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As explained at the start of this chapter, the STL supports (and extensively utilizes) function objects Function objects are simply classes that define operator( ) The STL provides many built-in function objects, such as less, minus, etc It also allows you to define your own function objects Frankly, it is beyond the scope of this book to fully describe all of the issues surrounding the creation and use of function objects Fortunately, as the preceding examples have shown, you can make significant use of the STL without ever creating a function object However, since function objects are a main ingredient of the STL, it is important to have a general understanding

This change ensures that any modifications made to the current item are taken into account when the application is closed. Note that there is also an EndEdit method in the DataGrid class, which performs similar functionality on the data grid control. Our approach is more general, and applies to all edits on any control related to the given binding manager. Compile and run the application and make sure this works correctly. Figure 17.8 shows the application with the message dialog for saving a change displayed.

Just as there are unary and binary predicates, there are unary and binary function objects A unary function object requires one argument; a binary function object requires two You must use the type of object required For example, if an algorithm is expecting a binary function object, you must pass it a binary function object

Figure 17.8 The Save Changes dialog permits the user to save any changes made to the individual controls.

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The STL provides a rich assortment of built-in function objects The binary function objects are shown here plus equal_to less_equal minus not_equal_to logical_and multiplies greater logical_or divides greater_equal modulus less

This completes our discussion on data binding. We end with the usual summary of our accomplishments.

The function objects perform the operations specified by their names The only one that may not be self-evident is negate( ), which reverses the sign of its argument The built-in function objects are template classes that overload operator( ), which returns the result of the specified operation on whatever type of data you select For example, to invoke the binary function object plus(), use this syntax

plus<float>()

In this chapter we investigated the concept of data binding and constructed a new MyAlbumData application. We began with the DataGrid class and saw how to create and fill a data grid, and how to customize the contents and appearance of the grid for a specific type of data. We then looked at the IEditableObject interface as a way to support transactional updates to bound data. We implemented this interface in our Photograph class in order to track and save any changes made by a user. Binding to data grids is referred to as complex data binding. We also examined simple data binding, used to bind a single property of a control to a value in a data source. We created a TabPage object in our application to hold a set of controls related to a Photograph object, and bound properties of our controls to the active Photograph in a PhotoAlbum collection. We finished our chapter by examining the update and storage of data sources as the user interacts with the application in general and the bound controls in particular. While the examples here did not use the System.Data namespace, the binding of data grids and controls to database objects was discussed along the way in order to provide some insight into how such binding might be performed.