{"id":2596,"date":"2018-07-31T22:22:51","date_gmt":"2018-07-31T14:22:51","guid":{"rendered":"https:\/\/nodelay.xyz\/?p=2596"},"modified":"2018-07-31T22:22:51","modified_gmt":"2018-07-31T14:22:51","slug":"c-template-handle-mutual-circular-dependencies","status":"publish","type":"post","link":"https:\/\/dayandcarrot.space\/?p=2596","title":{"rendered":"C++ Template: Handle mutual (circular) dependencies"},"content":{"rendered":"

Sometimes we have two classes that need to call each other’s somewhat function, for example<\/p>\n

template <typename A>\nstruct B\n{\n    A* p_a_ = nullptr;\n};\ntemplate <typename B>\nstruct A\n{\n    B* p_b_ = nullptr;\n};<\/pre>\n
but as you can see A depends on B’s type to initialize and vise versa. If we declare the types as above we’ll not get a chance to create any instance.
\nThere are at least two ways to resolve the issue.<\/p>\n
I.Template template<\/h3>\n
A quick introduction about template template can be found here<\/a>.
\nDeclare either of {A, B} with template of template which accepts the other’s template argument.<\/p>\n
template <template<typename> typename A_t>\nstruct B\n{\n    using A = A_t<B>;\n    A* p_a_ = nullptr;\n};\ntemplate <typename B>\nstruct A\n{\n    B* p_b_ = nullptr;\n};\n\/\/and declare like this\nusing B_t = B<A>;\nB_t b_instance;\nA<B_t> A_instance;<\/pre>\n
II.Type traits<\/h3>\n
The other resolution is create a type trait struct which acts like a bridge that links the type of each other.<\/p>\n
template <typename foobar_tratis>\nstruct foobar_notifier\n{\n\tusing inst_t = typename foobar_tratis::foobar_inst_t;\n\tinst_t* inst_ = nullptr;\n\tvoid foobar()\n\t{\n\t\tstd::cout << \"foobar_notifier\" << std::endl;\n\t}\n};\ntemplate <typename foobar_tratis>\nstruct foobar_inst\n{\n\tusing notifier_t = typename foobar_tratis::foobar_notifier_t;\n\tnotifier_t* notifier_ = nullptr;\n\tvoid foobar()\n\t{\n\t\tstd::cout << \"foobar_inst\" << std::endl;\n\t}\n};\nstruct foobar_tratis\n{\n\tusing foobar_notifier_t = foobar_notifier<md_inst_traits>;\n\tusing foobar_inst_t = foobar_inst<md_inst_traits>;\n};\n\/\/\/ and declare like this\nfoobar_inst<foobar_tratis> inst2;\nfoobar_notifier<foobar_tratis> notifier2;<\/pre>\n
 <\/p>\n","protected":false},"excerpt":{"rendered":"
Sometimes we have two classes that need to call each other’s somewhat function, for example template <typename A> struct B { A* p_a_ = nullptr; }; template <typename B> struct A { B* p_b_ = nullptr; }; but as you can see A depends on B’s type to initialize and vise versa. If we declare […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[28,29],"class_list":["post-2596","post","type-post","status-publish","format-standard","hentry","category-study","tag-c","tag-c-template"],"_links":{"self":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/posts\/2596","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2596"}],"version-history":[{"count":0,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=\/wp\/v2\/posts\/2596\/revisions"}],"wp:attachment":[{"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2596"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2596"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/dayandcarrot.space\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2596"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}