D Programming Language
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# Deprecated Features

Sometimes it becomes clear that a feature is just a bad idea. These are called deprecated features and once they are judged to merit removal from the language, they follow a procedure in order to allow plenty of time for users to adjust to the change.

Deprecated Features
Feature | Spec | Warn | Dep | Error | Gone
--- | --- | --- | --- | --- | ---
Cast T[] to integral type | ? | N/A | 2.060 | 2.061 |
Base Class Protection | 2.058 | N/A | 2.058 |  |
Using length in index expressions | ? | N/A | 2.041 | 2.061 |
typedef | 2.057 | N/A | 2.057 |  |
Variable shadowing inside functions | ? | N/A | 0.161 | 2.061 |
invariant as an alias for immutable | 2.057 | N/A | 2.057 |  |
Using * to dereference arrays | ? | N/A | 2.057 |  |
Removing an item from an associative array with delete | ? | N/A | 0.127 | 2.061 |
.offset property | ? | N/A | 0.107 | 2.061 |
.size property | ? | N/A | 0.107 | 0.107 | 2.061
Escape string literals | ? | N/A | 2.026 | 2.061 |
Lower case 'l' suffix for integer literals | ? | N/A | 1.054 | 0.174 | 2.061
Octal literals | 2.054 | N/A | 2.053 |  |
Upper case 'I' suffix for imaginary literals | ? | N/A | 0.154 | 2.061 |
HTML source files | ? | N/A | 2.013 | N/A | 2.061
.typeinfo property | ? | N/A | 0.093 | 2.061 |
C-style function pointers | ? | N/A | 2.050 |  |
C-style array pointers | ? | N/A | 2.050 |  |
if (v; e) | ? | N/A | 0.149 | 2.061 |
volatile | 2.013 | N/A | 2.013 |  |
Non-final switch statements without a default case | 2.054 | N/A | 2.054 |  |
Hiding base class functions | 2.054 | N/A | 2.054 |  |
Windows 3.x and Windows 9x support | 2.058 | N/A | N/A | N/A | 2.058
.min property for floating point types | N/A | 2.061 |  |  |
imaginary and complex types | future |  |  |  |
floating point NCEG operators | future |  |  |  |
.sort and .reverse properties for arrays | future |  |  |  |
delete | future |  |  |  |
overriding without override | future |  |  |  |
scope for allocating classes on the stack | future |  |  |  |

Spec
  Removal from the Specification
Warn
  The compiler emits a warning if the feature is used
Dep
  The compiler issues an error if not compiled with the -d switch
Error
  It is an error to use the feature
Gone
  The feature is completely gone

### Cast T[] to integral type

At some point in time you could do:

    ulong u = cast(ulong)[1,2];

To get the length of the array.

#### Corrective Action

Use the .length property instead.

#### Rationale

Using a cast to get the length of an array is just confusing.

### Base Class Protection

Base class protections are things like:

    class A : protected B {
       ...
    }

#### Corrective Action

Delete the protection attribute keyword from in front of the base class and base interfaces.

#### Rationale

With D's module system, it doesn't seem to serve any useful purpose, and has never worked correctly.

### Using length in index expressions

When used inside an indexing or slicing expression, length is rewritten to be the length of the array being sliced.

    auto a = new int[5];
    a = a[0..length-1];

#### Corrective Action

Replace length with the equivalent '$'

#### Rationale

The implicitly defined length variable shadows existing declarations, and is less concise than the alternative.

### typedef

typedef can be used to construct a strongly-typed alias of a type.

    typedef int myint;
    static assert(!is(myint == int));

#### Corrective Action

Replace use of typedef with alias or use std.typecons.Typedef.

#### Rationale

typedef is not flexible enough to cover all use cases. This is better done with a library solution.

### Variable shadowing inside functions

Variable shadowing is when a variable in an inner scope has the same name as a variable in an enclosing scope.

    void myFun()
    {
        int var;
        if (x)
        {
            int var;
            var = 3; // which var was meant?
        }
    }

#### Corrective Action

Rename shadowing variables so they don't conflict.

#### Rationale

Variable shadowing can introduce hard to find bugs where the wrong variable is modified.

### invariant as an alias for immutable

The invariant storage class and type modifier is an alias for immutable.

    static assert(is(invariant(int) == immutable(int)));

#### Corrective Action

Replace all uses of invariant as a storage class or type modifier with immutable. The invariant() syntax for struct and class invariants is still supported.

