Template Comparison - D Programming Language
Description: Comparison of templates between the D programming language and C++
Keywords: D Programming Language, template metaprogramming, variadic templates, type deduction, dependent base class

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# Template Comparison

C++ pioneered templates and template metaprogramming and has continued to improve on it. The D programming language is the first to comprehensively reengineer templates based on the C++ experience.

Template Comparison Table
Feature | D | C++
--- | --- | ---
Argument list delineation | Uses !( ), as in Foo!(int). Can omit parens when the argument is a single lexical token: Foo!int | Uses < > as in Foo<int>
Class Templates | Yes:
    class Foo(T)
    {
        T x;
    }
| Yes:
    template<class T>
    class Foo
    {
        T x;
    };
Function Templates | Yes:
    T foo(T)(T i)
    {
        ...
    }
| Yes:
    template<class T>
    T foo(T i)
    {
        ...
    }
Member Templates | Yes | Yes
Constructor Templates | No | Yes
Parameterize any Declaration | Yes, classes, functions, typedefs, variables, enums, etc. can be parameterized, such as this variable:
    template Foo(T)
    {
        static T* p;
    }
| C++98 No, only classes and functions C++11 Yes:
    template<class T>
    constexpr T pi = T(3.1415926535897932385L);
Template Typedefs: Create an alias that binds to some but not all of the template parameters | Yes:
    class Foo(T, U) { }
    template MyFoo(T)
    {
        alias MyFoo = Foo!(T, int);
    }
    MyFoo!(uint) f;
| C++98 No C++11 Yes:
    template<class T, class U> class Foo { };
    template<class T> using MyFoo = Foo<T, int>;
    MyFoo<unsigned> f;
Sequence Constructors | No | C++98 No C++11 Yes:
    template <class T>
    class Foo {
        Foo(std::initializer_list);
    };

    Foo<double> f = { 1.2, 3.0, 6.8 };
Concepts | Yes: Constraints | C++98 No C++20 Yes: Constraints and Concepts
Recursive Templates | Yes:
    template factorial(int n)
    {
        const factorial = n * factorial!(n-1);
    }
    template factorial(int n : 1)
    {
        const factorial = 1;
    }
| Yes:
    template<int n> class factorial
    {
      public:
        enum { result = n * factorial<n-1>::result };
    };
    template<> class factorial<1>
    {
      public:
        enum { result = 1 };
    };
Conditional Compilation based on Template Arguments | Yes:
    template void foo(T)(T i)
    {
      static if (can_fast_foo!(T))
        FastFoo f = fast_foo(i);
      else
        SlowFoo f = slow_foo(i);
      use_foo(f);
    }

    class HashTable(T, int maxLength)
    {
        static if (maxLength < 0xFFFE)
            alias CellIdx = ushort;
        else
            alias CellIdx = uint;
        CellIdx index;
    }
| C++98 No, but workarounds exist:
    template<class T> void foo(T i)
    {
      // Differentiate using a
      // Helper specialization
      Helper>::use_foo(i);
    };

    template<class T, int maxLength> class HashTable {
        typedef typename std::conditional<
                maxLength < 0xFFFE, uint16_t, uint32_t>
            ::type CellIdx;
        CellIdx index;
    };
C++17 Yes, but is limited to block scope:
    template<class T> void foo(T i)
    {
      if constexpr (can_fast_foo)
        FastFoo foo = fast_foo(i);
      else
        SlowFoo foo = slow_foo(i);
      use_foo(foo); // error
    }

    template<class T, int maxLength> class HashTable {
        // error
        if constexpr (maxLength < 0xFFFE)
            using CellIdx = ushort;
        else
            using CellIdx = uint;
        CellIdx index;
    };
Template Declarations (with no definition) | No | Yes:
    template<class T>
    class Foo;
Grouping templates with the same parameters together | Yes:
    template Foo(T, U)
    {
        class Bar { ... }
        T foo(T t, U u) { ... }
    }
    Foo!(int,long).Bar b;
    return Foo!(char,int).foo('c',3);
| Sort of, using a class' members:
    template<class T, class U>
    class Foo {
        class Bar { ... };
        static T foo(T t, U u) { ... }
    };
    Foo::bar b;
    return Foo::foo('c', 3);
Compile time execution of functions | Yes:
    int factorial(int i)
    {
        if (i == 0)
            return 1;
        else
            return i * factorial(i - 1);
    }
    static f = factorial(6);
| C++98 No C++11 Yes, but only for constexpr functions. constexpr was highly restricted in C++11, but each subsequent standard has eased these restrictions.
Parameters | D | C++
Type Parameters | Yes:
    class Foo(T)
    {
        T x;
    }
    Foo!(int) f;
| Yes:
    template<class T>
    class Foo
    {
        T x;
    };
    Foo<int> f;
Integral Parameters | Yes:
    void foo(int i)()
    {
        int v = i;
    }
| Yes:
    template<int i>
    void foo()
    {
        int v = i;
    }
Pointer Parameters | Yes, a pointer to object or function | Yes, a pointer to object or function
Reference Parameters | No, D does not have a general reference type | Yes:
    template<double& D>
    void foo()
    {
        double y = D;
    }
Pointer to Member Parameters | No, D does not have pointers to members. It does have delegates however, which can be used as parameters | Yes
Template Template Parameters | Yes:
    class Foo(T, alias C)
    {
        C!(T) x;
    }
| Yes:
    template<class T,
             template<class U> class C>
    class Foo
    {
        C<T> x;
    };
Alias Parameters | Yes, any symbol can be passed to a template as an alias:
    void bar(int);
    void bar(double);
    void foo(T, alias S)(T t)
    {
        S(t);
    }
    // calls bar(double)
    foo!(double, bar)(1);
| No
Floating Point Parameters | Yes:
    class Foo(double D)
    {
        double x = D;
    }
    ...
    Foo!(1.6) F;
| C++98 No C++11 Yes:
    template<float f>
    void foo()
    {
        int v = f;
    }
String Parameters | Yes:
    void foo(char[] format)(int i)
    {
        writefln(format, i);
    }
    ...
    foo!("i = %s")(3);
| C++98 No C++17 Only indirectly:
    template <const char*>
    struct S {};

