{"id":6230,"date":"2021-09-29T20:27:12","date_gmt":"2021-09-29T20:27:12","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/special-friendship-of-templates\/"},"modified":"2024-07-22T15:21:12","modified_gmt":"2024-07-22T15:21:12","slug":"special-friendship-of-templates","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/special-friendship-of-templates\/","title":{"rendered":"The Special Friendship of Templates"},"content":{"rendered":"

A friend has unrestricted access to the members of a class. Consequently, friendship should be given wisely. Regarding templates, friendship is special.<\/p>\n

<\/p>\n

\"templates\"<\/p>\n

Before I write about the friendship rules for templates, I want to present the general rules about friendship.<\/p>\n

    \n
  1. The friend declaration can be made in any place in the class.<\/li>\n
  2. For friendship, access rights in the class are not considered.<\/li>\n
  3. Friendship is not inherited. When a class grants friendship to a class Derived, a\u00a0 Derived<\/code> derived class is not automatically a friend to Base.<\/li>\n
  4. Friendship is not transitive. When class B<\/code> is a friend of class A and Class C<\/code> is a friend of class B<\/code>, class C<\/code> is not automatically a friend of class A<\/code>.<\/li>\n<\/ol>\n

    A class or a class template can have a friendship to class or class templates, function or function templates, or types.<\/p>\n

    General Friendship<\/h2>\n

    A class or a class template can grant friendship to each instance of a class template or a function template.<\/p>\n

