Editing Template (C++) (section)

== Generic programming features in other languages ==
Initially, the concept of templates was not included in some languages, such as [[Java (programming language)|Java]] and [[C Sharp (programming language)|C#]] 1.0. [[Generics in Java|Java's adoption of generics]] mimics the behavior of templates, but is technically different. C# added generics (parameterized types) in [[.NET Framework|.NET]] 2.0. The generics in Ada predate C++ templates.

Although C++ templates, Java generics, and [[.NET]] generics are often considered similar, generics only mimic the basic behavior of {{nowrap|C++}} templates.<ref>{{cite web| url=http://msdn.microsoft.com/en-us/library/c6cyy67b.aspx| title=Differences Between C++ Templates and C# Generics (C# Programming Guide)| date=12 March 2024}}</ref> Some of the advanced template features utilized by libraries such as [[Boost C++ Libraries|Boost]] and [[STLSoft C++ Libraries|STLSoft]], and implementations of the STL, for [[template metaprogramming]] (explicit or partial specialization, default template arguments, template non-type arguments, template template arguments, ...) are unavailable with generics.

In C++ templates, compile-time cases were historically performed by pattern matching over the template arguments. For example, the template base class in the Factorial example below is implemented by matching 0 rather than with an inequality test, which was previously unavailable. However, the arrival in C++11 of standard library features such as std::conditional has provided another, more flexible way to handle conditional template instantiation.

<syntaxhighlight lang="cpp">

// Induction
template<unsigned N> 
struct Factorial {
  static constexpr unsigned value = N * Factorial<N - 1>::value;
};

// Base case via template specialization:
template<> struct Factorial<0> {
  static constexpr unsigned value = 1;
};
</syntaxhighlight>

With these definitions, one can compute, say 6! at compile time using the expression <code>Factorial<6>::value</code>.

Alternatively, <code>[[C++11#constexpr – Generalized constant expressions|constexpr]]</code> in C++11 / <code>if constexpr</code> in C++17 can be used to calculate such values directly using a function at compile-time:

<syntaxhighlight lang="cpp">
template<unsigned N>
unsigned factorial() {
    if constexpr(N<=1)
        return 1;
    else
        return N * factorial<N-1>();
}
</syntaxhighlight>

Because of this, template meta-programming is now mostly used to do operations on types.