The C keyword 'struct' can be used to define a hierarchical data structure when parsing a file. Structures can be defined using C/C++ syntax. For example:

    struct myStruct {
        int a;
        int b;
        int c;
    };

See Declaring Template Variables for information on declaring the variables within a struct. Note the semi-colon after the struct definition is required. This syntax would actually generate a new type myStruct, but would not declare any variables until an instance of type myStruct is declared:

    myStruct s;

After this declaration, the Template Results would have an entry 'myStruct s' with a '+' beside it. Clicking the '+' would display the variables a, b, and c beneath.

Instances of structures can be declared at the same time the structure is defined. For example:

    struct myStruct {
        int a;
        int b;
        int c;
    } s1, s2;

would generate two instances of myStruct. s1 would cover the first 12 bytes of the file (4 bytes for each of the 3 integers) and s2 would cover the next 12 bytes of the file.

These structs are more powerful than typical C structs since they may contain control statements such as if, for, or while. For example:

    struct myIfStruct {
        int a;
        if( a > 5 )
            int b;
        else
            int c;
    } s;

In this example, when s is declared only two variables are generated: a, and one of b or c. Remember that templates are executed as an interpreter would, evaluating each line before stepping to the next. The value of a is read directly from the current file.

Structures can be nested and array of structures can also be declared. For example:

    struct {
        int width;

        struct COLOR {
            uchar r, g, b;
        } colors[width];

    } line1;

Note that forward-declared structs are supported and structs can even be nested recursively. Typedefs can be used with structs as an alternate way to define a structure. For example:

    typedef struct {
        ushort id;
        int    size;
    } 
    myData;

Unions

    union myUnion {
        ushort s;
        double d;
        int    i;
    } u;

all three variables would be read starting from the same position and the size of the union would be 8 bytes to contain the double.