![\"TimelineCpp20\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2017\/02\/TimelineCpp20.png\")
<\/p>\nDesignated initialization is an extension of aggregate initialization and empowers you to directly initialize the members of a class type using their names.<\/p>\n
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Designated initialization is a special case of aggregate initialization. Writing about designated initialization means, therefore, writing about aggregate initialization.<\/p>\n
First: what is an aggregate? Aggregates are arrays and class types. A class type is a class<\/span>, a struct<\/span>, or a union<\/span>.<\/p>\n With C++20, the following condition must hold class types:<\/p>\n The next program exemplifies aggregate initialization.<\/p>\n <\/p>\n <\/p>\n <\/p>\n (1) and (2) directly initialize the aggregates using curly braces. The sequence of the initializers in the curly braces has to match the declaration order of the members.<\/p>\n Based on aggregate initialization in C++11, we get designed initializers in C++20. So far, only the Microsoft compiler support designated initializers completely.<\/p>\n<\/p>\n Designated initializers enable it to initialize members of a class type using their name directly. For a union, only one initializer can be provided. As for aggregate initialization, the sequence of initializers in the curly braces has to match the declaration order of the members.<\/p>\n <\/p>\n <\/p>\n <\/p>\n (1) and (2) use designated initializers to initialize the aggregates. The initializers, such as .x<\/span> or .y<\/span> are often called designators.<\/p>\n <\/p>\n The members of the aggregate can already have a default value. This default value is used when the initializer is missing. This does not hold for a union.<\/p>\n <\/p>\n <\/p>\n <\/p>\n (1) initializes all members, but (2) does not provide a value for member x<\/span>. Consequently, x <\/span>is not initialized. It is fine if you only initialize the members who don’t have a default value such as in (3) or (5). The expression (4) would not compile because z<\/span> and y<\/span> are in the wrong order.<\/p>\n Designated initializers detect narrowing conversion. Narrowing conversion is a conversion of a value, including the loss of its precision.<\/p>\n <\/p>\n\n
\n\/\/ aggregateInitialization.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n\r\nstruct<\/span> Point2D{\r\n int<\/span> x; \r\n int<\/span> y;\r\n};\r\n\r\nclass<\/span> Point3D<\/span>{\r\npublic:<\/span>\r\n int<\/span> x;\r\n int<\/span> y;\r\n int<\/span> z;\r\n};\r\n\r\nint<\/span> main<\/span>(){\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n \r\n Point2D point2D{1<\/span>, 2<\/span>}; \/\/ (1)<\/span>\r\n Point3D point3D{1<\/span>, 2<\/span>, 3<\/span>}; \/\/ (2)<\/span>\r\n\r\n std::<\/span>cout <<<\/span> \"point2D: \"<\/span> <<<\/span> point2D.x <<<\/span> \" \"<\/span> <<<\/span> point2D.y <<<\/span> std::<\/span>endl;\r\n std::<\/span>cout <<<\/span> \"point3D: \"<\/span> <<<\/span> point3D.x <<<\/span> \" \"<\/span> <<<\/span> point3D.y <<<\/span> \" \"<\/span> <<<\/span> point3D.z <<<\/span> std::<\/span>endl;\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n}\r\n<\/pre>\n<\/div>\n![\"aggregateInitialization\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/07\/aggregateInitialization.png\")
<\/p>\nDesignated Initializers<\/h2>\n
