Re: [eigen] Pass POD by reference or by pointer?

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 16.02.2013 07:19, 何震邦 wrote:
> Many C++ books say pass by reference whenever possible. However, I find the
> code below misleading.
>
> ### unsupported/Eigen/src/MatrixFunctions/MatrixPower.h
>
>   81 template<typename Derived, typename ResultType>
>   82 void apply(const Derived&, ResultType&, bool&);
>
> 209 apply(b, res, init);
>
> ###
>
> At line 209, it looks as if init would never be written. This is also why Qt
> prefers pointers to references.

And how do you see from line 209 alone whether b or res are written to 
or from? (Assuming the variable name 'res' would not suggest it is an 
output variable) Maybe it's rather an argument to rename 'init' to sth 
like 'is_initialized'
To see which variables are in-/output parameters you need to look at 
least at the signature anyways and if the function is neither 
documented, nor has a descriptive name you need to look into it anyways 
to see what it actually does (which is ok for functions that short).

When looking at a signature I'd rather say that passing pointers is 
misleading, because I would think that the function actually expects an 
array (as for example the Map constructors) rather than a single variable.

Actually, in that same file I'd suggest also changing modfAndInit that 
it takes a reference instead of a pointer and you might even consider 
passing RealScalar by const reference instead of by value (RealScalar is 
not guaranteed to be a POD).

Regards,
Christoph


> I think there is nothing wrong to pass a reference to a non-POD, but passing a
> reference to a POD is functionally safe but syntactically misleading. Even if
> we want the functional safety of a reference, we can pass a pointer-const
> instead, but the constness seems unnecessary.
>
> If passing a pointer is better, feel free to alter the code. ;)
>
> Thanks,
> Chen-Pang


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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
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