Variadic template resolution in VS2013 - Error C3520

问题

What's wrong with this code?

enum LogLevel {
    LogLevel_Error = 1,
    LogLevel_Warning = 2,
    LogLevel_Info = 3,
    LogLevel_Debug = 4
};

LogLevel GetLogLevel() {return LogLevel_Debug;};

void Write(const std::string& message) {};

void Write(LogLevel level, std::stringstream& ss) {
    if (level > GetLogLevel())
        return;
    Write(ss.str());
}

template<typename Arg> void Write(LogLevel level, std::stringstream& ss, Arg arg) {
    if (level > GetLogLevel())
        return;
    ss << arg;
    Write(ss.str());
}

template<typename First, typename... Rest> void Write(LogLevel level, std::stringstream& ss, First first, Rest... rest) {
    if (level > GetLogLevel())
        return;
    ss << first;
    Write(level, ss, rest); // Error C3520, see below
}

Write(std::stringstream(), "Hello", (const char*)" World!", 1);

I tried to create a recursive/variadic template for logging just as in MSDN, but I just can't rid of error C3520 (C3520: 'P' : parameter pack must be expanded in this context.). Is there something I'm doing wrong, or is it (god forbid) a compiler bug?

回答1:

I see the following issues:

1. rest needs to be expanded:

   Write(level, ss, rest...);
   

2. Write is supposed to take a log level as its first argument:

   Write(GetLogLevel(), std::stringstream(), "Hello", (const char*)" World!", 1);
   

3. You can't pass a temporary std::stringstream to an lvalue reference:

   std::stringstream ss;
   Write(GetLogLevel(), ss, "Hello", (const char*)" World!", 1);
   

回答2:

You probably need

Write(level, ss, rest...);
//                   ^^^ Note these dots!

来源：https://stackoverflow.com/questions/24683732/variadic-template-resolution-in-vs2013-error-c3520