[cctbxbb] Idiot C++ question

[Phil Evans]

I can see that problem with pointers but it doesn't seem to apply to double

I am naive about this, I suppose

1) I never learnt C, just C++
2) I don't use pointers (or very rarely)
3) I read "double(i)" as a constructor of a double from an int ie

class double {
public:
	double(const int& i);
};

thus it shouldn't accept eg

thing* instance;
d = double(instance);

which should be a compiler error, just as an illegal construction of any other object should be

and indeed
  double* pd;
  double d = double(pd);

gives
t.cpp:4:23: error: invalid cast from type 'double*' to type 'double'

Phil


On 15 May 2012, at 11:38, Luc Bourhis wrote:

> Hi Phil,
> 
>> Looking at some cctbx C++ code, I note use of syntax like
>> 
>> int i = 1234;
>> double d = static_cast<double>(i);
>> 
>> How is this different or indeed preferable to
>> 
>> double d = double(i);
>> 
>> or
>> 
>> d = i;
>> 
>> ?
> 
> 
> Short answer: For conversions between non-pointer types, it does not matter but for pointer types, static_cast shall always be used to the exclusion of any other conversion.
> 
> The language lawyer:
> 
> The construct
> 
> double d = double(i)
> 
> which is known as functional-style cast is completely equivalent to the so-call cast notation, which comes from C:
> 
> double d = (double)i
> 
> You may see both in a few places in the cctbx code actually. 
> 
> The problem with those two constructs is that the compiler will let use convert a pointer to anything into a pointer to anything else. Consider:
> 
> #include <iostream>
> 
> struct foo { 
>  int i,j; 
>  foo(int i, int j): i(i), j(j) {} 
> };
> 
> struct bar { 
>  double d; 
>  bar(double d): d(d) {} 
> };
> 
> int main() {
>  foo x(1, 2);
>  bar *y = (bar *)&x;
>  std::cout << y->d << std::endl;
>  return 0;
> }
> 
> This prints 4.24399e-314 on my computer but this depends on the compiler. On the contrary,
> 
> bar *y = static_cast<bar *>(&x)
> 
> is not legit and the compiler will bug you.
> 
> HtH,
> 
> Luc
> 
> 
> 
> 
> _______________________________________________
> cctbxbb mailing list
> cctbxbb at phenix-online.org
> http://phenix-online.org/mailman/listinfo/cctbxbb