使用模板variadic函数将多个参数传递给另一个函数

Question

Let us consider the following function:

static void Print(const Type& type, const std::string& message, const std::string& variable) {
    Log(type, message + ": " + variable);
}

I'd like it to pass arbitrary number of variables (I mean std::string & variable - this holds a variable name) and then send them via Log() function together and for this reason, I've considered using template variadic function (an overloaded Print() ). I'd define it like this:

template <typename Arg, typename ...Args)
static void Print(const Type& type, const std::string& message,
                  const Arg& arg, const Args&... args);

and then:

Print(type, message, args...);
Log(type, message + ": " + arg);

Just an idea, this would work most likely like this:

• args... would be passed and Print() function would be called recursively until there's no arguments left,
• but at the same time, Log() function would be called which would basically log it every time.

What I would need to do is to somehow remember arg value but it would require calling Print() with an additional argument and I don't really like this idea. Do you have any other clues?

Answer 1

Depending on the desired format, you might be able to get away with a fold expression:

template<class... Args>
void Print(const Type& type, const std::string& message, const Args&... arg)
{
    std::stringstream strstr;
    strstr << message << ": "; // Or your prefix computation, whatever you want.

    ((strstr << arg << ", "), ...);

    std::string toLog = strstr.str();
    // Remove last separator characters.
    toLog.erase(toLog.end() - 2, toLog.end());
    Log(type, strstr.str());
}

Demo

Answer 2

It seems to me that the Max Langhof's solution is simple and elegant.

Unfortunately it uses template folding that is available only starting from C++17.

I propose a C++11/C++14 version that, instead template folding, uses the old trick of the initialization of an unused array

template <typename ... Args>
void Print (Type const & type, std::string const & message,
            Args const & ... arg)
 {
   using unused = int[];

   std::stringstream strstr;

   strstr << message << ": ";

   (void)unused { 0, (strstr << arg << ", ", 0)... };

    std::string toLog = strstr.str();

    // Remove last separator characters.
    toLog.erase(toLog.end() - 2, toLog.end());
    Log(type, strstr.str());
 }

Answer 3

I simplified your example a bit, so assuming I correctly understood what you want to do, you can do one of the 2 following solutions, if the C++17 folds suggested by @Max Langhof are not supported by your compiler.

Both of them work on any type that supports operator+ for doing the correct thing, but are simple to modify if your concat function is something else.

Option 1, recursive unpacking:

template <typename Arg>
static void Print(const Arg& message, const Arg& arg1)
{
    Log(message + ": " + arg1);
}

template <typename Arg, typename... Args>
static void Print(const Arg& message, const Arg& arg1, const Arg& arg2, const Args&... variables)
{
    Print(message, arg1 + ", " + arg2, variables...);
}

Option 2, unpacking into a std:vector:

template <typename Arg, typename... Args>
static void Print2(const Arg& message, const Arg& arg1, const Args&... variables)
{
    std::vector<Arg> args = { variables... };
    Arg result = std::accumulate(args.begin(), args.end(), arg1, [](const Arg& a, const Arg& b) {
        return a + ", " + b;});
    Log(message + ": " + result);
}

Be aware that this version will create copies of the arguments within the std::vector, unlike the other solution which will not.

Both examples can be used in the following fashion:

static void Log(const std::string& m)
{
    std::cout << m << std::endl;
}

int main()
{
    std::string msg = "MyMessage1";
    std::string var1 = "Var1";
    std::string var2 = "Var2";
    std::string var3 = "Var3";
    std::string var4 = "Var4";
    std::string var5 = "Var5";

    Print(msg, var1);
    Print(msg, var1, var2);
    Print(msg, var1, var2, var3);
    Print(msg, var1, var2, var3, var4);
    Print(msg, var1, var2, var3, var4, var5);
}