Chapter 7. Templates

C++ is more than a mere object-oriented programming language. The full power of C++ is seen when programming with templates. Templates lie at the heart of the standard library: strings, I/O streams, containers, iterators, algorithms, and more.

A template is a pattern for creating classes or functions as instances of the template at compile time, similar to the manner in which a class is a pattern for creating objects as instances of the class at runtime. A template takes one or more parameters, and when you instantiate a class or function template, you must supply arguments for the parameters. The classes and functions can have different behaviors or implementations , depending on the arguments. This style of programming is often called generic programming . Templates can also be used to select code at compile time, affect the behavior of generated code, and set policies. This style of programming is often known as template metaprogramming .

Programming with templates is unlike traditional object-oriented programming. Object-oriented programming centers around type polymorphism (requiring classes, objects, and virtual functions). Template-based programming centers around parametric polymorphism , in which a function or class is defined independently of its parameters (which can be values, types, or even other templates).

This chapter describes the syntax and semantics of declaring, specializing , instantiating, and using templates. See Chapter 8 for information about some typical uses of templates in the standard library. See Appendix B for information about template-oriented projects.

The syntax descriptions in this chapter are informal. See Chapter 12 for a precise BNF grammar.