Records and Other Languages

Records - Other Languages

• Java: You know ...


• C and C++: Records are called structs
• Example:

        // Define a struct type (ie a record type)
    struct Pair {int x; int y;};  // Declare type Pair (semi after }!)

    struct Pair pair1, pair2, pair3;    // Declare three Pairs

    pair1.x = 1;

    const int size = 3;    // int const size = 3 also works
    struct Pair a[size];   // Array of Pairs

    // //////////////////////////////////////////////////////
    // A Line is a nested record
    // Typedef defines Line_t as a new name for this type

    typedef struct Line {struct Pair p1; struct Pair p2;} Line_t;

    Line_t l;   //  Declare a Line_t
        

• A c example and prettified


• C++:
• Can also use classes for composite types
• Structs and classes are very similar (ie almost identical)
• Fields are default public in structs, default private in classes
• Keyword struct not needed on variable declarations
• Structs can have functions and constructors, like classes


• Could include some other languages ...

Variant Records in Other Languages

• C and C++ have Union - similar but less or no checking


• Unions do not have to be records - they simply allow sharing of memory locations


• Example:

        // Define a union type
        union mytypes_t {
            int i;
            float f;
            char c[4];
        };

        // Declare whatIsThis, a variable of union type
        union mytypes_t  whatIsThis;

        // These all share the same memory location:
        whatIsThis.i
        whatIsThis.f
        whatIsThis.c 
        

• Let's look at some real programs (programs under construction):
• union1.c which shows a union used for memory sharing and getting around type checking rules (prettified: union1.c.html)
• A union that shows a transaction struct, which is similar to a variant record (union2.c and a prettified version)
• But unions are not type safe: (union3.c and a prettified version)

The Term Union

• A union type is the union of the types that make it up
• In a similar vein, a record is a cross product (ie a set of all tuples)