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.