CODEWITHSUNDEEP
parameter-passingd
DS_London
DS_London

Reputation: 4261

Returning by reference from struct method in D

I'm starting my journey in D from C++. In C++ passing by reference or value is quite explicit, but in D it seems to vary between structs and classes.

My question is how can I force a return by reference?

I have a simple XmlNode class for building Xml trees (which is a lift from my C++ code):

import std.stdio; 

struct XmlNode
{
    string _name;
    string _data;
    XmlNode[] _children;

    this(string name, string data="")
    {
        _name = name;
        _data = data;
    }
    
    //Trying to return a reference to the added Node
    ref XmlNode addChild(string name,string data = "")
    {
        _children ~= XmlNode(name,data);
        return _children[$-1]; 
    }

    string toString(bool bPlain = true, string indent = "")
    {
        //Omitted for brevity
    }
}

And here is the testing code:

int main()
{
    auto root = XmlNode("root");
    
    //Chained call
    root.addChild("Level 1").addChild("Level 2","42");

    //Call in two parts
    auto n = root.addChild("Level 1");
    n.addChild("Level 2","101"); //n seems to be a copy not a reference

    //Chained call
    root.addChild("Level 1").addChild("Level 2","999");

    writeln(root.toString(false));

    return 0;
}

which gives the following output:

root
  Level 1
    Level 2
      42
  Level 1
  Level 1
    Level 2
      999

As you can see the 'chained' use of addChild() performs as hoped. But if I try to break it up into two separate calls, only the first has an effect, and the second seems to operate on a copy of the first, not a reference. I optimistically added a ref qualifier to the addChild() signature, but that doesn't seem to help.

As ever, I'd be grateful for any advice (using DMD / Visual D / Visual Studio / Windows 10).

Upvotes: 2

Views: 195

Answers (1)

Vladimir Panteleev
Vladimir Panteleev

Reputation: 25187

    auto n = root.addChild("Level 1");

Here, though addChild returns a reference, it is assigned to a variable, and thus dereferenced and copied. Instead, you probably want:

    auto n = &root.addChild("Level 1");

Note that D does not have reference variables, like in C++. Variables can be only pointers (though it's possible to write a wrapper template with reference-like semantics).

Also note that in the current design of XmlNode, the returned reference will only be valid until the next time _children is modified (as that may cause a reallocation and thus move the contents to another address, making any extant references outdated). It is a common footgun, which could be avoided by storing references of XmlNode (or making it a reference type i.e. a class), at the cost of extra dereferences and allocations.

Upvotes: 3

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