Classes

A class is nothing more than a structure with an associated set of functions that take an implicit first parameter, namely a pointer to the structure. Therefore, it is very trivial to map a class to LLVM IR:

#include <stddef.h>

class Foo
{
public:
    Foo()
    {
        _length = 0;
    }

    size_t GetLength() const
    {
        return _length;
    }

    void SetLength(size_t value)
    {
        _length = value;
    }

private:
    size_t _length;
};

We first transform this code into two separate pieces:

• The structure definition.
• The list of methods, including the constructor.

; The structure definition for class Foo.
%Foo = type { i32 }

; The default constructor for class Foo.
define void @Foo_Create_Default(%Foo* %this) nounwind {
    %1 = getelementptr %Foo, %Foo* %this, i32 0, i32 0
    store i32 0, i32* %1
    ret void
}

; The Foo::GetLength() method.
define i32 @Foo_GetLength(%Foo* %this) nounwind {
    %1 = getelementptr %Foo, %Foo* %this, i32 0, i32 0
    %2 = load i32, i32* %1
    ret i32 %2
}

; The Foo::SetLength() method.
define void @Foo_SetLength(%Foo* %this, i32 %value) nounwind {
    %1 = getelementptr %Foo, %Foo* %this, i32 0, i32 0
    store i32 %value, i32* %1
    ret void
}

Then we make sure that the constructor (Foo_Create_Default) is invoked

whenever an instance of the structure is created:

Foo foo;

%foo = alloca %Foo
call void @Foo_Create_Default(%Foo* %foo)