\n\/\/ designatedInitializer.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n\r\nstruct<\/span> Point2D{\r\n int<\/span> x;\r\n int<\/span> y;\r\n};\r\n\r\nclass<\/span> Point3D<\/span>{\r\npublic:<\/span>\r\n int<\/span> x;\r\n int<\/span> y;\r\n int<\/span> z;\r\n};\r\n\r\nint<\/span> main<\/span>(){\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n \r\n Point2D point2D{.x =<\/span> 1<\/span>, .y =<\/span> 2<\/span>}; \/\/ (1)<\/span>\r\n Point3D point3D{.x =<\/span> 1<\/span>, .y =<\/span> 2<\/span>, .z =<\/span> 3<\/span>}; \/\/ (2)<\/span>\r\n\r\n std::<\/span>cout <<<\/span> \"point2D: \"<\/span> <<<\/span> point2D.x <<<\/span> \" \"<\/span> <<<\/span> point2D.y <<<\/span> std::<\/span>endl;\r\n std::<\/span>cout <<<\/span> \"point3D: \"<\/span> <<<\/span> point3D.x <<<\/span> \" \"<\/span> <<<\/span> point3D.y <<<\/span> \" \"<\/span> <<<\/span> point3D.z <<<\/span> std::<\/span>endl;\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n}\r\n<\/pre>\n<\/div>\n![\"designatedInitializer\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/07\/designatedInitializer.png\")
<\/p>\n\n\/\/ designatedInitializersDefaults.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n\r\nclass<\/span> Point3D<\/span>{\r\npublic:<\/span>\r\n int<\/span> x;\r\n int<\/span> y =<\/span> 1<\/span>; \r\n int<\/span> z =<\/span> 2<\/span>;\r\n};\r\n\r\nvoid<\/span> needPoint<\/span>(Point3D p) {\r\n std::<\/span>cout <<<\/span> \"p: \"<\/span> <<<\/span> p.x <<<\/span> \" \"<\/span> <<<\/span> p.y <<<\/span> \" \"<\/span> <<<\/span> p.z <<<\/span> std::<\/span>endl;\r\n}\r\n\r\nint<\/span> main<\/span>(){\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n \r\n Point3D point1{.x =<\/span> 0<\/span>, .y =<\/span> 1<\/span>, .z =<\/span> 2<\/span>}; \/\/ (1)<\/span>\r\n std::<\/span>cout <<<\/span> \"point1: \"<\/span> <<<\/span> point1.x <<<\/span> \" \"<\/span> <<<\/span> point1.y <<<\/span> \" \"<\/span> <<<\/span> point1.z <<<\/span> std::<\/span>endl;\r\n \r\n Point3D point2; \/\/ (2)<\/span>\r\n std::<\/span>cout <<<\/span> \"point2: \"<\/span> <<<\/span> point2.x <<<\/span> \" \"<\/span> <<<\/span> point2.y <<<\/span> \" \"<\/span> <<<\/span> point2.z <<<\/span> std::<\/span>endl;\r\n \r\n Point3D point3{.x =<\/span> 0<\/span>, .z =<\/span> 20<\/span>}; \/\/ (3)<\/span>\r\n std::<\/span>cout <<<\/span> \"point3: \"<\/span> <<<\/span> point3.x <<<\/span> \" \"<\/span> <<<\/span> point3.y <<<\/span> \" \"<\/span> <<<\/span> point3.z <<<\/span> std::<\/span>endl;\r\n \r\n \/\/ Point3D point4{.z = 20, .y = 1}; ERROR \/\/ (4) <\/span>\r\n \r\n needPoint({.x =<\/span> 0<\/span>}); \/\/ (5)<\/span>\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n\r\n}\r\n<\/pre>\n<\/div>\n![\"designatedInitializerDefaults\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2020\/07\/designatedInitializerDefaults.png\")
<\/p>\n\n\/\/ designatedInitializerNarrowingConversion.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n\r\nstruct<\/span> Point2D{\r\n int<\/span> x;\r\n int<\/span> y;\r\n};\r\n\r\nclass<\/span> Point3D<\/span>{\r\npublic:<\/span>\r\n int<\/span> x;\r\n int<\/span> y;\r\n int<\/span> z;\r\n};\r\n\r\nint<\/span> main<\/span>(){\r\n \r\n std::<\/span>cout <<<\/span> std::<\/span>endl;\r\n \r\n Point2D point2D{.x =<\/span> 1<\/span>, .y =<\/span> 2.5<\/span>}; \/\/ (1)<\/span>\r\n Point3D point3D{.x =<\/span> 1<\/span>, .y =<\/span> 2<\/span>, .z =<\/span> 3.5f<\/span>}; \/\/ (2)<\/span>\r\n\r\n std::<\/span>cout <<<\/span> \"point2D: \"<\/span> <<<\/span> point2D.x <<<\/span> \" \"<\/span> <<<\/span> point2D.y <<<\/span> std::<\/span>endl;\r\n std::<\/span>cout <<<\/span> \"point3D: \"<\/span> <<<\/span> point3D.x <<<\/span> \" \"<\/span>