tmp$ cat > str.cc
struct Foo {
    Foo() : str("foo"), len(3) { }
    const char* str;
    int len;
};

struct Bar {
    Bar() : str("bar"), len(3) { }
    typedef const char* pointer;
    pointer str;
    int len;
};

int main()
{
  Foo foo;
  Bar bar;
  return 0;
}
tmp$ cat > a.out-gdb.py
import gdb

class StrPrinter:
    "Print a stringy thingie"

    def __init__ (self, val):
        self.val = val

    def to_string (self):
        s = self.val['str']
        l = self.val['len']
        return s.lazy_string(length = l)

    def display_hint(self):
        return 'string'

def str_lookup_function(val):
    lookup_tag = val.type.tag
    if lookup_tag == "Foo" or lookup_tag == "Bar":
        return StrPrinter(val)
    return None

gdb.current_objfile().pretty_printers.append(str_lookup_function)
tmp$ g++ -g str.cc
tmp$ ~/gcc/gdb/7.8.50/bin/gdb 
GNU gdb (GDB) 7.8.50.20141216-cvs
Copyright (C) 2014 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) set auto-load safe-path /
(gdb) file a.out
Reading symbols from a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x4005b8: file str.cc, line 16.
Starting program: /tmp/a.out 

Temporary breakpoint 1, main () at str.cc:16
16        Foo foo;
(gdb) n
17        Bar bar;
(gdb) p foo
$1 = "foo"
(gdb) n
18        return 0;
(gdb) p bar
$2 = "\x4006b4\003\000\000\000\000\000\000\000\x4006b0"
(gdb) 

The LazyString seems to be confused by the fact that Bar::str is declared with a typedef, so it prints the contents of &bar.str instead of bar.str

The only solution I've found is to strip the typedef first:

    def to_string (self):
        s = self.val['str']
        s = s.cast(s.type.strip_typedefs())
        l = self.val['len']
        return s.lazy_string(length = l)