Converting between std::wstring and std::string

While researching ways to convert back and forth between std::wstring and std::string, I found this conversation on the MSDN forums.

There were two functions that, to me, looked good. Specifically, these:

std::wstring s2ws(const std::string& s)
{
    int len;
    int slength = (int)s.length() + 1;
    len = MultiByteToWideChar(CP_ACP, 0, s.c_str(), slength, 0, 0); 
    wchar_t* buf = new wchar_t[len];
    MultiByteToWideChar(CP_ACP, 0, s.c_str(), slength, buf, len);
    std::wstring r(buf);
    delete[] buf;
    return r;
}

std::string ws2s(const std::wstring& s)
{
    int len;
    int slength = (int)s.length() + 1;
    len = WideCharToMultiByte(CP_ACP, 0, s.c_str(), slength, 0, 0, 0, 0); 
    char* buf = new char[len];
    WideCharToMultiByte(CP_ACP, 0, s.c_str(), slength, buf, len, 0, 0); 
    std::string r(buf);
    delete[] buf;
    return r;
}

However, the double allocation and the need to delete the buffer concern me (performance and exception safety) so I modified them to be like this:

std::wstring s2ws(const std::string& s)
{
    int len;
    int slength = (int)s.length() + 1;
    len = MultiByteToWideChar(CP_ACP, 0, s.c_str(), slength, 0, 0); 
    std::wstring r(len, L'\0');
    MultiByteToWideChar(CP_ACP, 0, s.c_str(), slength, &r[0], len);
    return r;
}

std::string ws2s(const std::wstring& s)
{
    int len;
    int slength = (int)s.length() + 1;
    len = WideCharToMultiByte(CP_ACP, 0, s.c_str(), slength, 0, 0, 0, 0); 
    std::string r(len, '\0');
    WideCharToMultiByte(CP_ACP, 0, s.c_str(), slength, &r[0], len, 0, 0); 
    return r;
}

Unit testing indicates that this works in a nice, controlled environment but will this be OK in the vicious and unpredictable world that is my client's computer?

I would, and have, redesign your set of functions to resemble casts:

std::wstring x;
std::string y = string_cast(x);

This can have a lot of benefits later when you start having to deal with some 3rd party library's idea of what strings should look like.

StackExchange Code Review Q#419, answer score: 13