std::tr1::bind gives error C2065: '_1' : undeclared identifier

以下

std::tr1::bind(&GameLevel::health,
               currLevel,
               _1)                              

报错

error C2065: '_1' : undeclared identifier

原因

见MSDN\Standard C++ Library TR1 Extensions Reference\<functional> (TR1)
Object
_1 Object
Description
 Placeholders for replaceable arguments.

namespace placeholders {
   extern unspecified _1, _2, ... _M
 } // namespace placeholders (within std::tr1)

结论

std::tr1::bind(&GameLevel::health,
               currLevel,
               std::tr1::placeholders::_1)                  ---effiective c++ ver.3---