#### Rationale

The alias is unnecessary.

### Using * to dereference arrays

D array variables can be dereferenced to get the first element, much like pointers.

    int[] arr = [1, 2, 3];
    assert(*arr == 1);

#### Corrective Action

Use indexing syntax to access first member.

    int[] arr = [1, 2, 3];
    assert(arr[0] == 1);

#### Rationale

D arrays are not pointers.

### Removing an item from an associative array with delete

delete can be used to remove an item from an associative array.

    int[int] aa = [1 : 2];
    delete aa[1];
    assert(1 !in aa);

#### Corrective Action

Use .remove instead.

    int[int] aa = [1 : 2];
    aa.remove(1);
    assert(1 !in aa);

#### Rationale

The delete syntax is confusing and conflicts with the normal delete syntax.

### .offset property

The .offset property can be used to get member offset information.

    struct S { int a, b; }
    static assert(S.b.offset == 4);

#### Corrective Action

Use .offsetof instead

    struct S { int a, b; }
    static assert(S.b.offsetof == 4);

#### Rationale

The .offset syntax has been superseded by .offsetof

### .size property

The .size property can be used to get type size information

    struct S { int a, b; }
    static assert(S.size == 8);

#### Corrective Action

Use .sizeof instead

    struct S { int a, b; }
    static assert(S.sizeof == 8);

#### Rationale

The .size syntax has been superseded by .sizeof

### Escape string literals

Escape string literals can be used to describe characters using escape sequences.

    // deprecated code
    // string x = "hello" ~ \n;

#### Corrective Action

Put escape sequences inside a regular string literal.

    string x = "hello\n";

#### Rationale

Escape string literals are unintuitive and unnecessary.

### Lower case 'l' suffix for integer literals

Lower case 'l' is an alternative suffix to denote 64 bit integer literals.

    // deprecated code
    // auto x = 123l;

#### Corrective Action

Use the upper case 'L' suffix.

    auto x = 123L;

#### Rationale

The lower case suffix is easily confused with the digit '1'.

### Octal literals

Octal literals can be used to enter literals in base 8.

    // deprecated code
    // auto x = 0123;

#### Corrective Action

Use the octal!num template.

    auto x = octal!123;

#### Rationale

The use of a leading zero is confusing, as 0123 != 123.

### Upper case 'I' suffix for imaginary literals

The 'I' suffix can be used to denote imaginary floating point values.

    // deprecated code
    // auto x = 1.234I;

#### Corrective Action

Use the lower case 'i' suffix.

    auto x = 1.234i;

#### Rationale

The 'I' suffix is easily confused with the digit '1'.

### HTML source files

The D compiler can parse html files by ignoring everything not contained in <code></code> tags.

    <html>
    <code>
        ... source ...
    </code>
    </html>

#### Corrective Action

Extract code to regular source files.

#### Rationale

This has been replaced for documentation by the introduction of ddoc

### .typeinfo property

The .typeinfo property can be used to get the associated TypeInfo class.

    T.typeinfo

#### Corrective Action

Use typeid() instead

    typeid(T)

#### Rationale

The .typeinfo syntax has been superseded by typeid()

### C-style function pointers

C-style function pointers can be used in D.

    alias void(*fptr)(int, long);

#### Corrective Action

Replace with D-style function pointers.

    alias void function(int, long) fptr;

#### Rationale

The D syntax is much cleaner and easier to use.

### C-style array pointers

C-style array pointers can be used in D.

    alias float *arrayptr[10][15];

#### Corrective Action

Replace with D-style array pointers.

    alias float[15][10]* arrayptr;

#### Rationale

The D syntax is much cleaner and easier to use.

### if (v; e)

This syntax can be used to declare a variable in an if statement condition.

    if (v; calculateAndReturnPointer()) { ... }

#### Corrective Action

Replace with an auto declaration.

    if (auto v = calculateAndReturnPointer()) { ... }

#### Rationale

The syntax is clearer with auto.

### volatile

volatile can be used to mark statement, in order to prevent some compiler optimizations.

    volatile
    {
        ... do something involving ghared variables ...
    }

#### Corrective Action

Convert the code to use synchronized statements instead.

