Programming in D for C++ Programmers

Every experienced C++ programmer accumulates a series of idioms and techniques which become second nature. Sometimes, when learning a new language, those idioms can be so comfortable it's hard to see how to do the equivalent in the new language. So here's a collection of common C++ techniques, and how to do the corresponding task in D.

Defining constructors

The C++ Way

class Foo
{
    Foo(int x);
};

The D Way

class Foo
{
    this(int x) { }
}

Base class initialization

The C++ Way

class A { A() {... } };
class B : A
{
    B(int x)
        : A()   // call base constructor
    {
        ...
    }
};

The D Way

class A { this() { ... } }
class B : A
{
    this(int x)
    {
        ...
        super();    // call base constructor
        ...
    }
}

class A
{
    int a;
    int b;
    this() { a = 7; b = foo(); }
    this(int x)
    {
        this();
        a = x;
    }
}

class A
{
    int a = 7;
    int b;
    this() { b = foo(); }
    this(int x)
    {
        this();
        a = x;
    }
}

Comparing structs

The C++ Way

struct A x, y;
...
x = y;

#include <string.h>

struct A x, y;

inline bool operator==(const A& x, const A& y)
{
    return (memcmp(&x, &y, sizeof(struct A)) == 0);
}
...
if (x == y)
    ...

Note that the operator overload must be done for every struct needing to be compared, and the implementation of that overloaded operator is free of any language help with type checking. The C++ way has an additional problem in that just inspecting the (x == y) does not give a clue what is actually happening, you have to go and find the particular overloaded operator==() that applies to verify what it really does.

There's a nasty bug lurking in the memcmp() implementation of operator==(). The layout of a struct, due to alignment, can have ‘holes’ in it. C++ does not guarantee those holes are assigned any values, and so two different struct instances can have the same value for each member, but compare different because the holes contain different garbage.

To address this, the operator==() can be implemented to do a memberwise compare. Unfortunately, this is unreliable because (1) if a member is added to the struct definition one may forget to add it to operator==(), and (2) floating point nan values compare unequal even if their bit patterns match.

The D Way

A x, y;
...
if (x == y)
    ...

Creating a new typedef'd type

The C++ Way

#define HANDLE_INIT     ((Handle)(-1))
typedef void *Handle;
void foo(void *);
void bar(Handle);

Handle h = HANDLE_INIT;
foo(h);         // coding bug not caught
bar(h);         // ok

#define HANDLE_INIT     ((void *)(-1))
struct Handle
{
    void *ptr;

    // default initializer
    Handle() { ptr = HANDLE_INIT; }

    Handle(int i) { ptr = (void *)i; }

    // conversion to underlying type
    operator void*() { return ptr; }
};
void bar(Handle);

Handle h;
bar(h);
h = func();
if (h != HANDLE_INIT)
    ...

The D Way

alias Handle = Typedef!(void*, cast(void*)-1);
void bar(Handle);

Handle h;
bar(h);
h = func();
if (h != Handle.init)
    ...

Friends

The C++ Way

class A
{
  private:
    int a;

  public:
    int foo(B *j);
    friend class B;
    friend int abc(A *);
};

class B
{
  private:
    int b;

  public:
    int bar(A *j);
    friend class A;
};

int A::foo(B *j) { return j->b; }
int B::bar(A *j) { return j->a; }

int abc(A *p) { return p->a; }

The D Way

module X;

class A
{
  private:
    static int a;

  public:
    int foo(B j) { return j.b; }
}

class B
{
  private:
    static int b;

  public:
    int bar(A j) { return j.a; }
}

int abc(A p) { return p.a; }

Operator overloading

The C++ Way

struct A
{
    int operator <  (int i);
    int operator <= (int i);
    int operator >  (int i);
    int operator >= (int i);
};

int operator <  (int i, A &a) { return a >  i; }
int operator <= (int i, A &a) { return a >= i; }
int operator >  (int i, A &a) { return a <  i; }
int operator >= (int i, A &a) { return a <= i; }

The D Way

struct A
{
    int opCmp(int i);
}

The compiler automatically interprets all the <, <=, > and >= operators in terms of the cmp function, as well as handling the cases where the left operand is not an object reference.