    S<"Foo"> foo1; error
    const char foo_str[] = "foo";
    S<foo_str> foo2; // Literal types are allowed, including arrays.
Local Class Parameters | Yes | C++98 No C++17 Yes
Local Variable Parameters | Yes | No
Parameter Default Values | Yes:
    class Foo(T = int)
    {
        T x;
    }
| Yes:
    template<class T = int>
    class Foo
    {
        T x;
    };
Variadic Parameters | Yes, Variadic Templates:
    void print(A...)(A a)
    {
        foreach(t; a)
            writeln(t);
    }
| C++98 No C++11 Yes:
    // Need a zero- or one-argument version
    // to handle the final argument.
    void print() {};

    template <class Arg, class... Args>
    void print(const Arg& arg, Args&&... args)
    {
        writeln(arg);
        print(std::forward<Args>(args)...);
    }
Note that the example above was improved somewhat in C++17 using Fold Expressions:
    template <class Args...>
    void print(Args...&& args)
    {
        (writeln(std::forward<Args.>(args)), ...);
    }
Specializations | D | C++
Explicit Specialization | Yes:
    class Foo(T : int)
    {
        T x;
    }
| Yes:
    template<>
    class Foo<int>
    {
        int x;
    };
Partial Specialization | Yes:
    class Foo(T : T*, U)
    {
        T x;
    }
| Yes:
    template<class T, class U>
    class Foo<T*, U>
    {
        T x;
    };
Partial specialization derived from multiple parameters | Yes:
    class Foo(T : Bar!(T, U), U)
    {
        ...
    }
| Yes:
    template<class T, class U>
    class Foo< Bar<T,U> >
    {
        ...
    };
Can specializations exist without a primary template? | Yes | No
Other | D | C++
Exported Templates | Yes, it falls out as a natural consequence of modules | C++98 Yes, but was only support in compilers based on EDG's front end. C++11 Was removed from the language due to a lack of support.
SFINAE (Substitution Failure Is Not An Error) | No | Yes
Parse Template Definition Bodies before Instantiation | Yes | Not required by the Standard, but some implementations do
Overloading Function Templates with Functions | No, but the equivalent can be done with explicitly specialized templates:
    void foo(T)(T t) { }
    void foo(T:int)(int t) { }
| Yes:
    template<class T>
    void foo(T i) { }
    void foo(int t) { }
Implicit Function Template Instantiation | Yes | Yes
Templates can be evaluated in scope of instantiation rather than definition | Yes, Template Mixins | No, but can be faked using macros
Can extract arguments of template instance | Yes:
    class Foo(T)
    {
        static if (is(T x : T!A, A...))
        {
            pragma(msg, A);  // (int, float)
        }
    }

    struct Bar(T1, T2) { }
    alias BarInst = Bar!(int, float);
    Foo!(BarInst) f;
See is expressions. | No
Parsing Idiosyncracies | D | C++
Context-Free Grammar | Yes:
    class Foo(int i)
    {
        ...
    }
    Foo!(3 > 4) f;
| No:
    template<int i> class Foo
    {
        ...
    };
    Foo<3 > 4> f; // error
Distinguish template arguments from other operators | Yes:
    class Foo(T)
    {
        ...
    }
    class Bar(int i)
    {
        ...
    }
    Foo!(Bar!(1)) x1;
| C++98 No:
    template<class T> class Foo
    {
        ...
    };
    template<int i> class Bar
    {
        ...
    };
    Foo<Bar<1>> x1; // error
    Foo<Bar<1> > x2;
C++11 Partially fixed by Right Angle Brackets N1757
Redeclaration of Template Parameter | Yes:
    class Foo(T)
    {
        int T;
        void foo()
        {
            int T;
        }
    }
| No:
    template<class T>
    class Foo
    {
        int T; // error
        void foo()
        {
            int T; // error
        }
    };
Dependent Base Class Lookup | Yes:
    class Foo(T)
    {
        alias A = int;
    }
    class Bar(T) : Foo(T)
    {
        A x;
    }
| No:
    template<class T>
    class Foo
    {
      public:
        typedef int A;
    };
    template<class T>
    class Bar : Foo<T>
    {
      public:
        A x; // error
    };
Forward Referencing | Yes:
    int g(void *);

    class Foo(T)
    {
        int foo()
        {
            return g(1);
        }
    }

    int g(int i);
| No:
    int g(void *);

    template<class T>
    class Foo
    {
        int foo()
        {
            return g(1); // error
        }
    };

    int g(int i);
Member templates parseable without hints | Yes:
    class Foo
    {
        Foo bar!(int I)();
    }
    void abd(T)(T f)
    {
        T f1 = f.bar!(3)();
    }
| No:
    class Foo
    {
      public:
        template<int> Foo *bar();
    };
    template<class T> void abc(T *f)
    {
        T *f1 = f->bar<3>(); // error
        T *f2 = f->template bar<3>();
    }
Dependent type members parseable without hints | Yes:
    class Foo(T)
    {
        T.A* a1;
    }
| No:
    template<class T> class Foo
    {
      public:
        T::A *a1; // error
        typename T::A *a2;
    };

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