extern_cpp_class_better_c.d

import core.stdc.stdio : printf;

extern (C) void main()
{
    Base b = create!Derived;
    b.foo();
}

extern (C++) class Base
{
    int x = 39;
    this() { ++x; }
    void foo() { printf("Base.\n"); }
}

extern (C++) class Derived : Base
{
    this() { x += 2; }
    override void foo() { printf("Derived - x = %d.\n", x); }
}

T create(T)() if (is(T == class))
{
    import core.stdc.stdlib : malloc;
    import core.stdc.string : memcpy;
    enum size = __traits(classInstanceSize, T);
    void[] mem = malloc(size)[0 .. size];
    auto init = typeid(T).initializer;
    memcpy(mem.ptr, init.ptr, size);
    T result = cast(T)mem.ptr;
    static if (__traits(compiles, result.__ctor()))
         result.__ctor();
    return result;
}

output.txt

$ dmd --version
DMD64 D Compiler v2.075.0-b1
Copyright (c) 1999-2017 by Digital Mars written by Walter Bright

$ dmd -betterC extern_cpp_class.d && ./extern_cpp_class
Derived - x = 42.