#### Rationale

volatile statements are a misfeature.

### Non-final switch statements without a default case

Switch statements can be declared without a default case, and the compiler automatically adds one.

    switch(a)
    {
    case 1:
        break;
    case 2:
        break;
    // the compiler adds
    // default:
    //     throw new SwitchError();
    }

#### Corrective Action

Add the default case manually.

    switch(a)
    {
    case 1:
        break;
    case 2:
        break;
    default:
        assert(0);
    }

#### Rationale

Missing default cases can hide bugs, and making the default case explicit should be mandatory.

### Hiding base class functions

Declaration functions in a derived class that can be called with the same arguments as a function in a base class, but do not override that function causes the base class function to be hidden.

    class A
    {
        void fun(int x) {}
    }
    class B : A
    {
        void fun(long x) {}
    }

#### Corrective Action

Add the function to the base class, or use an alias to bring the base class overload into the derived class.

    class A
    {
        void fun(int x) {}
        void fun(long x) {} // this fixes it
    }
    class B : A
    {
        void fun(long x) {}
        alias A.fun fun; // so does this
    }

#### Rationale

This is an error that is already detected at runtime, and is being extended to compile time.

### Windows 3.x and Windows 9x support

There is some code in Phobos for Windows 3.x/9x support.

#### Corrective Action

Upgrade Windows or switch to another supported OS.

#### Rationale

Supporting such outdated and rarely used OS-es isn't worth the trouble.

### .min property for floating point types

Floating point types have the .min property to access the smallest value.

    enum m = real.min;

#### Corrective Action

Replace with .min_normal

    enum m = real.min_normal;

#### Rationale

The name min_normal is more accurate, as .min does not include denormalized floating point values.

### imaginary and complex types

D currently supports imaginary and complex versions of all floating point types.

    float a = 2;
    ifloat b = 4i;
    cfloat c = a + b;
    assert(c == 2 + 4i);

#### Corrective Action

Use the library types in std.complex.

#### Rationale

These types are too specialized to be a part of the core language.

### floating point NCEG operators

D currently supports the NCEG floating point operators (!<>=, <>, <>=, !>, !>=, !<, !<=, !<>) for comparisons involving NaNs.

#### Corrective Action

Use the normal operators and std.math.isNaN.

#### Rationale

These operators are too specialized to be a part of the core language.

### .sort and .reverse properties for arrays

D arrays can be manipulated using these built-in properties.

    int[] x = [2, 3, 1];
    assert(x.sort == [1, 2, 3]);

#### Corrective Action

Use the generic functions in std.algorithm.

#### Rationale

These operations are better implemented in the standard library.

### delete

Memory allocated on the GC heap can be freed with delete.

    auto a = new Class();
    delete a;

#### Corrective Action

Use object.destroy() instead.

    auto a = new Class();
    destroy(a);

#### Rationale

delete makes assumptions about the type of garbage collector available that limits which implementations can be used, and can be replaced by a library solution.

### overriding without override

Virtual functions can currently override a function in a base class without the 'override' attribute.

    class A
    {
        void fun() {}
    }
    class B : A
    {
        // overrides but is not marked with override
        void fun() {}
    }

#### Corrective Action

Mark overriding functions with 'override'

    class A
    {
        void fun() {}
    }
    class B : A
    {
        override void fun() {}
    }

#### Rationale

Making the 'override' attribute mandatory makes it explicit, and can catch errors when a base class function is accidentally overridden.

### scope for allocating classes on the stack

The scope keyword can be used to allocate class objects on the stack:

    class A
    {
        int x;
        this(int x) { this.x = x; }
    }

    void main()
    {
        A obj;
        {
            scope A a = new A(1);
            obj = a;
        }
        assert(obj.x == 1);  // fails, 'a' has been destroyed
    }

#### Corrective Action

Use scoped from std.typecons

    class A
    {
        this(int x) { }
    }
    void main()
    {
        A a = scoped!A(1);
    }

#### Rationale

scope was an unsafe feature. A reference to a scoped class could easily be returned from a function without errors, which would make using such an object undefined behavior due to the object being destroyed after exiting the scope of the function it was allocated in. To discourage it from general-use but still allow usage when needed a library solution was implemented.

Note that scope for other usages (e.g. scoped variables) is unrelated to this feature and will not be deprecated.

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