Node:Temporaries, Next:Copy Assignment, Previous:Static Definitions, Up:C++ Misunderstandings

#### Temporaries May Vanish Before You Expect

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 ());