import std.stdio;

​

class A {}

​

class B { A a; }

​

class C { B b; }

​

class D { C c; }

​

class E { D d; }

​

string createCondition(string[] entries)

{

    string result = "";

    string lastEntry = "";

    foreach (entry; entries)

    {

        if (lastEntry && lastEntry.length) lastEntry ~= "." ~ entry;

        else lastEntry = entry;

        result ~= lastEntry ~ " !is null &&";

    }

    result.length -= 2;

    return result;

}

​

bool isDefined(alias symbol,T)(T arg)

{

    import std.array : split;

    enum symbolEntries = symbol.split(".");

    enum entriesCondition = createCondition(["arg"] ~ symbolEntries[1..$]);

    mixin("return " ~ entriesCondition ~ ";");

}

​

​

void main()

{

    auto e = new E;

    e.d = new D;

    e.d.c = new C;

    e.d.c.b = new B;

    e.d.c.b.a = new A;

    if (isDefined!((e.d.c.b.a).stringof)(e))

    {

        writeln("e.d.c.b.a is defined.");

    }

    else

    {

        writeln("e.d.c.b.a is not defined.");

    }

    e = new E;

    e.d = new D;

    e.d.c = new C;

    if (isDefined!((e.d.c.b.a).stringof)(e))

    {

        writeln("e.d.c.b.a is defined.");

    }

    else

    {

        writeln("e.d.c.b.a is not defined.");

    }

}