C++11 Variadic Template

Question

I got some example code to make a c++ variadic template from here:

http://en.wikipedia.org/wiki/Variadic_template

My code is as follows.

#ifdef DEBUG
    #define logDebug(x, ...) streamPrintf( x, ##__VA_ARGS__ );
#else
    #define logDebug(x, ...)
#endif

void streamPrintf(const char *s);
template<typename T, typename... Args>
void streamPrintf(const char *s, T value, Args... args)
{
while (*s) {
    if (*s == '%') {
        if (*(s + 1) == '%') {
            ++s;
        }
        else {
            std::cout << value;
            streamPrintf(s + 1, args...); 
            return;
        }
    }
    std::cout << *s++;
}
throw std::logic_error("extra arguments provided to printf");
}

void streamPrintf(const char *s)
{
while (*s) {
    if (*s == '%') {
        if (*(s + 1) == '%') {
            ++s;
        }
        else {
            throw std::runtime_error("invalid format string: missing arguments");
        }
    }
    std::cout << *s++;
    }
}

But it prints only junk. The main reason to use this is so I can print out std::string. How can I print out the correct values?

I call the function like this:

logDebug("Event is, event=%", value);

Peter T found the problem via chat. It does not print uint8_t correctly as it treats it like a ASCII. It needs to be type cast to e.g. uint16_t. When I have a solution, I will post it here.

Did you compile with all warnings and debug info (`gcc -std=c++11 -Wall -g`)? Did you use the debugger (`gdb`)? — Basile Starynkevitch, Oct 06 '14 at 11:08
Looks just [fine](http://coliru.stacked-crooked.com/a/3cb5daa3767e0984) to me. — Columbo, Oct 06 '14 at 11:14
Did you use the `gdb` debugger to run your program step by step? — Basile Starynkevitch, Oct 06 '14 at 11:15
@user1876942 What do you mean? I am passing values into the function (template), two `std::string`s and an integer. Edit:Alright, i see your edit. — Columbo, Oct 06 '14 at 11:22
@user1876942 You do realize that this `streamPrintf`-template doesn't consider format specifiers? All it takes is the percentage sign `'%'`, anything after that is just printed out. — Columbo, Oct 06 '14 at 11:24
@Loopunroller - Yes, I tried that but still problems, then I just tried anything. I will edit my question. — user1876942, Oct 06 '14 at 11:25
@user1876942 What **are** the problems? When i compile your code like this it works *just* like I expected. Do you use VC++? — Columbo, Oct 06 '14 at 11:27
Nope, even VC++ is [doing the job](http://rextester.com/XVVHX51461). I really can't reproduce your problems, could you provide us with a complete test program? — Columbo, Oct 06 '14 at 11:29
@Loopunroller it compiles OK but does not print the values, I see blank or a square box. — user1876942, Oct 06 '14 at 11:29
@Loopunroller OK, it works for you, I will need to investigate why not for me. I did exactly what you did. — user1876942, Oct 06 '14 at 11:31
is this correct? #define logDebug(x, ...) streamPrintf( x, ##__VA_ARGS__ ); — user1876942, Oct 06 '14 at 11:35
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/62518/discussion-between-loopunroller-and-user1876942). — Columbo, Oct 06 '14 at 11:38
"But it prints only junk." What does that mean? Just post complete program and the output you get. — D Drmmr, Oct 06 '14 at 11:53

Laura Maftei · Answer 1 · 2014-10-06T13:29:53.303

A good example for using variadic templates with printf can be found here:

http://msdn.microsoft.com/en-us/library/dn439779.aspx

void print() {
    cout << endl;
}

template <typename T> void print(const T& t) {
    cout << t << endl;
}

template <typename First, typename... Rest> void print(const First& first, const Rest&... rest) {
    cout << first << ", ";
    print(rest...); // recursive call using pack expansion syntax
}

int main()
{
    print(); // calls first overload, outputting only a newline
    print(1); // calls second overload

    // these call the third overload, the variadic template, 
    // which uses recursion as needed.
    print(10, 20);
    print(100, 200, 300);
    print("first", 2, "third", 3.14159);
}

Answers that only have a link aren't very good; the links rot. Try to at least sum up the article. — Bartek Banachewicz, Oct 06 '14 at 12:27

C++11 Variadic Template

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