...P> myProperty(P &&... p) : m_valuestd::forward<P>(p)...
It makes myProperty<Vec3> vp21, 2, 3; compile.
Also it stops myProperty<Vec3> vp11, 2, 3; from compiling (which seems to match your intentions).
The problem with this option is that it prevents copy construction from working propertly.
(If the parameter is a non-const myProperty<T> lvalue, then this variadic constructor is a better match than myProperty(const myProperty &).)
This can be solved with SFINAE:
C++17 with <experimental/type_traits>:
#include <experimental/type_traits>
#include <utility>
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<std::experimental::is_detected_v<list_constructible, T, P...>>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
C++14:
#include <type_traits>
#include <utility>
template <typename...> using void_t = void;
template <typename DummyVoid, template <typename...> class A, typename ...B> struct is_detected : std::false_type ;
template <template <typename...> class A, typename ...B> struct is_detected<void_t<A<B...>>, A, B...> : std::true_type ;
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<is_detected<void, list_constructible, T, P...>::value>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
add a comment |
up vote
6
down vote
A simple solution is to use a variadic template constructor:
template <typename ...P> myProperty(P &&... p) : m_valuestd::forward<P>(p)...
It makes myProperty<Vec3> vp21, 2, 3; compile.
Also it stops myProperty<Vec3> vp11, 2, 3; from compiling (which seems to match your intentions).
The problem with this option is that it prevents copy construction from working propertly.
(If the parameter is a non-const myProperty<T> lvalue, then this variadic constructor is a better match than myProperty(const myProperty &).)
This can be solved with SFINAE:
C++17 with <experimental/type_traits>:
#include <experimental/type_traits>
#include <utility>
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<std::experimental::is_detected_v<list_constructible, T, P...>>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
C++14:
#include <type_traits>
#include <utility>
template <typename...> using void_t = void;
template <typename DummyVoid, template <typename...> class A, typename ...B> struct is_detected : std::false_type ;
template <template <typename...> class A, typename ...B> struct is_detected<void_t<A<B...>>, A, B...> : std::true_type ;
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<is_detected<void, list_constructible, T, P...>::value>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
add a comment |
up vote
6
down vote
up vote
6
down vote
A simple solution is to use a variadic template constructor:
template <typename ...P> myProperty(P &&... p) : m_valuestd::forward<P>(p)...
It makes myProperty<Vec3> vp21, 2, 3; compile.
Also it stops myProperty<Vec3> vp11, 2, 3; from compiling (which seems to match your intentions).
The problem with this option is that it prevents copy construction from working propertly.
(If the parameter is a non-const myProperty<T> lvalue, then this variadic constructor is a better match than myProperty(const myProperty &).)
This can be solved with SFINAE:
C++17 with <experimental/type_traits>:
#include <experimental/type_traits>
#include <utility>
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<std::experimental::is_detected_v<list_constructible, T, P...>>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
C++14:
#include <type_traits>
#include <utility>
template <typename...> using void_t = void;
template <typename DummyVoid, template <typename...> class A, typename ...B> struct is_detected : std::false_type ;
template <template <typename...> class A, typename ...B> struct is_detected<void_t<A<B...>>, A, B...> : std::true_type ;
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<is_detected<void, list_constructible, T, P...>::value>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
A simple solution is to use a variadic template constructor:
template <typename ...P> myProperty(P &&... p) : m_valuestd::forward<P>(p)...
It makes myProperty<Vec3> vp21, 2, 3; compile.
Also it stops myProperty<Vec3> vp11, 2, 3; from compiling (which seems to match your intentions).
The problem with this option is that it prevents copy construction from working propertly.
(If the parameter is a non-const myProperty<T> lvalue, then this variadic constructor is a better match than myProperty(const myProperty &).)
This can be solved with SFINAE:
C++17 with <experimental/type_traits>:
#include <experimental/type_traits>
#include <utility>
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<std::experimental::is_detected_v<list_constructible, T, P...>>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
C++14:
#include <type_traits>
#include <utility>
template <typename...> using void_t = void;
template <typename DummyVoid, template <typename...> class A, typename ...B> struct is_detected : std::false_type ;
template <template <typename...> class A, typename ...B> struct is_detected<void_t<A<B...>>, A, B...> : std::true_type ;
template <typename T, typename ...P> using list_constructible = decltype(Tstd::declval<P>()...);
// ...
template
<
typename ...P,
typename = std::enable_if_t<is_detected<void, list_constructible, T, P...>::value>
>
myProperty(P &&... p) : m_valuestd::forward<P>(p)...
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
edited Nov 10 at 12:37
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
answered Nov 10 at 12:30
HolyBlackCat
14.9k23361
14.9k23361
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
add a comment |
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
Nov 12 at 12:15
add a comment |
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1
Is making
myPropertyitself an aggregate an option? (I.e. remove the constructor and makem_valuepublic.) Otherwise the only solution is see is a templated constructor, that forwards its arguments to theTconstructor:template <typename ...P> myProperty(P &&... p) : m_value(std::forward<P>(p)...).– HolyBlackCat
Nov 10 at 11:01
@HolyBlackCat, that won't compile. To make it work, you'll have to put extra parentheses, like
m_value(std::forward<P>(p)...), that wayvp2case will compile butvp1not. And makingmyPropertyan agregate is not an option.– nVxx
Nov 10 at 11:35
Oops. It should be
: m_valuestd::forward<P>(p)..., otherwisemyProperty<int> ip1;doesn't compile. "vp2 case will compile but vp1 not" Does it mean you want bothvp1andvp2to compile, not onlyvp2?– HolyBlackCat
Nov 10 at 11:38
@HolyBlackCat, actually no, I don't want vp1 to compile :) My bad, didn't test your suggestion thoroughly, assumed something like
vp4myVec3Objetwon't compile also, but it does, so this looks good :) Would accept the answer if you post it.– nVxx
Nov 10 at 11:52
1
Turns out it breaks copy-construction (because it's a better match than
myProperty(const myProperty &)if the parameter is non-const). I'll post an answer if I figure out how to fix that in a neat way.– HolyBlackCat
Nov 10 at 12:07