How Templates Work: An Analogy

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When you declare and define a template you are creating a generic version of whatever piece of code you are writing, be it a function or a class. In the declaration and definition of the template you will use one or more identifiers as type placeholders. These type placeholders are similar in function to the placeholders in a form letter generated with a word processor.

Figure 15.1 illustrates a simple mail merge operation. A master letter is created with placeholders for certain data elements. The structure of the data source is mapped to the master letter by the placeholder identifiers name and age. When the mail merge function is executed, the data source is merged with the master letter to yield the finished letters. The master letter is a generic document that can be reused to generate many specific letter instances.

Figure 15-1: Placeholder Use In Mail Merge

Class and function templates work in similar fashion. A generic function or class is declared and defined. Placeholders are inserted into the code to reserve spots for actual data types. When specific versions of a template function are required the type substitutions are made based on the types of the arguments used to call the function. In the case of template classes, a special syntax is used when a new template class is declared.

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