Variadic function without specified first parameter? - c++

Question

Out of curiosity, I thought I'd try and write a basic C++ class that mimics C#'s multiple delegate pattern. The code below mostly does the job, with the nasty sacrifice of losing almost all type-safety, but having to use the initial dummy parameter to set up the va_list really seems a bit off. Is there a way to use va_list without this?
I do realize there are ways to do this with (for example) boost, but I was aiming for something dead simple that used just the standard library.
#include <vector>
#include <iostream>
#include <string>
#include <stdarg.h>
#include <algorithm>
using namespace std;
class CDelegate
{
public:
virtual bool operator()(va_list params) = 0;
};
class CMultipleDelegateCaller
{
public:
typedef vector<CDelegate*> CDelegateVector;
CMultipleDelegateCaller& operator+=(CDelegate &rDelegate)
{
m_apDelegates.push_back(&rDelegate);
return (*this);
}
CMultipleDelegateCaller& operator-=(CDelegate &rDelegate)
{
CDelegateVector::iterator iter =
find(m_apDelegates.begin(), m_apDelegates.end(), &rDelegate);
if (m_apDelegates.end() != iter) m_apDelegates.erase(iter);
return (*this);
}
bool Call(int iDummy, ...)
{
va_list params;
CDelegate* pDelegate;
CDelegateVector::iterator iter;
for (iter = m_apDelegates.begin(); iter != m_apDelegates.end(); ++iter)
{
pDelegate = *iter;
va_start(params, iDummy);
if (!(*pDelegate)(params)) return false;
va_end(params);
}
return true;
}
private:
CDelegateVector m_apDelegates;
};
class CTestDelegate:
public CDelegate
{
public:
CTestDelegate():m_iId(++s_iCount) {}
virtual bool operator()(va_list params)
{
int iIntParam = va_arg(params, int);
char* szCharPtrParam = va_arg(params, char*);
string* psStringParam = va_arg(params, string*);
cout<<m_iId<<"{"
<<iIntParam<<", "
<<szCharPtrParam<<", "
<<*psStringParam<<"}"<<endl;
return true;
}
int m_iId;
static int s_iCount;
};
int CTestDelegate::s_iCount = 0;
int main(int argc, char* argv[])
{
CMultipleDelegateCaller cDelegateCaller;
CTestDelegate cTestDelegate1;
CTestDelegate cTestDelegate2;
cout<<"--------------------"<<endl;
cDelegateCaller += cTestDelegate1;
cDelegateCaller += cTestDelegate2;
string sString("World");
cDelegateCaller.Call(1, 2, "Hello", &sString);
cout<<"--------------------"<<endl;
cDelegateCaller -= cTestDelegate1;
cDelegateCaller.Call(1, 2, "Hello", &sString);
cout<<"--------------------"<<endl;
cDelegateCaller -= cTestDelegate2;
cDelegateCaller.Call(1, 2, "Hello", &sString);
cout<<"--------------------"<<endl;
cin>>sString;
return 0;
}

Answer 1

Functions with ellipsis in C++ is only for compatibility with C. Using C++ I'd return temporary helper object in Call function and add template operator% to pass variable number of arguments. To use it in the following way:
cDelegateCaller.Call() % 2 % "Hello" % sString; // dummy argument isn't required
As to your question, Standard requires to invoke va_start before any access to the unnamed arguments. And va_start requires second argument which is the identifier of the rightmost parameter in the variable parameter list in the function definition.

Answer 2

Out of Kirill's answer you can conclude that it's possible to create a type-safe delegate, using a template argument-combining function. This function also needs a dummy starting point, but has the benefit of type-safety.
The FastFormat library uses this, boost uses this, and I once provided another example in an answer to another question.