(C++) Template metaprogram

 

Template metaprogramming is a technique to perform calculations during compiling only.

 

There is no semantic difference between class and typename in a template-parameter [1].

 

Below is an example to calculate the factorial of an (unsigned) integer during compile-time. So when the program starts to run, all factorials are already calculated.

 

• Download the Qt Creator project 'CppTemplateMetaprogram' (zip)

 

Other examples:

 

• Compile-time Strategy Design Pattern

 

 

 

 

 

Technical facts

 

Application type(s)

• Desktop application

Operating system(s) or programming environment(s)

• Lubuntu 12.10 (quantal)

IDE(s):

• Qt Creator 2.5.2

Project type:

• Console application

C++ standard:

• C++98

Compiler(s):

• G++ 4.7.2

Libraries used:

• STL: GNU ISO C++ Library, version 4.7.2

 

 

 

 

 

Qt project file: CppTemplateMetaprogram.pro

 

---

TEMPLATE = app CONFIG += console CONFIG -= qt QMAKE_CXXFLAGS += -Wall -Wextra -Weffc++ -Werror SOURCES += main.cpp

 

 

 

 

 

main.cpp

 

---

template <bool> struct CtAssert; template <> struct CtAssert<true> {}; //The template metaprogram for factorial template <unsigned int N> struct factorial {   static unsigned const value = N * factorial<N-1>::value; }; template <> struct factorial<0> {   static unsigned const value = 1; }; int main() {   CtAssert<(factorial<0>::value==1)>();   CtAssert<(factorial<1>::value==1)>();   CtAssert<(factorial<2>::value==2)>();   CtAssert<(factorial<3>::value==6)>();   CtAssert<(factorial<4>::value==24)>();   CtAssert<(factorial<5>::value==120)>(); }

 

 

 

 

 

References

 

1. C++. International Standard. ISO/IEC 14882. Second edition. Paragraph 14.1.2.