12.1 Structures

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Suppose you are writing an inventory program for a warehouse. The warehouse is filled with bins , each containing a bunch of parts. All the parts in a bin are identical, so you don't have to worry about mixed bins or partials .

For each bin you need to know:

• The name of the part it holds (30-character string).

• The quantity on hand (integer).

• The price (integer cents ).

In previous chapters you have used arrays for storing a group of similar data types, but in this example you have a mixed bag: two integers and a string.

Instead of an array, you will use a new data type called a structure. In an array, all the elements are of the same type and are numbered. In a structure, each element, or member, is named and has its own data type.

The general form of a structure definition is:

 struct   structure-name   {   member-type     member-name;   //   Comment     member-type     member-name   ;  //   Comment   . . . .  }   variable-name   ; 

For example, say you want to define a bin to hold printer cables. The structure definition is:

 struct bin {      char    name[30];   // Name of the part     int     quantity;   // How many are in the bin     int     cost;       // The cost of a single part (in cents) } printer_cable_box;    // Where we put the print cables 

This definition actually tells C++ two things. The first is what a struct bin looks like. This statement defines a new data type that can be used in declaring other variables . This statement also declares the variable printer_cable_box . Since the structure of a bin has been defined, you can use it to declare additional variables:

 struct bin terminal_cable_box;  // Place to put terminal cables 

The structure-name part of the definition may be omitted:

 struct {      char    name[30];   // Name of the part     int     quantity;   // How many are in the bin     int     cost;       // The cost of a single part (in cents) } printer_cable_box;    // Where we put the print cables 

The variable printer_cable_box is still to be defined, but no data type is created. The data type for this variable is an anonymous structure .

The variable-name part also may be omitted. This would define a structure type but no variables:

 struct bin {      char    name[30];   // Name of the part     int     quantity;   // How many are in the bin     int     cost;       // The cost of a single part (in cents) }; 

In an extreme case, both the variable-name and the structure-name parts may be omitted. This creates a section of correct but totally useless code.

Once the structure type has been defined you can use it to define variables:

 struct bin printer_cable_box; // Define the box holding printer cables 

C++ allows the struct to be omitted, so you can use the following declaration:

 bin printer_cable_box; // Define the box holding printer cables 

You have defined the variable printer_cable_box containing three named members : name , quantity, and cost . To access them you use the syntax:

   variable   .   member   

For example, if you just found out that the price of the cables went up to $12.95, you would do the following:

 printer_cable_box.cost = 1295;   // .95 is the new price 

To compute the value of everything in the bin, you can simply multiply the cost by the number of items using the following:

 total_cost = printer_cable_box.cost * printer_cable_box.quantity; 

Structures may be initialized at declaration time by putting the list of elements in curly braces ({ }):

 /*   * Printer cables   */  struct bin {      char    name[30];   // Name of the part     int     quantity;   // How many are in the bin     int     cost;       // The cost of a single part (in cents) }; struct bin printer_cable_box = {      "Printer Cables",   // Name of the item in the bin     0,                  // Start with empty box     1295                // Cost -- .95 }; 

The definition of the structure bin and the variable printer_cable_box can be combined in one step:

 struct bin {      char    name[30];   // Name of the part     int     quantity;   // How many are in the bin     int     cost;       // The cost of a single part (in cents) } printer_cable_box = {      "Printer Cables",   // Name of the item in the bin     0,                  // Start with empty box     1295                // Cost -- .95 }; 

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