C++ Templates

C++ has a mechanism called templates to reduce code duplication when supporting numerous data types. A C++ function or C++ class with functions which operates on integers, float and double data types can be unified with a single template function or class with functions which is flexible enough to use all three data types. This mechanism in C++ is called the "Template".

C++ templates fall under the category of "meta-programming" and auto code generation although one never sees the code generated.

In the following example we have a single template represent the code to square a number with the data types int, float and double.

2: 4
2.2: 4.84
2.2: 4.84

2: 4
2.2: 4.84
2.2: 4.84
2.2: 4.84

• The code used in the overloaded C++ functions example above, is repeated for each data type. The templated function is specified once.

• The templated type keyword specifier can be either "class" or "typename":
  • template<class T>
  • template<typename T>
  Both are valid and behave exactly the same. I prefer "typename".

• The templated function works using either the explicit or implicit template expression square<int>(value) or square(value).

• In the template definition, "T" represents the data type. The compiler will generate the type specific functions required. This results in a more compact code base which is easier to maintain.

• The code and logic of the functions only has to be specified once in the templated function and the parameter "T" is used to represent the argument type.

• The template declaration and definition must reside in the same file, typically an include header file.

• A "C" macro function could also perform this purpose: #define square(x) (x * x)
  The advantage of the template is that it performs type checking while the macro does not.

Template Specialization:

The following is an extended example of the square function using template specialization to support type string which requires special handling.

2: 4
AaaAaa

Note that a specialized template was created to handle the string class.

Template specialization is used when a different and specific implementation is to be used for a specific data type.

Multiple Templated Types:

The following is an example of a template supporting multiple types:

Compile: g++ test.cpp
Run: ./a.out

X: 2 4
Y: 2.1 4.41

Non-type parameters:

Non-type template parameters provide the ability to pass a constant expression at compile time. The constant expression may also be an address of a function, object or static class member.
The following is an example of a template function supporting a non-type parameter "count" used for the array size and loop count:

Compile: g++ test.cpp
Run: ./a.out

2.1
3.1
4.1

Specify a default type parameter and default non-type parameter:

Compile: g++ test.cpp -std=c++0x
Run: ./a.out

2.1: 378.228

Note that multIt<> with no type specified, deferred to the default type: float.

[Potential Pitfall]: You must specify the compiler argument -std=c++0x to avoid the following error:
test.cpp:5:13: error: default template arguments may not be used in function templates without -std=c++0x or -std=gnu++0x

The concept of template functions can be extended to template classes.

• A template class takes the form: template <class T> class MyTemplateClass { ... };
• Class template specialization takes the form: template <> class MyTemplateClass <specific-data-type> { ... };

An example of a C++ template class applied to a 2 by 2 matrix:

Compile: g++ -o TestMatrix2x2 TestMatrix2x2.cpp

X:
|1  2|
|3  4|
Y:
|5  6|
|7  8|
A:
|6  8|
|10  12|
B:
|19  22|
|43  50|

Static member variables of a template class:

Shown side by side with a regular C++ class for comparison.

1
1
2

1
1
1.2

Note that each version or "specialization" (data type) of the class will have its own unique copy of the static member. All objects of that specialization will share the same static member. Static member variables are initialized for each specialization/type.

Template template parameters:

1. A templated class may be derived from a regular non-templated C++ base class
2. A regular non-templated C++ class can be derived from a templated base class
3. A templated class may be derived from another templated class

g++ -o testCircle testCircle.cpp
./testCircle
Area: 314.159
Color: white

    (Note that this example uses Color.hpp and Circle.hpp from example one)

g++ -o testSphere testSphere.cpp
./testSphere
Volume: 4188.79
Color: blue

    (Note that this example uses Color.hpp and Circle.hpp from example one)

g++ -o testSphere testSphere.cpp
./testSphere
Volume: 4188.79
Color: blue

Template Argument Deduction refers to the deduction of a template argument type from its implied use.

1. Template argument deduction from the function call expression
2. Template argument deduction from the address of a member function template specialization
3. Template argument deduction from specialization of a member function template

• Curiously Recurring Template Pattern

Books:

	C++ How to Program by Harvey M. Deitel, Paul J. Deitel ISBN #0131857576, Prentice Hall Fifth edition. The first edition of this book (and Professor Sheely at UTA) taught me to program C++. It is complete and covers all the nuances of the C++ language. It also has good code examples. Good for both learning and reference.
	Exceptional C++: 47 Engineering Puzzles, Programming Problems and Solutions by Herb Sutter ISBN #0201615622, Addison-Wesley Professional Advanced C++ features and STL.
	More Exceptional C++ by Herb Sutter ISBN #020170434X, Addison-Wesley Professional
	Effective C++: 50 Specific Ways to Improve Your Programs and Design (2nd Edition) by Scott Meyers ISBN #0201924889, Addison-Wesley Professional
	More Effective C++: 35 New Ways to improve your Programs and Designs by Scott Meyers ISBN #020163371X, Addison-Wesley Professional