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(gcc-300.info)Temporaries

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Temporaries May Vanish Before You Expect
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   It is dangerous to use pointers or references to _portions_ of a
temporary object.  The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage.  The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type `char *' or
`const char *'--which is one reason why the standard `string' class
requires you to call the `c_str' member function.  However, any class
that returns a pointer to some internal structure is potentially
subject to this problem.

   For example, a program may use a function `strfunc' that returns
`string' objects, and another function `charfunc' that operates on
pointers to `char':

     string strfunc ();
     void charfunc (const char *);
     
     void
     f ()
     {
       const char *p = strfunc().c_str();
       ...
       charfunc (p);
       ...
       charfunc (p);
     }

In this situation, it may seem reasonable to save a pointer to the C
string returned by the `c_str' member function and use that rather than
call `c_str' repeatedly.  However, the temporary string created by the
call to `strfunc' is destroyed after `p' is initialized, at which point
`p' is left pointing to freed memory.

   Code like this may run successfully under some other compilers,
particularly obsolete cfront-based compilers that delete temporaries
along with normal local variables.  However, the GNU C++ behavior is
standard-conforming, so if your program depends on late destruction of
temporaries it is not portable.

   The safe way to write such code is to give the temporary a name,
which forces it to remain until the end of the scope of the name.  For
example:

     string& tmp = strfunc ();
     charfunc (tmp.c_str ());