Re: [eigen] is returning Eigen::Ref objects legit?

[何震邦 <jdh8@xxxxxxxxxxxxxx>]

在 2013 五月 20 週一 15:38:12，Sébastien Barthélémy 寫道:
> Hello,
> 
> I'm giving new Eigen::Refs a try and thought they might be a handy
> type to return (by const ref), especially when the implementation
> knows the matrix size at compile time but the caller does not.

Yes, it's a wrapper of reference.

> However, I'm not sure they are intended to be returned. The doc says
> "This class permits to write non template functions taking Eigen's
> object as parameters".
> 
> Moreover my small test gets a "returning reference to temporary"
> warning at compile time and a std::bad_alloc exception at runtime.
> This does not sound good ;).

Temporary variables are deconstructed at the end of the statement (;), local
ones at the end of the block (}).  When returning a reference to an in-scope
variable, the variable is deconstructed, making the reference pointing to
'nothing'.  One says the reference is 'lost'.  ("Lost" as in "lost in space")

However, returning a reference to an out-scope variable is safe.  The variable
is not deconstructed after the execution of the function.

### example.cpp
#include <cstdio>

int glob = 3;

int& safe1(int& arg)
{ return arg; }

int& safe2()
{ return glob; }

int& dangerous1(int arg)  // arg 
{ return arg; }

int& dangerous2()
{
  int tmp = 2;
  return tmp;
}

int main()
{
  printf("%d\n", safe1(glob));  // &glob == &arg
  printf("%d\n", safe2());
  printf("%d\n", dangerous1(glob));  // &glob != &arg
  printf("%d\n", dangerous2());
  return 0;
}
### EOF

> Could you please confirm in either way?
> 
> Example and backtrace are attached just in case.
> 
> Thanks!

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何震邦 | Chen-Pang He | 0918-319823
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