D Programming Language
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# Conditional Compilation

Conditional compilation is the process of selecting which code to compile and which code to not compile. (In C and C++, conditional compilation is done with the preprocessor directives #if / #else / #endif.)

    ConditionalDeclaration:
        Condition DeclarationBlock
        Condition DeclarationBlock else DeclarationBlock
        Condition : DeclDefsopt
        Condition DeclarationBlock else : DeclDefsopt

    ConditionalStatement:
        Condition NoScopeNonEmptyStatement
        Condition NoScopeNonEmptyStatement else NoScopeNonEmptyStatement

If the Condition is satisfied, then the following DeclarationBlock or Statement is compiled in. If it is not satisfied, the DeclarationBlock or Statement after the optional else is compiled in.

Any DeclarationBlock or Statement that is not compiled in still must be syntactically correct.

No new scope is introduced, even if the DeclarationBlock or Statement is enclosed by { }.

ConditionalDeclarations and ConditionalStatements can be nested.

The StaticAssert can be used to issue errors at compilation time for branches of the conditional compilation that are errors.

Condition comes in the following forms:

    Condition:
        VersionCondition
        DebugCondition
        StaticIfCondition

### Version Condition

    VersionCondition:
        version ( IntegerLiteral )
        version ( Identifier )
        version ( unittest )
        version ( assert )

Versions enable multiple versions of a module to be implemented with a single source file.

The VersionCondition is satisfied if the IntegerLiteral is greater than or equal to the current version level, or if Identifier matches a version identifier.

The version level and version identifier can be set on the command line by the -version switch or in the module itself with a VersionSpecification, or they can be predefined by the compiler.

Version identifiers are in their own unique name space, they do not conflict with debug identifiers or other symbols in the module. Version identifiers defined in one module have no influence over other imported modules.

    int k;
    version (Demo) // compile in this code block for the demo version
    {
        int i;
        int k;    // error, k already defined

        i = 3;
    }
    x = i;      // uses the i declared above

    version (X86)
    {
        ... // implement custom inline assembler version
    }
    else
    {
        ... // use default, but slow, version
    }

The version(unittest) is satisfied if and only if the code is compiled with unit tests enabled (the -unittest option on dmd).

### Version Specification

    VersionSpecification:
        version = Identifier ;
        version = IntegerLiteral ;

The version specification makes it straightforward to group a set of features under one major version, for example:

    version (ProfessionalEdition)
    {
        version = FeatureA;
        version = FeatureB;
        version = FeatureC;
    }
    version (HomeEdition)
    {
        version = FeatureA;
    }
    ...
    version (FeatureB)
    {
        ... implement Feature B ...
    }

Version identifiers or levels may not be forward referenced:

    version (Foo)
    {
        int x;
    }
    version = Foo;  // error, Foo already used

VersionSpecifications may only appear at module scope.

While the debug and version conditions superficially behave the same, they are intended for very different purposes. Debug statements are for adding debug code that is removed for the release version. Version statements are to aid in portability and multiple release versions.

Here's an example of a full version as opposed to a demo version:

    class Foo
    {
        int a, b;

        version(full)
        {
            int extrafunctionality()
            {
                ...
                return 1;  // extra functionality is supported
            }
        }
        else // demo
        {
            int extrafunctionality()
            {
                return 0;  // extra functionality is not supported
            }
        }
    }

Various different version builds can be built with a parameter to version:

    version(n) // add in version code if version level is >= n
    {
        ... version code ...
    }

    version(identifier) // add in version code if version
                             // keyword is identifier
    {
        ... version code ...
    }

These are presumably set by the command line as -version=n and -version=identifier.

### Predefined Versions

Several environmental version identifiers and identifier name spaces are predefined for consistent usage. Version identifiers do not conflict with other identifiers in the code, they are in a separate name space. Predefined version identifiers are global, i.e. they apply to all modules being compiled and imported.

