第一种方法：调用WideCharToMultiByte()和MultiByteToWideChar()，代码如下（关于详细的解释，可以参考《windows核心编程》）：

#include
#include
using namespace std;
//Converting a WChar string to a Ansi string
std::string WChar2Ansi(LPCWSTR pwszSrc)
{
int nLen = WideCharToMultiByte(CP_ACP, 0, pwszSrc, -1, NULL, 0, NULL, NULL);

if (nLen<= 0) return std::string("");

char* pszDst = new char[nLen];
if (NULL == pszDst) return std::string("");

WideCharToMultiByte(CP_ACP, 0, pwszSrc, -1, pszDst, nLen, NULL, NULL);
pszDst[nLen -1] = 0;

std::string strTemp(pszDst);
delete [] pszDst;

return strTemp;
}

string ws2s(wstring& inputws){ return WChar2Ansi(inputws.c_str()); }

//Converting a Ansi string to WChar string

std::wstring Ansi2WChar(LPCSTR pszSrc, int nLen)

{
int nSize = MultiByteToWideChar(CP_ACP, 0, (LPCSTR)pszSrc, nLen, 0, 0);
if(nSize <= 0) return NULL;

WCHAR *pwszDst = new WCHAR[nSize+1];
if( NULL == pwszDst) return NULL;

MultiByteToWideChar(CP_ACP, 0,(LPCSTR)pszSrc, nLen, pwszDst, nSize);
pwszDst[nSize] = 0;

if( pwszDst[0] == 0xFEFF) // skip Oxfeff
for(int i = 0; i < nSize; i ++) 
pwszDst[i] = pwszDst[i+1];

wstring wcharString(pwszDst);
delete pwszDst;

return wcharString;
}

std::wstring s2ws(const string& s){ return Ansi2WChar(s.c_str(),s.size());}

第二种方法：采用ATL封装_bstr_t的过渡：（注，_bstr_是Microsoft Specific的，所以下面代码可以在VS2005通过，无移植性）；
#include
#include
using namespace std;
#pragma comment(lib, "comsuppw.lib")

string ws2s(const wstring& ws);
wstring s2ws(const string& s);

string ws2s(const wstring& ws)
{
_bstr_t t = ws.c_str();
char* pchar = (char*)t;
string result = pchar;
return result;
}

wstring s2ws(const string& s)
{
_bstr_t t = s.c_str();
wchar_t* pwchar = (wchar_t*)t;
wstring result = pwchar;
return result;
}

第三种方法：使用CRT库的mbstowcs()函数和wcstombs()函数，平台无关，需设定locale。
#include
#include
using namespace std;
string ws2s(const wstring& ws)
{
string curLocale = setlocale(LC_ALL, NULL); // curLocale = "C";

setlocale(LC_ALL, "chs");

const wchar_t* _Source = ws.c_str();
size_t _Dsize = 2 * ws.size() + 1;
char *_Dest = new char[_Dsize];
memset(_Dest,0,_Dsize);
wcstombs(_Dest,_Source,_Dsize);
string result = _Dest;
delete []_Dest;

setlocale(LC_ALL, curLocale.c_str());

return result;
}

wstring s2ws(const string& s)
{
setlocale(LC_ALL, "chs");

const char* _Source = s.c_str();
size_t _Dsize = s.size() + 1;
wchar_t *_Dest = new wchar_t[_Dsize];
wmemset(_Dest, 0, _Dsize);
mbstowcs(_Dest,_Source,_Dsize);
wstring result = _Dest;
delete []_Dest;

setlocale(LC_ALL, "C");

return result;
}