Suppose you have a class declare in a namespace, like this:<\/p>\n
\r\n#pragma once\r\n\r\nnamespace Test\r\n{\r\n\tclass foo\r\n\t{\r\n\tpublic:\r\n\t\tfoo(void);\r\n\t\t~foo(void);\r\n\t};\r\n};\r\n<\/pre>\n\r\n#include \"foo.h\"\r\n\r\nnamespace Test\r\n{\r\n\tfoo::foo(void)\r\n\t{\r\n\t}\r\n\r\n\tfoo::~foo(void)\r\n\t{\r\n\t}\r\n};\r\n<\/pre>\nAnd you want to use that foo class in a another class, with a forward declaration:<\/p>\n
\r\n#pragma once\r\n\r\nclass foo;\r\n\r\nclass bar\r\n{\r\n\tfoo * m_foo;\r\npublic:\r\n\tbar(void);\r\n\t~bar(void);\r\n};\r\n<\/pre>\n\r\n#include \"bar.h\"\r\n\r\n#include \"foo.h\"\r\nusing namespace Test;\r\n\r\nbar::bar(void)\r\n{\r\n\tm_foo = new foo;\r\n}\r\n\r\nbar::~bar(void)\r\n{\r\n}\r\n<\/pre>\nMost likely you’ll get some errors, like this (issues by VS2008 compiler)<\/p>\n
\nerror C2872: ‘foo’ : ambiguous symbol
\nerror C2512: ‘foo’ : no appropriate default constructor available\n<\/p><\/blockquote>\n
The problem is that for the compiler it is ambiguous whether that foo and Test::foo are the same or not. To remove the ambiguity you should specify that the forward declared foo type is the same with the one from the namespace Test. The best way to do that is like this:<\/p>\n
\r\nnamespace Test\r\n{\r\n\tclass foo;\r\n};\r\n\r\nclass bar\r\n{\r\n\tTest::foo * m_foo;\r\npublic:\r\n\tbar(void);\r\n\t~bar(void);\r\n};\r\n<\/pre>\nThe only thing that I should mention is that approach is not allowed for the types from namespace std, as Herb Sutter explains here<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"Suppose you have a class declare in a namespace, like this: #pragma once namespace Test { class foo { public: foo(void); ~foo(void); }; }; #include “foo.h” namespace Test { foo::foo(void) { } foo::~foo(void) { } }; And you want to use that foo class in a another class, with a forward declaration: #pragma once class … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","advgb_blocks_editor_width":"","advgb_blocks_columns_visual_guide":"","_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":false,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[7],"tags":[],"class_list":["post-99","post","type-post","status-publish","format-standard","hentry","category-c"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t