D Programming Language 2.0

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Last update Wed Oct 17 15:41:33 2007

# 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 : Declarations

    DeclarationBlock:
        Declaration
        { Declarations }
        { }

    Declarations:
        Declaration
        Declaration Declarations

    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

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

    VersionCondition:
    	version ( Integer )
    	version ( Identifier )

The VersionCondition is satisfied if the Integer 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
    }

### Version Specification

    VersionSpecification
        version = Identifier ;
        version = Integer ;

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.

DigitalMars
  Digital Mars is the compiler vendor
X86
  Intel and AMD 32 bit processors
X86_64
  AMD and Intel 64 bit processors
Windows
  Microsoft Windows systems
Win32
  Microsoft 32 bit Windows systems
Win64
  Microsoft 64 bit Windows systems
linux
  All linux systems
LittleEndian
  Byte order, least significant first
BigEndian
  Byte order, most significant first
D_Coverage
  Coverage analyser is implemented and the -cov switch is thrown
D_InlineAsm_X86
  Inline assembler for X86 is implemented
D_Version2
  This is a D version 2 compiler
none
  Never defined; used to just disable a section of code
all
  Always defined; used as the opposite of none

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.

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

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.

    DebugCondition:
        debug
        debug ( Integer )
        debug ( Identifier )

The debug condition is satisfied when the -debug switch is thrown on the compiler.

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

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 = Integer ;

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) writefln("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(Integer) { }    // add in debug code if debug level is >= Integer
    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 short INT;
        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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