Question

这段代码出了什么问题？

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);

我尝试创建一个用于记录just as in MSDN的递归/可变参数模板，但我无法摆脱错误C3520（C3520: 'P' : parameter pack must be expanded in this context.）。有什么我做错了，还是（上帝禁止）编译错误？

Answer 1

我看到以下问题：

rest需要扩展：
```
Write(level, ss, rest...);
```

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);

您无法将临时std::stringstream传递给左值参考：

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

Answer 2

您可能需要

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

VS2013中的可变参数模板分辨率 - 错误C3520

2 个答案: