Re: [boost] [c++-sig] A question regarding arguments bindings

Subject: Re: [boost] [c++-sig] A question regarding arguments bindings
From: Stefan Seefeld (seefeld_at_[hidden])
Date: 2008-11-13 17:15:02

Sandeep Gupta wrote:
> Hi,
>  My question is regarding the default behavior of the compiler when there is
> mismatch in the argument type
>  between binding declaration and function declaration. By default no warning
> is generated.
>
> Consider following example:
> class A{
> public:
>    A(const B&);
> };
>
> In wrapper, if by mistake (which I made in my code), i write
> class_<A>("A", init<B>)
> ;
> The compiler does not complain about no constructor found.  Instead for
> reasons, beyond my understanding of boost.python, B get passed by value.
>
> My question why doesn't  compiler issue  at least a warning. Needless to say
> i am new to boost.python. My apologies if this is something trivial or well
> known.
>   

I'm not entirely certain about how it works, but can easily imagine...:

As A's constructor is called from Python, there needs to be code that 
will unpack the C++ arguments from the Python callable argument tuple, 
and pass it down to the constructor. That code gets instantiated by 
means of 'init<signature>'. Since you indicate 'Signature' to be 'B', 
the code will extract a B instance (by value) from the Python argument 
list, and pass that down to A's constructor, which the compiler happily 
accepts.
There is just no reason the compiler would guess that this is not the 
intended behavior. If it was the other way around, i.e. if the compiler 
expected a (non-const) reference to 'B', but was fed a const B 
reference, it would loudly complain.

HTH,
       Stefan

-- 
      ...ich hab' noch einen Koffer in Berlin...