История изменений

ЕМНИП, в Си можно обращаться к неинициализированным компонентам union. Т.е., в Си это вроде как легально

Не совсем.

6.5.2.3 Structure and union members

95. If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a trap representation.

Т.е. может рвануть, а может и нет. Сишный стандарт вообще жопой писан. Но в самом доступе UB нет, если нет проблем с выравниванием и подобным. Зависит от типов внутри union.

ЕМНИП, в Си можно обращаться к неинициализированным компонентам union. Т.е., в Си это вроде как легально

Не совсем.

6.5.2.3 Structure and union members

95. If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a trap representation.

Т.е. может рвануть, а может и нет. Сишный стандарт вообще жопой писан.