C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
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?
[35.2] What's the syntax / semantics for a "class template"?

Consider a container class Array that acts like an array of integers:

// This would go into a header file such as "Array.h"
class Array {
public:
  Array(int len=10)                  : len_(len), data_(new int[len]) { }
 ~Array()                            { delete[] data_; }
  int len() const                    { return len_;     }
  int const& operator[](int i) const { return data_[check(i)]; }   subscript operators often come in pairs
  int&       operator[](int i)       { return data_[check(i)]; }   subscript operators often come in pairs
  Array(Array const&);
  Array& operator= (Array const&);
private:
  int  len_;
  int* data_;
  int  check(int i) const
    {
      if (i < 0 || i >= len_)
        throw BoundsViol("Array", i, len_);
      return i;
    }
};
Repeating the above over and over for Array of float, of char, of std::string, of Array-of-std::string, etc, will become tedious.
// This would go into a header file such as "Array.h"
template<typename T>
class Array {
public:
  Array(int len=10)                : len_(len), data_(new T[len]) { }
 ~Array()                          { delete[] data_; }
  int len() const                  { return len_;     }
  T const& operator[](int i) const { return data_[check(i)]; }
  T&       operator[](int i)       { return data_[check(i)]; }
  Array(const Array<T>&);
  Array<T>& operator= (const Array<T>&);
private:
  int len_;
  T*  data_;
  int check(int i) const
    {
      if (i < 0 || i >= len_)
        throw BoundsViol("Array", i, len_);
      return i;
    }
};
Just as with a normal class, you can optionally define your methods outside the class:
template<typename T>
class Array {
public:
  int len() const;
  ...
};

template<typename T>
inline       see below if you want to make this non-inline
int Array<T>::len() const
{
  ...
}
Unlike template functions, template classes (instantiations of class templates) need to be explicit about the parameters over which they are instantiating:
int main()
{
  Array<int>           ai;
  Array<float>         af;
  Array<char*>         ac;
  Array<std::string>   as;
  Array< Array<int> >  aai;
  ...
}
Note the space between the two >'s in the last example. Without this space, the compiler would see a >> (right-shift) token instead of two >'s.