{"id":3367,"date":"2019-03-18T03:17:44","date_gmt":"2019-03-18T01:17:44","guid":{"rendered":"https:\/\/mariusbancila.ro\/blog\/?p=3367"},"modified":"2021-09-17T11:18:30","modified_gmt":"2021-09-17T09:18:30","slug":"little-known-cpp-operator-auto","status":"publish","type":"post","link":"https:\/\/mariusbancila.ro\/blog\/2019\/03\/18\/little-known-cpp-operator-auto\/","title":{"rendered":"Little-known C++: operator auto"},"content":{"rendered":"\n

A user-defined conversion function enables an implicit or explicit conversion between types. Such, a function has the following form (no return type and no parameters):<\/p>\n\n\n\n

struct foo\n{\n   operator int() const {return 42;}\n};\n\nfoo f;\nint i = f;  \/\/ implicit conversion<\/pre>\n\n\n\n
struct foo\n{\n   explicit operator int() const {return 42;}\n};\n\nfoo f;\nint i = static_cast<int>(f);  \/\/ explicit conversion<\/pre>\n\n\n\n
Conversion functions must be non-static but can be virtual. However, instead of specifying an explicit type, you can use the auto<\/code> placeholder to indicate a deduced return type (since C++14). In the following example, the deduced type is int<\/code>.<\/p>\n\n\n\n
struct foo\n{\n   foo(int const d) : data(d) {}\n   operator auto() {return data;}\n   \nprivate: \n   int data;\n};<\/pre>\n\n\n\n
Conversion functions with deduced return type cannot have a trailing return type and cannot be templetized.<\/p>\n\n\n\n
The catch with the deduced type is that if you return a reference, the type that is deduced is not the reference type but the referenced type.<\/p>\n\n\n\n
struct foo\n{\n   foo(int& d) : data(d) {}\n   operator auto() {return data;}  \/\/ deduced type is int\n    \nprivate:\n   int& data;\n};\n\nfoo f;\nint& r = f; \/\/ error: non-const lvalue reference to type 'int' cannot bind to a value of unrelated type 'foo'<\/pre>\n","protected":false},"excerpt":{"rendered":"
A user-defined conversion function enables an implicit or explicit conversion between types. Such, a function has the following form (no return type and no parameters):<\/p>\n
struct foo\r\n{\r\n   operator int() const {return 42;}\r\n};\r\n\r\nfoo f;\r\nint i = f;  \/\/ implicit conversion<\/pre>\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":true,"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":[75,451,334],"class_list":["post-3367","post","type-post","status-publish","format-standard","hentry","category-c","tag-auto","tag-c","tag-operators"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\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\n\t\t\n\t\t