variable parameter function, how to make it type safe and more meaningful?
I am a newer for C++, and my first language is Chinese, so my words with English may be unmeaningful, say sorry first. I know there is a way to write a function with variable parameters which number or type maybe different each calling, we can use the macros of va_list,va_start and va_end. But as everyone know, it is the C style. When we use the macros, we will lose the benefit of type-safe and auto-inference, then I try do it whit C++ template. My work is followed:
#include<iostream>
#include<vector>
#include<boost/any.hpp>
struct Argument
{
typedef boost::bad_any_cast bad_cast;
template<typename Type>
Argument& operator,(const Type& v)
{
boost::any a(v);
_args.push_back(a);
return *this;
}
size_t size() const
{
return _args.size();
}
template<typename Type>
Type value(size_t n) const
{
return boost::any_cast<Type>(_args[n]);
}
template<typename Type>
const Type* piont(size_t n) const
{
return boost::any_cast<Type>(&_args[n]);
}
private:
std::vector<boost::any> _args;
};
int sum(const Argument& arg)
{
int sum=0;
for(size_t s=0; s<arg.size(); ++s)
{
sum += arg.value<int>(s);
}
return sum;
}
int main()
{
std::cout << sum((Argument(), 1, 3, 4, 5)) << std::endl;
return 0;
}
I think it's ugly, I want to there is a way to do better? Thanks, and sorry for language errors.
You can do something like this:
template <typename T>
class sum{
T value;
public:
sum ()
: value() {};
// Add one argument
sum<T>& operator<<(T const& x)
{ value += x; return *this; }
// to get funal value
operator T()
{ return value;}
// need another type that's handled differently? Sure!
sum<T>& operator<<(double const& x)
{ value += 100*int(x); return *this; }
};
#include <iostream>
int main()
{
std::cout << (sum<int>() << 5 << 1 << 1.5 << 19) << "n";
return 0;
}
Such technique (operator overloading and stream-like function class) may solve different problems with variable arguments, not only this one. For example:
create_window() << window::caption - "Hey" << window::width - 5;
// height of the window and its other parameters are not set here and use default values
I wrote an article about a typesafe printf C++ implementation on DDJ.com some years ago. It propably deals with comparable problems. Maybe that helps.
See http://www.ddj.com/cpp/184401999
After giving it some thought, I found a way to do it using a typelist. You don't need an any type that way, and your code becomes type-safe.
It's based on building a template structure containing a head (of a known type) and a tail, which is again a typelist. I added some syntactic sugar to make it more intuitive: use like this:
// the 1 argument processing function
template< typename TArg > void processArg( const TArg& arg ) {
std::cout << "processing " << arg.value << std::endl;
}
// recursive function: processes
// the first argument, and calls itself again for
// the rest of the typelist
// (note: can be generalized to take _any_ function
template< typename TArgs >
void process( const TArgs& args ) {
processArg( args.head );
return process( args.rest );
}
template<> void process<VoidArg>( const VoidArg& arg ){}
int main() {
const char* p = "another string";
process( (arglist= 1, 1.2, "a string", p ) );
}
And here is the argument passing framework:
#include <iostream>
// wrapper to abstract away the difference between pointer types and value types.
template< typename T > struct TCont {
T value;
TCont( const T& t ):value(t){}
};
template<typename T, size_t N> struct TCont< T[N] > {
const T* value;
TCont( const T* const t ) : value( t ) { }
};
template<typename T> struct TCont<T*> {
const T* value;
TCont( const T* t ): value(t){}
};
// forward definition of type argument list
template< typename aT, typename aRest >
struct TArgList ;
// this structure is the starting point
// of the type safe variadic argument list
struct VoidArg {
template< typename A >
struct Append {
typedef TArgList< A, VoidArg > result;
};
template< typename A >
typename Append<A>::result append( const A& a ) const {
Append<A>::result ret( a, *this );
return ret;
}
//syntactic sugar
template< typename A > typename Append<A>::result operator=( const A& a ) const { return append(a); }
} const arglist;
// typelist containing an argument
// and the rest of the arguments (again a typelist)
//
template< typename aT, typename aRest >
struct TArgList {
typedef aT T;
typedef aRest Rest;
typedef TArgList< aT, aRest > Self;
TArgList( const TCont<T>& head, const Rest& rest ): head( head ), rest( rest ){}
TCont<T> head;
Rest rest;
template< typename A > struct Append {
typedef TArgList< T, typename Rest::Append<A>::result > result;
};
template< typename A >
typename Append< A >::result append( const A& a ) const {
Append< A >::result ret ( head.value, (rest.append( a ) ) );
return ret;
}
template< typename A > typename Append<A>::result operator,( const A& a ) const { return append(a); }
};
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