Member initialization while using delegated constructor

Question

I've started trying out the C++11 standard and i found this question which describes how to call your ctor from another ctor in the same class to avoid having a init method or the like. Now i'm trying the same thing with code that looks like this:

hpp:

class Tokenizer
{
public:
  Tokenizer();
  Tokenizer(std::stringstream *lines);
  virtual ~Tokenizer() {};
private:
  std::stringstream *lines;
};

cpp:

Tokenizer::Tokenizer()
  : expected('=')
{
}

Tokenizer::Tokenizer(std::stringstream *lines)
  : Tokenizer(),
    lines(lines)
{
}

But this is giving me the error: In constructor ‘config::Tokenizer::Tokenizer(std::stringstream*)’: /path/Tokenizer.cpp:14:20: error: mem-initializer for ‘config::Tokenizer::lines’ follows constructor delegation I've tried moving the Tokenizer() part first and last in the list but that didn't help.

What's the reason behind this and how should i fix it? I've tried moving the lines(lines) to the body with this->lines = lines; instead and it works fine. But i would really like to be able to use the initializer list.

jogojapan · Accepted Answer · 2012-08-30T05:34:23.190

When you delegate the member initialization to another constructor, there is an assumption that the other constructor initializes the object completely, including all members (i.e. including the lines member in your example). You can't therefore initialize any of the members again.

The relevant quote from the Standard is (emphasis mine):

(§12.6.2/6) A mem-initializer-list can delegate to another constructor of the constructor’s class using any class-or-decltype that denotes the constructor’s class itself. If a mem-initializer-id designates the constructor’s class, it shall be the only mem-initializer; the constructor is a delegating constructor, and the constructor selected by the is the target constructor. [...]

You can work-around this by defining the version of the constructor that takes arguments first:

Tokenizer::Tokenizer(std::stringstream *lines)
  : lines(lines)
{
}

and then define the default constructor using delegation:

Tokenizer::Tokenizer()
  : Tokenizer(nullptr)
{
}

As a general rule, you should fully specify that version of the constructor that takes the largest number of arguments, and then delegate from the other versions (using the desired default values as arguments in the delegation).

It seems counter-intuitive at first but is really helping actually! — Korchkidu, Aug 20 '14 at 07:55
@JacobWaters Then most likely you'll use whatever is the appropriate default initialization. Let's say, `lines` were of type `A`, you could say `: Tokenizer(A())` for the initializer list. This could be problematic if `A` is large and has no move contructor or somehow does not lend itself to efficient/optimizable copying. In that case, it might be best not to use this kind of delegation at all. — jogojapan, Aug 29 '21 at 10:58

Member initialization while using delegated constructor

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