    <\/p>\n

    \n
    \/\/ generalFriendship.cpp<\/span>\n\n#include <iostream><\/span>\n\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>                            \/\/ (1)<\/span><\/em>\nvoid<\/span> myFriendFunction(T);\n\ntemplate<\/span> <<\/span>typename<\/span> U><\/span>                            \/\/ (2)<\/span><\/em>\nclass<\/span> MyFriend<\/span>;\n\nclass<\/span> GrantingFriendshipAsClass<\/span> {\n\n  template<\/span> <<\/span>typename<\/span> U><\/span> friend<\/span> void<\/span> myFriendFunction(U);\n  template<\/span> <<\/span>typename<\/span> U><\/span> friend<\/span> class<\/span> MyFriend<\/span>;\n\n  std::<\/span>string secret{\"Secret from GrantingFriendshipAsClass.\"<\/span>};\n\n};\n\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\nclass<\/span> GrantingFriendshipAsClassTemplate<\/span>{\n\n  template<\/span> <<\/span>typename<\/span> U><\/span> friend<\/span> void<\/span> myFriendFunction(U);\n  template<\/span> <<\/span>typename<\/span> U><\/span> friend<\/span> class<\/span> MyFriend<\/span>;\n\n  std::<\/span>string secret{\"Secret from GrantingFriendshipAsClassTemplate.\"<\/span>};\n\n};\n\ntemplate<\/span> <<\/span>typename<\/span> T>                                \/\/ (3)<\/span><\/em><\/span>\nvoid<\/span> myFriendFunction(T){\n  GrantingFriendshipAsClass myFriend;\n  std::<\/span>cout <<<\/span> myFriend.secret <<<\/span> '\\n'<\/span>;\n\n  GrantingFriendshipAsClassTemplate<<\/span>double<\/span>><\/span> myFriend1;\n  std::<\/span>cout <<<\/span> myFriend1.secret <<<\/span> '\\n'<\/span>;\n}\n\ntemplate<\/span> <<\/span>typename<\/span> T>                              \/\/ (4)<\/span><\/em><\/span>\nclass<\/span> MyFriend<\/span>{\npublic:<\/span>\n  MyFriend(){\n    GrantingFriendshipAsClass myFriend;\n    std::<\/span>cout <<<\/span> myFriend.secret <<<\/span> '\\n'<\/span>;\n\n    GrantingFriendshipAsClassTemplate<<\/span>T><\/span> myFriend1;\n    std::<\/span>cout <<<\/span> myFriend1.secret <<<\/span> '\\n'<\/span>;\n  }\n};\n\nint<\/span> main<\/span>(){\n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n  int<\/span> a{2011<\/span>};\n  myFriendFunction(a);\n\n  MyFriend<<\/span>double<\/span>><\/span> myFriend;\n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre>\n<\/div>\n
    Line (1) and line (2) forward declare the function template myFriendFunction<\/code> and the class template MyFriend.<\/code> The function template myFriendFunction<\/code> is defined in line (3), and the class template MyFriend<\/code> in line (4). The classes GrantingFriendshipAsClass<\/code> and GrantingFriendshipAsClassTemplate<\/code> grant the function template myFriendFunction <\/code>and the class template MyFriend <\/code>friendship. <\/code>Due to the friendship, both templates can directly invoke the private member secrete <\/code>of the class and the class template.<\/p>\n
    \"generalFriendship\"
    \n<\/code><\/p>\n
    There is a pitfall involved in the class template GrantingFriendShipAsClassTemplate<\/code>. Usually, you call the first type parameter of a template T<\/code>. An error occurs when you use – such as in the following code snippet – the same type parameter name for the class template and the function template
    \nor the class template. The name T<\/code> of myFriendFunction<\/code> or MyFriend<\/code> shadows the name T<\/code> of the class template GrantingFriendshipAsClassTemplate<\/code>.<\/p>\n
    The following code snippet displays the pitfall.<\/p>\n
    <\/p>\n
    \n
    template<\/span> <<\/span>typename<\/span> T><\/span>\nclass<\/span> GrantingFriendshipAsClassTemplate<\/span>{\n\n  template<\/span> <<\/span>typename<\/span> T><\/span> friend<\/span> void<\/span> myFriendFunction(T);\n  template<\/span> <<\/span>typename<\/span> T><\/span> friend<\/span> class<\/span> MyFriend<\/span>;\n\n  std::<\/span>string secret{\"Secret from GrantingFriendshipAsClassTemplate.\"<\/span>};\n\n};\n<\/pre>\n<\/div>\n
    Special Friendship<\/h2>\n
    A special friendship is a friendship that depends on the type of template parameter.<\/p>\n
    <\/p>\n
    \n
    \/\/ specialFriendship.cpp<\/span>\n\n#include <iostream><\/span>\n\ntemplate<\/span> <<\/span>typename<\/span> T><\/span> void<\/span> myFriendFunction(T);\ntemplate<\/span> <<\/span>typename<\/span> U><\/span> class<\/span> MyFriend<\/span>;\n\n\nclass<\/span> GrantingFriendshipAsClass<\/span> {\n\n  friend<\/span> void<\/span> myFriendFunction<><\/span>(int<\/span>);             \/\/ (1)<\/span>\n  friend<\/span> class<\/span> MyFriend<\/span><<\/span>int<\/span>><\/span>;                      \/\/ (2)<\/span>\n\nprivate:<\/span>\n  std::<\/span>string secret{\"Secret from GrantingFriendshipAsClass.