ChefZ -- 磨劍錄 (A Coder's Log)

• Start from something simple:

#include <stdarg.h> //	va_list
#include <stdio.h>

void func( const char *str, ... );

template< class TYPE, typename TO > bool FormTest( const TYPE str, TO i )
{
	func( str, i );
	return true;
}

int main( void )
{
	FormTest< char*, char* >( _("%s\n"), _("string") );
}

void func( const char *str, va_list &ap );

void func( const char *str, ... )
{
	va_list ap;
        va_start( ap, str );
	func( str, ap );
	va_end( ap );
}

void func( const char *str, va_list &ap )
{
	const char *pek; int i;
	pek=va_arg( ap, const char* );
	printf( str, pek );
}

• From something more flexible yet still deterministic:
  

template<typename T>
struct SVaPassNext{
    SVaPassNext<T> sibs;
    T dwObject;
};
template<> struct SVaPassNext<0>{};
//SVaPassNext - is generator of structure of any size at compile time.


class CVaPassNext{
public:
    SVaPassNext<typename T> svapassnext;
    CVaPassNext(va_list & args){
		try{//to avoid access violation

			memcpy(&svapassnext, args, sizeof(svapassnext));
		} catch (...) {}
    }
};
#define va_pass(valist, T) CVaPassNext(valist).svapassnext<T>

void func(const char *szFormat, ...)
{
    va_list args;
    va_start(args, szFormat);
    func(szFormat, args);
    va_end(args);
}
void func( const char *str, va_list &ap )
{
	wxObject*pek;
	pek = va_arg( va_pass(ap, wxObject*), wxObject* );
	....
}
int main( void )
{
    FormTest< char*, wxObject* >( _("id_of_MyObject"), &m_MyObject );
}

• How about from some underministic cases:

#include <iostream>
#include <cstdarg>
using namespace std;

/*
*  The challenge: find the maximum value of a arbitrary list of values without
*      the use of the conditional-control structures.  
*  NOTE: The use of the iif<T>() function was inspired
*        by Bench's int greatest(int, int) function using arrays to resolve a binary condition.
*        Bench also inspired the combination of a iif() and function pointers to make a function table to
*        avoid the use of any conditional control structure in the recursion.
*/

//This simulates the ?: operator 
template <class T>
T iif(bool condition, T true_val, T false_val) {
    T table[2];
    table[0] = false_val;
    table[1] = true_val;
    return table[condition]; 
}    

// This will return the greater of the two values
template <class T>
T greatest( T first, T second )
{
    return iif((first > second), first, second); 
}

// This function marks the end of the recursion used in maximum_loop()
template <class T>
T terminatingFunction(va_list &args, int num, T theMax) { return theMax; }

//This is a recursive function used to scroll thought the list or parameters
template <class T>
T maximum_loop(va_list &args, int num, T theMax) {
    T current = va_arg(args, T);
    theMax = greatest(theMax, current);
    //This is the ugliest line in the code, the "T (*)(va_list&, int, T)" is the type for the function pointers.
    // this line simulates the conditional function call:
    //  return ((num > 1) ? &maximum_loop: &terminatingFunction)(args, num-1, theMax);
    return iif<T (*)(va_list&, int, T)>((num > 1), &maximum_loop, &terminatingFunction )(args, num-1, theMax);
}

//This function will return the maximum value of a variable number of elements.
template <class T, int len>
T maximum(T first, ...) {
    T theMax = first;
    va_list args;
    va_start(args, first);
    int length = len - 1; //One is used up by "first"
    theMax = maximum_loop<T>(args, length, theMax);
    va_end(args);
    return theMax;
}


int main() {
    double a = maximum<double, 11>(2.1, 1.2, 65.3, 10.4, 37.5, 6.0, 9.0, 24.0, 89.3, 89.5, 89.9);
    cout << endl << a << endl;
    system("Pause");
    return 0;
}

• How about these:

template<typename T>
void print( const T& t )
{
        std::cout << t;
}
 
template< typename T, typename... Args>
void print( const T& t, const Args&... args )
{
       print( t );
       print( args... );
}
 
template<typename... T>
class tuple;
template<typename T, typename... Ts>
class tuple<T, Ts...>;
template<>
class tuple<>;