c++ - VS2013 中的 Variadic 模板解析 - 错误 C3520

Question

这段代码有什么问题？

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);

我试图创建一个用于日志记录的递归/可变参数模板 just as in MSDN ，但我无法摆脱错误 C3520(C3520:“P”:必须在此上下文中扩展参数包。)。是我做错了什么，还是(上帝保佑)编译器错误？

Answer 1

我看到以下问题:

1. rest需要展开:

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

2. Write 应该将日志级别作为其第一个参数:

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

3. 您不能将临时 std::stringstream 传递给左值引用:

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