What's the idea behind templates?, C++ FAQ

C++ FAQ / Section 35 / FAQ 35.1

Section 35:

35.1	What's the idea behind templates?
35.2	What's the syntax / semantics for a "class template"?
35.3	What's the syntax / semantics for a "function template"?
35.4	How do I explicitly select which version of a function template should get called?
35.5	What is a "parameterized type"?
35.6	What is "genericity"?
35.7	My template function does something special when the template type `T` is `int` or `std::string`; how do I write my template so it uses the special code when `T` is one of those specific types?
35.8	Huh? Can you provide an example of template specialization that doesn't use `foo` and `bar`?
35.9	But most of the code in my template function is the same; is there some way to get the benefits of template specialization without duplicating all that source code?
35.10	All those templates and template specializations must slow down my program, right?
35.11	So templates are overloading, right?
35.12	Why can't I separate the definition of my templates class from its declaration and put it inside a .cpp file?
35.13	How can I avoid linker errors with my template functions? Updated!
35.14	How does the C++ keyword `export` help with template linker errors? Updated!
35.15	How can I avoid linker errors with my template classes? Updated!
35.16	Why do I get linker errors when I use template friends?
35.17	How can any human hope to understand these overly verbose template-based error messages?
35.18	Why am I getting errors when my template-derived-class uses a nested type it inherits from its template-base-class?
35.19	Why am I getting errors when my template-derived-class uses a member it inherits from its template-base-class?
35.20	Can the previous problem hurt me silently? Is it possible that the compiler will silently generate the wrong code?
35.21	How can I create a container-template that allows my users to supply the type of the underlying container that actually stores the values?
35.22	Follow-up to previous: can I pass in the underlying structure and the element-type separately?
35.23	Related: all those proxies must negatively reflect on the speed of my program. Don't they?

[35.1] What's the idea behind templates?

A template is a cookie-cutter that specifies how to cut cookies that all look pretty much the same (although the cookies can be made of various kinds of dough, they'll all have the same basic shape). In the same way, a class template is a cookie cutter for a description of how to build a family of classes that all look basically the same, and a function template describes how to build a family of similar looking functions.

Class templates are often used to build type safe containers (although this only scratches the surface for how they can be used).