Initialize class' template (aggregate type) member with aggregate initializer but without extra parenthesis

up vote
4
down vote

favorite

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.

asked Nov 10 at 10:55

nVxx

244212

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)...) .
– 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)...), 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)..., 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

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

add a comment |

up vote
4
down vote

favorite

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.

asked Nov 10 at 10:55

nVxx

244212

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)...) .
– 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)...), 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)..., 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

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

add a comment |

up vote
4
down vote

favorite

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.

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

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

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)...) .
– 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)...), 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)..., 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

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

add a comment |

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)...) .
– 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)...), 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)..., 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

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

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)...) .
– 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)...), 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)..., 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

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)...

edited Nov 10 at 12:37

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
yesterday

add a comment |

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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).

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)...

edited Nov 10 at 12:37

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
yesterday

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).

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)...

edited Nov 10 at 12:37

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
yesterday

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).

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)...

edited Nov 10 at 12:37

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

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).

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)...

edited Nov 10 at 12:37

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

edited Nov 10 at 12:37

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

answered Nov 10 at 12:30

HolyBlackCat

14.6k23361

Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
yesterday

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
yesterday

Yet only an upvote, to have some time to experiment with this solution and also keep the question open for alternative solutions.
– nVxx
yesterday

add a comment |

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