{"id":110365,"date":"2024-10-11T07:00:00","date_gmt":"2024-10-11T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=110365"},"modified":"2024-10-11T13:50:29","modified_gmt":"2024-10-11T20:50:29","slug":"20241011-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20241011-00\/?p=110365","title":{"rendered":"How can I explicitly specialize a templated C++ constructor?"},"content":{"rendered":"

C++ allows constructors to be templated, but there is no syntax for explicitly specializing the constructor. Here’s a rather artificial example:<\/p>\n

\/\/ Assume derived classes by convention have a constructor\r\n\/\/ whose first parameter is an ObjectManager&.\r\nstruct CommonBase\r\n{\r\n    virtual ~CommonBase(){}\r\n    virtual void initialize(int reason) = 0;\r\n};\r\n\r\nstruct ObjectManager\r\n{\r\n    \/\/ Concrete should derive from CommonBase\r\n    template<typename Concrete, typename...Args>\r\n    ObjectManager(int reason, Args&&...args) :\r\n        m_base(std::make_unique<Concrete>(\r\n                *this, std::forward<Args>(args)...))\r\n    {\r\n        m_base->initialize(reason);\r\n    }\r\n\r\n    std::unique_ptr<CommonBase> m_base;\r\n};\r\n<\/pre>\n
The idea here is that you have some type, and you want to templatize the constructor. It is legal to have a templated constructor, but there is no way to explicitly specialize a constructor.<\/p>\n
struct Widget : CommonBase\r\n{\r\n    Widget(int param);\r\n    \u27e6 ... \u27e7\r\n};\r\n\r\n\/\/ This is not allowed\u00b9\r\nauto manager = ObjectManager::ObjectManager<Widget>(42);\r\n<\/pre>\n
So how do you tell the constructor, “I want you to use this type for Concrete<\/code>?”<\/p>\n
Your only option is type inference, so you’ll have to make it inferrable from a parameter.<\/p>\n
Enter std::in_place_type<\/code> and friends.<\/p>\n
We start with std::in_place_type_t<\/code>, which is an empty type that takes a single type as a template parameter. You can use this as a dummy parameter and deduce the template type parameter from it.<\/p>\n
struct ObjectManager\r\n{\r\n    \/\/ Concrete should derive from CommonBase\r\n    template<typename Concrete, typename...Args>\r\n    ObjectManager(int reason,\r\n        std::in_place_type_t<Concrete>,<\/span>\r\n        Args&&...args) :\r\n        m_base(std::make_unique<Concrete>(\r\n                *this, std::forward<Args>(args)...))\r\n    {\r\n        m_base->initialize(reason);\r\n    }\r\n\r\n    std::unique_ptr<CommonBase> m_base;\r\n};\r\n\r\n\/\/ Example usage:\r\nauto manager = ObjectManager(9, std::in_place_type_t<Derived>{}, 42);\r\n<\/pre>\n
The in_place_type_t<\/code> is an empty class that is default-constructible. As a convenience, the standard library also defines a premade value:<\/p>\n
template<T>\r\ninline constexpr std::in_place_type_t in_place_type{};\r\n<\/pre>\n
Which lets you simplify the usage to<\/p>\n
auto manager = ObjectManager(9, std::in_place_type<Derived>, 42);\r\n<\/pre>\n
Note that there is no member type type<\/code> inside the std::in_place_type_t<\/code>, so you have to use deduction to pull it out. You can’t say<\/p>\n
    \/\/ Concrete should derive from CommonBase\r\n    template<typename Trait<\/span>, typename...Args>\r\n    ObjectManager(int reason,\r\n        Trait<\/span>,\r\n        Args&&...args) :\r\n        m_base(std::make_unique<typename Trait::type<\/span>>(\r\n                *this, std::forward<Args>(args)...))\r\n    {\r\n        m_base->initialize(reason);\r\n    }\r\n<\/pre>\n
You might be tempted to use std::type_identity<\/code>\u00b2 as the type holder:<\/p>\n