Predefined Version Identifiers
Version Identifier | Description
--- | ---
DigitalMars | DMD (Digital Mars D) is the compiler
GNU | GDC (GNU D Compiler) is the compiler
LDC | LDC (LLVM D Compiler) is the compiler
SDC | SDC (Stupid D Compiler) is the compiler
Windows | Microsoft Windows systems
Win32 | Microsoft 32-bit Windows systems
Win64 | Microsoft 64-bit Windows systems
linux | All Linux systems
OSX | Mac OS X
FreeBSD | FreeBSD
OpenBSD | OpenBSD
NetBSD | NetBSD
DragonFlyBSD | DragonFlyBSD
BSD | All other BSDs
Solaris | Solaris
Posix | All POSIX systems (includes Linux, FreeBSD, OS X, Solaris, etc.)
AIX | IBM Advanced Interactive eXecutive OS
Haiku | The Haiku operating system
SkyOS | The SkyOS operating system
SysV3 | System V Release 3
SysV4 | System V Release 4
Hurd | GNU Hurd
Android | The Android platform
Cygwin | The Cygwin environment
MinGW | The MinGW environment
FreeStanding | An environment without an operating system (such as Bare-metal targets)
CRuntime_Bionic | Bionic C runtime
CRuntime_Digitalmars | DigitalMars C runtime
CRuntime_Glibc | Glibc C runtime
CRuntime_Microsoft | Microsoft C runtime
X86 | Intel and AMD 32-bit processors
X86_64 | Intel and AMD 64-bit processors
ARM | The ARM architecture (32-bit) (AArch32 et al)
ARM_Thumb | ARM in any Thumb mode
ARM_SoftFloat | The ARM soft floating point ABI
ARM_SoftFP | The ARM softfp floating point ABI
ARM_HardFloat | The ARM hardfp floating point ABI
AArch64 | The Advanced RISC Machine architecture (64-bit)
Epiphany | The Epiphany architecture
PPC | The PowerPC architecture, 32-bit
PPC_SoftFloat | The PowerPC soft float ABI
PPC_HardFloat | The PowerPC hard float ABI
PPC64 | The PowerPC architecture, 64-bit
IA64 | The Itanium architecture (64-bit)
MIPS32 | The MIPS architecture, 32-bit
MIPS64 | The MIPS architecture, 64-bit
MIPS_O32 | The MIPS O32 ABI
MIPS_N32 | The MIPS N32 ABI
MIPS_O64 | The MIPS O64 ABI
MIPS_N64 | The MIPS N64 ABI
MIPS_EABI | The MIPS EABI
MIPS_SoftFloat | The MIPS soft-float ABI
MIPS_HardFloat | The MIPS hard-float ABI
NVPTX | The Nvidia Parallel Thread Execution (PTX) architecture, 32-bit
NVPTX64 | The Nvidia Parallel Thread Execution (PTX) architecture, 64-bit
SPARC | The SPARC architecture, 32-bit
SPARC_V8Plus | The SPARC v8+ ABI
SPARC_SoftFloat | The SPARC soft float ABI
SPARC_HardFloat | The SPARC hard float ABI
SPARC64 | The SPARC architecture, 64-bit
S390 | The System/390 architecture, 32-bit
SystemZ | The System Z architecture, 64-bit
HPPA | The HP PA-RISC architecture, 32-bit
HPPA64 | The HP PA-RISC architecture, 64-bit
SH | The SuperH architecture, 32-bit
SH64 | The SuperH architecture, 64-bit
Alpha | The Alpha architecture
Alpha_SoftFloat | The Alpha soft float ABI
Alpha_HardFloat | The Alpha hard float ABI
LittleEndian | Byte order, least significant first
BigEndian | Byte order, most significant first
D_Coverage | Code coverage analysis instrumentation (command line switch -cov) is being generated
D_Ddoc | Ddoc documentation (command line switch -D) is being generated
D_InlineAsm_X86 | Inline assembler for X86 is implemented
D_InlineAsm_X86_64 | Inline assembler for X86-64 is implemented
D_LP64 | Pointers are 64 bits (command line switch -m64). (Do not confuse this with C's LP64 model)
D_X32 | Pointers are 32 bits, but words are still 64 bits (x32 ABI) (This can be defined in parallel to X86_64)
D_HardFloat | The target hardware has a floating point unit
D_SoftFloat | The target hardware does not have a floating point unit
D_PIC | Position Independent Code (command line switch -fPIC) is being generated
D_SIMD | Vector extensions (via __simd) are supported
D_Version2 | This is a D version 2 compiler
D_NoBoundsChecks | Array bounds checks are disabled (command line switch -noboundscheck)
D_ObjectiveC | The target supports interfacing with Objective-C
unittest | Unit tests are enabled (command line switch -unittest)
assert | Checks are being emitted for assert expressions
none | Never defined; used to just disable a section of code
all | Always defined; used as the opposite of none

The following identifiers are defined, but are deprecated:

Predefined Version Identifiers (deprecated)
Version Identifier | Description
--- | ---
darwin | The Darwin operating system; use OSX instead
Thumb | ARM in Thumb mode; use ARM_Thumb instead
S390X | The System/390X architecture | 64-bit; use SystemZ instead

Others will be added as they make sense and new implementations appear.

