String joining an array of structs or class instances implementing toString?

[pineapple via Digitalmars-d-learn]

It feels like there should be an out-of-the box way to do this 
but I haven't been able to find it? Help?

This is the thing that I want to do:
struct example{
    const string str;
    //this(string str){ this.str = str; }
    string toString(){
        return this.str;
    }
}

public void main(){
    import std.stdio;
    import std.string;
    example[] parts = [example("hello"), example("world")];
    writeln(std.string.join(parts, " "));
}