Question

Could you help me in fixing the compile error in the following code:

#include <sstream>
#include <iostream>

using namespace std;

template<typename T, typename ...P>
class Mystrcat{
 public:
 Mystrcat(T t, P... p){init(t,p...);}

 ostringstream & get(){return o;}         

 private:
 ostringstream o;
 void init(){}
 void init(T t, P... p);
};

template<typename T, typename ...P>
void Mystrcat<T,P...>::init(T t, P ...p){
  o << t;
  if (sizeof...(p)) init(p...);
}

int main(){
 Mystrcat<char*,char *> m("Amma","Namasivayah");
 cout << m.get().str();
}

I get the error, no matching function for call to

‘Mystrcat<char*, char*>::init(char*&)’

note: candidates are:

void Mystrcat<T, P>::init() [with T = char*, P = char*]
void Mystrcat<T, P>::init(T, P ...) [with T = char*, P = char*]

gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)

Thanks
suresh

Answer 1

You’re getting this error because there isn’t a way to unpack your p into either init function. In your instantiation Mystrcat<char*, char *>, unpacking a P... will yield a single thing in the type: char*, which doesn’t have an init with that signature (the instantiated version will have a void init() and void init(char*, char*), whereas you are trying to call init(char*)).

In fact, your template is impossible to instantiate, since init will always take one more argument than you give it in void Mystrcat<T,P...>::init(T t, P ...p). If you change the definition to call what you have defined:

template<typename T, typename ...P>
void Mystrcat<T,P...>::init(T t, P ...p){
  o << t;
  if (sizeof...(p)) init(t, p...);
}

then this will work (at least in g++-4.5.2).

---

EDIT: I think this is what you’re actually looking for:

#include <sstream>
#include <iostream>

class MyCollector
{
public:
    template <typename... T>
    explicit MyCollector(const T&... args)
    {
        init(args...);
    }

    std::string str()
    {
        return _stream.str();
    }

private:
    void init()
    { }

    template <typename First, typename... Rest>
    void init(const First& first, const Rest&... rest)
    {
        _stream << first;
        init(rest...);
    }

    std::ostringstream _stream;
};

int main()
{
    MyCollector collector("Whatever ", "stuff like ", 2, " or ", 3.14, "\n");
    std::cout << collector.str();
    return 0;
}