It is inevitable that the D language will evolve over time. Therefore, the version identifier namespace beginning with "D_" is reserved for identifiers indicating D language specification or new feature conformance. Further, all identifiers derived from the ones listed above by appending any character(s) are reserved. This means that e.g. ARM_foo and Windows_bar are reserved while foo_ARM and bar_Windows are not.

Furthermore, predefined version identifiers from this list cannot be set from the command line or from version statements. (This prevents things like both Windows and linux being simultaneously set.)

Compiler vendor specific versions can be predefined if the trademarked vendor identifier prefixes it, as in:

    version(DigitalMars_funky_extension)
    {
        ...
    }

It is important to use the right version identifier for the right purpose. For example, use the vendor identifier when using a vendor specific feature. Use the operating system identifier when using an operating system specific feature, etc.

## Debug Condition

    DebugCondition:
        debug
        debug ( IntegerLiteral )
        debug ( Identifier )

Two versions of programs are commonly built, a release build and a debug build. The debug build includes extra error checking code, test harnesses, pretty-printing code, etc. The debug statement conditionally compiles in its statement body. It is D's way of what in C is done with #ifdef DEBUG / #endif pairs.

The debug condition is satisfied when the -debug switch is passed to the compiler or when the debug level is >= 1.

The debug ( IntegerLiteral ) condition is satisfied when the debug level is >= IntegerLiteral.

The debug ( Identifier ) condition is satisfied when the debug identifier matches Identifier.

    class Foo
    {
        int a, b;
    debug:
        int flag;
    }

### Debug Specification

    DebugSpecification:
        debug = Identifier ;
        debug = IntegerLiteral ;

Debug identifiers and levels are set either by the command line switch -debug or by a DebugSpecification.

Debug specifications only affect the module they appear in, they do not affect any imported modules. Debug identifiers are in their own namespace, independent from version identifiers and other symbols.

It is illegal to forward reference a debug specification:

    debug(foo) writeln("Foo");
    debug = foo;    // error, foo used before set

DebugSpecifications may only appear at module scope.

Various different debug builds can be built with a parameter to debug:

    debug(IntegerLiteral) { } // add in debug code if debug level is >= IntegerLiteral
    debug(identifier) { } // add in debug code if debug keyword is identifier

These are presumably set by the command line as -debug=n and -debug=identifier.

## Static If Condition

    StaticIfCondition:
        static if ( AssignExpression )

AssignExpression is implicitly converted to a boolean type, and is evaluated at compile time. The condition is satisfied if it evaluates to true. It is not satisfied if it evaluates to false.

It is an error if AssignExpression cannot be implicitly converted to a boolean type or if it cannot be evaluated at compile time.

StaticIfConditions can appear in module, class, template, struct, union, or function scope. In function scope, the symbols referred to in the AssignExpression can be any that can normally be referenced by an expression at that point.

    const int i = 3;
    int j = 4;

    static if (i == 3)    // ok, at module scope
        int x;

    class C
    {
        const int k = 5;

        static if (i == 3) // ok
            int x;
        else
            long x;

        static if (j == 3) // error, j is not a constant
            int y;

        static if (k == 5) // ok, k is in current scope
            int z;
    }

    template INT(int i)
    {
        static if (i == 32)
            alias INT = int;
        else static if (i == 16)
            alias INT = short;
        else
            static assert(0); // not supported
    }

    INT!(32) a;  // a is an int
    INT!(16) b;  // b is a short
    INT!(17) c;  // error, static assert trips

A StaticIfConditional condition differs from an IfStatement in the following ways:

1. It can be used to conditionally compile declarations, not just statements.
2. It does not introduce a new scope even if { } are used for conditionally compiled statements.
3. For unsatisfied conditions, the conditionally compiled code need only be syntactically correct. It does not have to be semantically correct.
4. It must be evaluatable at compile time.

## Static Assert

    StaticAssert:
        static assert ( AssignExpression );
        static assert ( AssignExpression , AssignExpression );

AssignExpression is evaluated at compile time, and converted to a boolean value. If the value is true, the static assert is ignored. If the value is false, an error diagnostic is issued and the compile fails.

Unlike AssertExpressions, StaticAsserts are always checked and evaluted by the compiler unless they appear in an unsatisfied conditional.

    void foo()
    {
        if (0)
        {
            assert(0);  // never trips
            static assert(0); // always trips
        }
        version (BAR)
        {
        }
        else
        {
            static assert(0); // trips when version BAR is not defined
        }
    }

StaticAssert is useful tool for drawing attention to conditional configurations not supported in the code.

The optional second AssignExpression can be used to supply additional information, such as a text string, that will be printed out along with the error diagnostic.

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