Template metaprogramming (TMP) is a metaprogramming technique in which templates are used by a compiler to generate temporary source code, which is merged by the compiler with the rest of the source code and then compiled. The output of these templates can include compile-time constants, data structures, and complete functions. The use of templates can be thought of as compile-time polymorphism. The technique is used by a number of languages, the best-known being C++, but also Curl, D, Nim, and XL.
Template metaprogramming was, in a sense, discovered accidentally.
Some other languages support similar, if not more powerful, compile-time facilities (such as Lisp macros), but those are outside the scope of this article.
Components of template metaprogramming
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 generic form of the generated source code is described in the template definition, and when the template is instantiated, the generic form in the template is used to generate a specific set of source code.
Template metaprogramming is Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram.
Templates are different from macros. A macro is a piece of code that executes at compile time and either performs textual manipulation of code to-be compiled (e.g. C++ macros) or manipulates the abstract syntax tree being produced by the compiler (e.g. Rust or Lisp macros). Textual macros are notably more independent of the syntax of the language being manipulated, as they merely change the in-memory text of the source code right before compilation.
Template metaprograms have no mutable variables— that is, no variable can change value once it has been initialized, therefore template metaprogramming can be seen as a form of functional programming. In fact many template implementations implement flow control only through recursion, as seen in the example below.
Using template metaprogramming
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.
Compile-time class generation
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: unsigned factorial(unsigned n) { return n == 0 ? 1 : n * factorial(n - 1); }
// Usage examples: // factorial(0) would yield 1; // factorial(4) would yield 24. 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: template <unsigned N> struct factorial { static constexpr unsigned value = N * factorial<N - 1>::value; };
template <>
struct factorial<0> {
static constexpr unsigned value = 1; };
// Usage examples: // factorial<0>::value would yield 1; // factorial<4>::value would yield 24. 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.
In C++11 and C++20, constexpr and consteval were introduced to let the compiler execute code. Using constexpr and consteval, one can use the usual recursive factorial definition with the non-templated syntax.
Compile-time code optimization
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.