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

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Having this code:

struct Vec3 {
    int x;
    int y;
    int z;
};

template <typename T>
class myProperty {
public:
   myProperty(const T& initValue) : m_value{initValue} {}
private:
    T m_value;
};

When creating myProperty type object:

myProperty<int> ip{1};
myProperty<Vec3> vp1{{1, 2, 3}};
// myProperty<Vec3> vp2{1, 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++14 templates class constructor c++
1 Answers

A simple solution is to use a variadic template constructor:

template <typename ...P> myProperty(P &&... p) : m_value{std::forward<P>(p)...} {}

It makes myProperty<Vec3> vp2{1, 2, 3}; compile.

Also it stops myProperty<Vec3> vp1{{1, 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(T{std::declval<P>()...});

// ...

template
<
    typename ...P,
    typename = std::enable_if_t<std::experimental::is_detected_v<list_constructible, T, P...>>
>
myProperty(P &&... p) : m_value{std::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(T{std::declval<P>()...});

// ...

template
<
    typename ...P,
    typename = std::enable_if_t<is_detected<void, list_constructible, T, P...>::value>
>
myProperty(P &&... p) : m_value{std::forward<P>(p)...} {}
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