    \/\/ Concrete should derive from CommonBase\r\n    template<typename Concrete, typename...Args>\r\n    ObjectManager(int reason,\r\n        std::type_identity<\/span><Concrete>,\r\n        Args&&...args) :\r\n        m_base(std::make_unique<Concrete>(\r\n                *this, std::forward<Args>(args)...))\r\n    {\r\n        m_base->initialize(reason);\r\n    }\r\n<\/pre>\n
but that is not allowed.<\/p>\n
According to the C++ standard, std::type_identity<\/code> is a Cpp17TransformationTrait, and  [meta.rqmts]<\/a> spells out the requirements of various trait types in the standard library.<\/p>\n\n\n\n\n\n\n
Trait<\/th>\nConstructible?<\/th>\nCopyable?<\/th>\nSpecial member<\/th>\n<\/tr>\n
Cpp17UnaryTypeTrait<\/td>\nYes<\/td>\nYes<\/td>\nvalue<\/code><\/td>\n<\/tr>\n
Cpp17BinaryTypeTrait<\/td>\nYes<\/td>\nYes<\/td>\nvalue<\/code><\/td>\n<\/tr>\n
Cpp17TransformationTrait<\/td>\nNo<\/td>\nNo<\/td>\ntype<\/code><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Since a Cpp17TransformationTrait is not constructible, and the language does not provide any pre-made instances, there is no legal way of gaining access to an instance of a Cpp17TransformationTrait. An implemention would be within its rights to define type_identity<\/code> as<\/p>\n
template<typename T>\r\nstruct type_identity\r\n{\r\n    using type = T;\r\n\r\n    \/\/ not constructible\r\n    type_identity() = delete;\r\n\r\n    \/\/ not copyable\r\n    type_identity(type_identity const&) = delete;\r\n}\r\n<\/pre>\n
\u00b9 Another place you cannot specialize a templated function is operator overloading.<\/p>\n
struct ObjectMaker\r\n{\r\n    ObjectMaker(std::string name) : m_name(std::move(name)) {}\r\n\r\n    template<typename Concrete>\r\n    Concrete operator()() { return Concrete(m_name); }\r\n\r\n    std::string m_name;\r\n};\r\n\r\nvoid sample()\r\n{\r\n    ObjectMaker maker(\"adam\");\r\n\r\n    \/\/ You can't do this\r\n    auto thing1 = maker<Thing1>();\r\n    auto thing2 = maker<Thing2>();\r\n}\r\n<\/pre>\n
You have to use more cumbersome syntax to specialize the overloaded operator:<\/p>\n
void sample()\r\n{\r\n    ObjectMaker maker(\"adam\");\r\n\r\n    \/\/ You have to write it like this\r\n    auto thing1 = maker.operator()<Thing1>();\r\n    auto thing2 = maker.operator()<Thing2>();\r\n}\r\n<\/pre>\n
It’s cumbersome, but at least it’s possible.<\/p>\n
But if you’re going to do that, you may as well  give it a name<\/a>:<\/p>\n
struct ObjectMaker\r\n{\r\n    ObjectMaker(std::string name) : m_name(std::move(name)) {}\r\n\r\n    template<typename Concrete>\r\n    Concrete make<\/span>() { return Concrete(m_name); }\r\n\r\n    std::string m_name;\r\n};\r\n\r\nvoid sample()\r\n{\r\n    ObjectMaker maker(\"adam\");\r\n\r\n    auto thing1 = maker.make<\/span><Thing1>();\r\n    auto thing2 = maker.make<\/span><Thing2>();\r\n}\r\n<\/pre>\n
\u00b2 For further reading:  What’s the deal with std::type_identity<\/code><\/a>?”<\/p>\n","protected":false},"excerpt":{"rendered":"
Looking for a C++ tag type for representing another type.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-110365","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Looking for a C++ tag type for representing another type.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/110365","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=110365"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/110365\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=110365"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=110365"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=110365"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}