Similar sensible rules hold for other operator overloads, making using operator overloading in D much less tedious and less error prone. Far less code needs to be written to accomplish the same effect.

Namespace using declarations

The C++ Way

namespace foo
{
    int x;
}
using foo::x;

The D Way

/** Module foo.d **/
module foo;
int x;

/** Another module **/
import foo;
alias x = foo.x;

RAII (Resource Acquisition Is Initialization)

The C++ Way

class File
{
    Handle *h;

    ~File()
    {
        h->release();
    }
};

The D Way

The bulk of resource release problems are simply keeping track of and freeing memory. This is handled automatically in D by the garbage collector. The second common resources used are semaphores and locks, handled automatically with D's synchronized declarations and statements.

The few RAII issues left are handled by structs. A struct gets its destructor run when it goes out of scope.

struct File
{
    Handle h;

    ~this()
    {
        h.release();
    }
}

void test()
{
    if (...)
    {
        auto f = File();
        ...
    } // f.~this() gets run at closing brace, even if
      // scope was exited via a thrown exception
}

classes are typically managed by the garbage collector which doesn't lend itself to RAII. If you need deterministic destruction with classes you can use std.typecons.scoped (which will also allocate the class on the stack instead of the garbage collector managed heap).

See also ScopeGuardStatement for a more generalized mechanism that lets you run arbitrary statements whenever leaving the current scope.

Properties

The C++ Way

class Abc
{
  public:
    void setProperty(int newproperty) { property = newproperty; }
    int getProperty() { return property; }

  private:
    int property;
};

Abc a;
a.setProperty(3);
int x = a.getProperty();

The D Way

class Abc
{
    // set
    @property void property(int newproperty) { myprop = newproperty; }

    // get
    @property int property() { return myprop; }

  private:
    int myprop;
}

Abc a = new Abc;
a.property = 3;
int x = a.property;

Recursive Templates

The C++ Way

template<int n> class factorial
{
  public:
    enum { result = n * factorial<n - 1>::result };
};

template<> class factorial<1>
{
  public:
    enum { result = 1 };
};

void test()
{
    printf("%d\n", factorial<4>::result); // prints 24
}

The D Way

template factorial(int n)
{
    enum factorial = n * .factorial!(n-1);
}

template factorial(int n : 1)
{
    enum factorial = 1;
}

void test()
{
    writeln(factorial!(4));  // prints 24
}

enum factorial(int n) = n * .factorial!(n-1);

enum factorial(int n : 1) = 1;

Meta Templates

The C++ Way

This example is simplified and adapted from one written by Dr. Carlo Pescio in Template Metaprogramming: Make parameterized integers portable with this novel technique.

There is no way in C++ to do conditional compilation based on the result of an expression based on template parameters, so all control flow follows from pattern matching of the template argument against various explicit template specializations. Even worse, there is no way to do template specializations based on relationships like "less than or equal to", so the example uses a clever technique where the template is recursively expanded, incrementing the template value argument by one each time, until a specialization matches. If there is no match, the result is an unhelpful recursive compiler stack overflow or internal error, or at best a strange syntax error.