\"<\/span>};\n\n};\n\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\nclass<\/span> GrantingFriendshipAsClassTemplate<\/span> {\n\n  friend<\/span> void<\/span> myFriendFunction<><\/span>(int<\/span>);\n  friend<\/span> class<\/span> MyFriend<\/span><<\/span>int<\/span>><\/span>;\n  friend<\/span> class<\/span> MyFriend<\/span><<\/span>T><\/span>;                        \/\/ (3)<\/span>\n\nprivate:<\/span>\n  std::<\/span>string secret{\"Secret from GrantingFriendshipAsClassTemplate.\"<\/span>};\n\n};\n\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\nvoid<\/span> myFriendFunction(T) {\n  GrantingFriendshipAsClass myFriend;\n  std::<\/span>cout <<<\/span> myFriend.secret <<<\/span> '\\n'<\/span>;             \/\/ (4)<\/span>\n\n  GrantingFriendshipAsClassTemplate<<\/span>T><\/span> myFriend1;\n  std::<\/span>cout <<<\/span> myFriend1.secret <<<\/span> '\\n'<\/span>;            \/\/ (5)<\/span>\n}\n\ntemplate<\/span> <<\/span>typename<\/span> T>                               \/\/ (6)<\/span><\/span>\nclass<\/span> MyFriend<\/span> {                      \npublic:<\/span>\n  MyFriend() {\n    GrantingFriendshipAsClass myFriend;                 \n    std::<\/span>cout <<<\/span> myFriend.secret <<<\/span> '\\n'<\/span>;\n\n    GrantingFriendshipAsClassTemplate<<\/span>int<\/span>><\/span> myFriendInt;  \n    std::<\/span>cout <<<\/span> myFriendInt.secret <<<\/span> '\\n'<\/span>;\n\n    GrantingFriendshipAsClassTemplate<<\/span>T><\/span> myFriendT;      \n    std::<\/span>cout <<<\/span> myFriendT.secret <<<\/span> '\\n'<\/span>;\n  }\n};\n\nint<\/span> main<\/span>() {\n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n  int<\/span> a{2011<\/span>};\n  myFriendFunction(a);                                    \n\n  MyFriend<<\/span>int<\/span>><\/span> myFriend;                                 \n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre>\n<\/div>\n
    The class GrantingFriendshipAsClass<\/code> grants friendship to the full specialization of the function template myFriendFunction<\/code> for int<\/code> (line 1) and the class template MyFriend<\/code> for int<\/code> (line 2). The same holds for the class template GrantingFrandshipAsClassTemplate<\/code>. Lines (3) is unique because it grants friendship to the full specialization for MyFriend<\/code> having the same type parameter as the class template GrantingFrandshipAsClassTemplate. <\/code>Consequently, the function template myFriendFunction<\/code> can invoke the secret of the class GrantingFriendshipAsClass <\/code>when myFriendFunctions<\/code> is a full specialization for int<\/code> (line 4) or GrantingFriendshipAsClassTemplate<\/code> has the same type, such as myFriendFunction<\/code> (line 5). The corresponding argumentation holds for the class template MyFriend<\/code> (line 6).<\/p>\n
    \"specialFriendship\"<\/p>\n
    Friend to Types<\/h2>\n
    A class template can also grant its friendship to a type parameter.<\/p>\n
    <\/p>\n
    \n
    \/\/ typeFriendship.cpp<\/span>\n\n#include <iostream><\/span>\n\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\nclass<\/span> Bank<\/span> {\n    std::<\/span>string secret{\"Secret from the bank.\"<\/span>};\n    friend<\/span> T;\n};\n\nclass<\/span> Account<\/span>{\n public:<\/span>\n    Account() {\n        Bank<<\/span>Account><\/span> bank;\n        std::<\/span>cout <<<\/span> bank.secret <<<\/span> '\\n'<\/span>;   \/\/ (1)<\/span>\n    }\n};\n\nint<\/span> main<\/span>(){\n\n    std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n    Account acc;\n\n    std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre>\n<\/div>\n
    The class Bank<\/code> grants friendship to its type parameter T. Consequently, an Account<\/code> can access the secret of the bank<\/code> instantiation for Account<\/code>: Bank<Account<\/code>> (line 1).<\/p>\n
    \"typeFriendship\"<\/p>\n
    What’s next?<\/h2>\n
    In my next post, I will write about dependent names, one of the more complicated corners of templates.<\/p>\n","protected":false},"excerpt":{"rendered":"
    A friend has unrestricted access to the members of a class. Consequently, friendship should be given wisely. Regarding templates, friendship is special.<\/p>\n","protected":false},"author":21,"featured_media":6208,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[376],"tags":[439],"class_list":["post-6230","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-templates","tag-friend"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/6230","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=6230"}],"version-history":[{"count":2,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/6230\/revisions"}],"predecessor-version":[{"id":9761,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/6230\/revisions\/9761"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/6208"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=6230"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=6230"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=6230"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}