boost thread warning C4191和algorithm warning C4242

今天抽出时间来整理一下项目中的警告信息，发现有几个警告很神奇，经过一番查证，终于找到了问题所在。

第一个警告：

boost/thread/win32/thread_primitives.hpp(315): warning C4191: “类型转换”: 从“boost::detail::win32::farproc_t”到“boost::detail::win32::detail::gettickcount64_t”的不安全转换1>          通过结果指针调用该函数可能导致程序失败

原因：因为我项目中第三方库除了boost库，还引用了cryptopp565这个库，通过头文件的逐级追查，发现在cryptopp565的头文件config.h中有如下定义

#ifdef _MSC_VER    // 4127: conditional expression is constant    // 4231: nonstandard extension used : 'extern' before template explicit instantiation    // 4250: dominance    // 4251: member needs to have dll-interface    // 4275: base needs to have dll-interface    // 4505: unreferenced local function    // 4512: assignment operator not generated    // 4660: explicitly instantiating a class that's already implicitly instantiated    // 4661: no suitable definition provided for explicit template instantiation request    // 4786: identifer was truncated in debug information    // 4355: 'this' : used in base member initializer list    // 4910: '__declspec(dllexport)' and 'extern' are incompatible on an explicit instantiation#   pragma warning(disable: 4127 4231 4250 4251 4275 4505 4512 4660 4661 4786 4355 4910)    // Security related, possible defects    // http://blogs.msdn.com/b/vcblog/archive/2010/12/14/off-by-default-compiler-warnings-in-visual-c.aspx#   pragma warning(once: 4191 4242 4263 4264 4266 4302 4826 4905 4906 4928)#endif

正是这里的pragma warning once释放了boost的这一警告，一般情况下是被boost屏蔽掉的

 inline detail::gettickcount64_t GetTickCount64_()            {                static detail::gettickcount64_t gettickcount64impl;                if(gettickcount64impl)                    return gettickcount64impl;                // GetTickCount and GetModuleHandle are not allowed in the Windows Runtime,                // and kernel32 isn't used in Windows Phone.#if BOOST_PLAT_WINDOWS_RUNTIME                gettickcount64impl = &GetTickCount64;#else                farproc_t addr=GetProcAddress(#if !defined(BOOST_NO_ANSI_APIS)                    GetModuleHandleA("KERNEL32.DLL"),#else                    GetModuleHandleW(L"KERNEL32.DLL"),#endif                    "GetTickCount64");                if(addr)                    gettickcount64impl=(detail::gettickcount64_t) addr;                else                    gettickcount64impl=&GetTickCount64emulation;#endif                return gettickcount64impl;            }

解决办法：改变头文件包含顺序，先include “boost/thread.hpp”;再include “config.h”;或者换标准库的thread，不过也奇怪了，std::thread不是从boost::thread搬来的吗，看样子并没有完全照搬，至少实现方法上肯定有差异，改天撸一遍源码看看

第二个警告：

1>C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\algorithm(1015): warning C4242: “=”: 从“int”转换到“char”，可能丢失数据1>  C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\include\algorithm(1027): note: 参见对正在编译的函数 模板 实例化“_OutIt std::_Transform<char*,_OutIt,int(__cdecl *)(int)>(_InIt,_InIt,_OutIt,_Fn1)”的引用1>          with1>          [1>              _OutIt=std::_String_iterator<std::_String_val<std::_Simple_types<char>>>,1>              _InIt=char *,1>              _Fn1=int (__cdecl *)(int)1>          ]1>  c:\program files\mysql\connector.c++ 1.1\include\cppconn\sqlstring.h(111): note: 参见对正在编译的函数 模板 实例化“_OutIt std::transform<std::_String_iterator<std::_String_val<std::_Simple_types<char>>>,std::_String_iterator<std::_String_val<std::_Simple_types<char>>>,int(__cdecl *)(int)>(_InIt,_InIt,_OutIt,_Fn1)”的引用1>          with1>          [1>              _OutIt=std::_String_iterator<std::_String_val<std::_Simple_types<char>>>,1>              _InIt=std::_String_iterator<std::_String_val<std::_Simple_types<char>>>,1>              _Fn1=int (__cdecl *)(int)1>          ]

原因：起初我以为是mysql connector中 sqlstring封装的问题，后来发现也是上边pragma warning once的问题，它打开了4242报警，不过不得不吐槽mysql connector c++封装的不那么如人意(目前考虑换下纯c的接口)…

解决办法：修改头文件顺序