#include <limits.h>

template< int nbits > struct Integer
{
    typedef Integer< nbits + 1 > :: int_type int_type ;
};

struct Integer< 8 >
{
    typedef signed char int_type ;
};

struct Integer< 16 >
{
    typedef short int_type ;
};

struct Integer< 32 >
{
    typedef long int_type ;
};

struct Integer< 64 >
{
    typedef long long int_type ;
};

// If the required size is not supported, the metaprogram
// will increase the counter until an internal error is
// signaled, or INT_MAX is reached. The INT_MAX
// specialization does not define a int_type, so a
// compiling error is always generated
struct Integer< INT_MAX >
{
};

// A bit of syntactic sugar
#define Integer( nbits ) Integer< nbits > :: int_type

#include <stdio.h>

int main()
{
    Integer( 8 ) i ;
    Integer( 16 ) j ;
    Integer( 29 ) k ;
    Integer( 64 ) l ;
    printf("%d %d %d %d\n",
        sizeof(i), sizeof(j), sizeof(k), sizeof(l));
    return 0 ;
}

The C++ Boost Way

#include <boost/mpl/if.hpp>
#include <boost/mpl/assert.hpp>

template <int nbits> struct Integer
    : mpl::if_c<(nbits <= 8), signed char
    , mpl::if_c<(nbits <= 16), short
    , mpl::if_c<(nbits <= 32), long
    , long long>::type >::type >
{
    BOOST_MPL_ASSERT_RELATION(nbits, <=, 64);
}

#include <stdio.h>

int main()
{
    Integer< 8 > i ;
    Integer< 16 > j ;
    Integer< 29 > k ;
    Integer< 64 > l ;
    printf("%d %d %d %d\n", sizeof(i), sizeof(j), sizeof(k), sizeof(l));
    return 0 ;
}

The D Way

import std.stdio;

template Integer(int nbits)
{
    static if (nbits <= 8)
        alias Integer = byte;
    else static if (nbits <= 16)
        alias Integer = short;
    else static if (nbits <= 32)
        alias Integer = int;
    else static if (nbits <= 64)
        alias Integer = long;
    else
        static assert(0);
}

int main()
{
    Integer!(8) i ;
    Integer!(16) j ;
    Integer!(29) k ;
    Integer!(64) l ;
    writefln("%d %d %d %d", i.sizeof, j.sizeof, k.sizeof, l.sizeof);
    return 0;
}

Type Traits

The C++ Way

template<typename T> class IsFunctionT
{
  private:
    typedef char One;
    typedef struct { char a[2]; } Two;
    template<typename U> static One test(...);
    template<typename U> static Two test(U (*)[1]);
  public:
    enum { Yes = sizeof(IsFunctionT<T>::test<T>(0)) == 1 };
};

void test()
{
    typedef int (fp)(int);

    assert(IsFunctionT<fp>::Yes == 1);
}

The D Way

template IsFunctionT(T)
{
    static if ( is(T[]) )
        const int IsFunctionT = 0;
    else
        const int IsFunctionT = 1;
}

void test()
{
    alias int fp(int);

    assert(IsFunctionT!(fp) == 1);
}

void test()
{
    alias int fp(int);

    assert( is(fp == function) );
}

Interfaces

Interfaces in C++

class Interface {
  public:
    virtual int method() = 0;
};

class FirstVariant : public Interface {
  public:
    int method() {
      ...
    }
};

class SecondVariant : public Interface {
  public:
    int method() {
      ...
    }
};

FirstVariant FirstVariant;
SecondVariant SecondVariant;

FirstVariant.method();
SecondVariant.method();

Interfaces in D

interface Interface {
  int method();
}

class FirstVariant : Interface {
  int method() {
    ...
  }
}

class SecondVariant : Interface {
  int method() {
    ...
  }
}

FirstVariant FirstVariant = new FirstVariant();
SecondVariant SecondVariant = new SecondVariant();

FirstVariant.method();
SecondVariant.method();

References in D

module test;

void main()
{
    struct Data
    {
        int id;
    }

    import std.container.array : Array;
    Array!Data gallery;

    Data d1;
    gallery.insertBack(d1);

    auto d2 = gallery[0];
    d2.id = 1;
    assert(d2.id == gallery[0].id, "neither ref nor pointer");
}

    auto d2 = &gallery[0];