View source for Porting 32 Bit Code to 64 Bits

Although D is designed to make it easy to port code between 32 and 64 bit modes, being a systems programming language, dependencies can creep in. This guide points out what changes between the two.

== Versions ==
Not all code can be made portable between 32 and 64 bits, and must be versioned. The following works:

<syntaxhighlight lang="D">
version (X86)
    ... 32 bit code ...
else version (X86_64)
    ... 64 bit code ...
else
    static assert("unsupported target")
</syntaxhighlight>

It is best to write versioning in that manner to give a compile time error on a new target, rather than guessing in advance what, for example, a 64 bit ARM target might require. Experience shows that guesses about how an unfamiliar platform might work always get it wrong. Save those decisions for when one is actually working on a 64 bit ARM.

== Size Changes ==
The size of pointers and references will increase from 4 to 8 bytes. The '''size_t''' alias moves from '''uint''' to '''ulong''', and the '''ptrdiff_t''' alias moves from '''int''' to '''long'''.

The sizes of compound types based on these will also increase. This includes dynamic arrays, associative arrays, delegates, and class references.