Another Technique for Assigning a Zero Value to All Members of a Struct

In addition to the technique described in tip #453, “Assigning A Zero Value to All Members of A Struct”, C++ (but not C) offers another method of assigning all the members of a struct to binary zeros at once. This method applies to small structs that occupy up to 8 bytes (10 bytes on platforms that support type long double).

This technique makes use of an anonymous union, which is an ordinary union except that it has no tag name and no instance name. The trick is to create an anonymous union that contains an instance of that struct and another built-in data type that occupies exactly the same size as that struct:

   struct Date  {    char day;    char month;    short year;  }; // Date occupies 4 bytes  int main()  {    union  //anonymous    {      Date d;      int num;    };  //use d    d.day = 13; d.month = 4; d.year = 2000; //now assign all members of d to binary zeros through num    num = 0;  }

Inside main(), the programmer defined an anonymous union that contains a Date object and an int. After filling d’s data members with values, the program resets these data members by assigning the value of 0 to num. This assignment actually resets all the bits of d to zeros, because d and num are stored at the same memory address and occupy the same size. Note that you use the members of an anonymous union directly, as if they were declared outside of a union. This convenience of referring to the union’s members directly cannot be achieved when using a named union.

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