lldb:无法实现:无法获取变量的值

我用这个命令行编译了一个cpp文件: g++ -g test.cpp

它在第28行引发异常.我想通过检查lldb中的变量来调查原因.我在第28行设置了一个断点并a.out在lldb中运行.

(lldb) n
Process 84233 stopped
* thread #1: tid = 0xa44b86, 0x00000001000017fb a.out`say(s=) + 987 at so.cpp:28, queue = 'com.apple.main-thread', stop reason = step over
    frame #0: 0x00000001000017fb a.out`say(s=) + 987 at so.cpp:28
   25       }
   26       else{
   27           s.insert(0, to_string(sz));
-> 28           s.erase(2, sz-1);
   29       }
   30       return s;
   31   }
(lldb) po s
error: Couldn't materialize: couldn't get the value of variable s: variable not available
Errored out in Execute, couldn't PrepareToExecuteJITExpression

为什么出现错误信息？我该如何检查变量s？

lldb版本:lldb-320.4.115.3

g ++版本:Configured with: --prefix=/Applications/Xcode6-Beta5.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 Apple LLVM version 6.0 (clang-600.0.45.3) (based on LLVM 3.5svn) Target: x86_64-apple-darwin13.3.0 Thread model: posix