The use of templates as a metaprogramming technique requires two distinct operations: a template must be defined, and a defined template must be instantiated. The template definition describes the generic form of the generated source code, and the instantiation causes a specific set of source code to be generated from the generic form in the template.
Though the syntax of template metaprogramming is usually very different from the programming language it is used with, it has practical uses. Some common reasons to use templates are to implement generic programming (avoiding sections of code which are similar except for some minor variations) or to perform automatic compile-time optimization such as doing something once at compile time rather than every time the program is run — for instance, by having the compiler unroll loops to eliminate jumps and loop count decrements whenever the program is executed.
What exactly "programming at compile-time" means can be illustrated with an example of a factorial function, which in non-template C++ can be written using recursion as follows:
The code above will execute at run time to determine the factorial value of the literals 0 and 4. By using template metaprogramming and template specialization to provide the ending condition for the recursion, the factorials used in the program—ignoring any factorial not used—can be calculated at compile time by this code:
The code above calculates the factorial value of the literals 0 and 4 at compile time and uses the results as if they were precalculated constants. To be able to use templates in this manner, the compiler must know the value of its parameters at compile time, which has the natural precondition that factorial<X>::value can only be used if X is known at compile time. In other words, X must be a constant literal or a constant expression.
The factorial example above is one example of compile-time code optimization in that all factorials used by the program are pre-compiled and injected as numeric constants at compilation, saving both run-time overhead and memory footprint. It is, however, a relatively minor optimization.
However, take care and exercise caution as this may cause code bloat as separate unrolled code will be generated for each 'N'(vector size) you instantiate with.
Polymorphism is a common standard programming facility where derived objects can be used as instances of their base object but where the derived objects' methods will be invoked, as in this code
Another similar use is the "Barton–Nackman trick", sometimes referred to as "restricted template expansion", where common functionality can be placed in a base class that is used not as a contract but as a necessary component to enforce conformant behaviour while minimising code redundancy.
The idea behind this is that the struct Helper recursively inherits from a struct with one more template argument (in this example calculated as INDEX * INDEX) until the specialization of the template ends the recursion at a size of 10 elements. The specialization simply uses the variable argument list as elements for the array. The compiler will produce code similar to the following (taken from clang called with -Xclang -ast-print -fsyntax-only).
To show a more sophisticated example the code in the following listing has been extended to have a helper for value calculation (in preparation for more complicated computations), a table specific offset and a template argument for the type of the table values (e.g. uint8_t, uint16_t, ...).* Specialization of the template to end the recursion when the table size reaches TABLE_SIZE.
Concepts allow programmers to specify requirements for the type, to make instantiation of template possible. The compiler looks for a template with the concept that has the highest requirements.
Here is an example of the famous Fizz buzz problem solved with Template Meta Programming.* By specializing `res` structure, with concepts requirements, proper instantation is performed std::tuple<number<1ul>, number<2ul>, Fizz, number<4ul>, Buzz, Fizz, number<7ul>, number<8ul>, Fizz, Buzz, number<11ul>, Fizz, number<13ul>, number<14ul>, FizzBuzz, number<16ul>, number<17ul>, Fizz, number<19ul>, Buzz, Fizz, number<22ul>, number<23ul>, Fizz, Buzz, number<26ul>, Fizz, number<28ul>, number<29ul>, FizzBuzz, number<31ul>, number<32ul>, Fizz, number<34ul>, Buzz, Fizz, number<37ul>, number<38ul>, Fizz, Buzz, number<41ul>, Fizz, number<43ul>, number<44ul>, FizzBuzz, number<46ul>, number<47ul>, Fizz, number<49ul>, Buzz, Fizz, number<52ul>, number<53ul>, Fizz, Buzz, number<56ul>, Fizz, number<58ul>, number<59ul>, FizzBuzz, number<61ul>, number<62ul>, Fizz, number<64ul>, Buzz, Fizz, number<67ul>, number<68ul>, Fizz, Buzz, number<71ul>, Fizz, number<73ul>, number<74ul>, FizzBuzz, number<76ul>, number<77ul>, Fizz, number<79ul>, Buzz, Fizz, number<82ul>, number<83ul>, Fizz, Buzz, number<86ul>, Fizz, number<88ul>, number<89ul>, FizzBuzz, number<91ul>, number<92ul>, Fizz, number<94ul>, Buzz, Fizz, number<97ul>, number<98ul>, Fizz, Buzz>