[boost] building container on stack (var arg)

From: Eric Zijlstra (ezijlstra_at_[hidden])
Date: 2005-04-29 20:38:44

I was looking for a convinient way to have variabele arguments for functions
and avoiding the memory allocations and copying that come with std
containers.  I didn't find such a container so I hacked one together myself.
The container builds a linked list by storing pointers to the arguments in
temporary node objects (StackArg). The list becomes invalid as soon as the
nodes go out of scope.  A test showed that this was 5 times faster than
using a std::vector. I don't have access to the code ATM, but look below how
it looks like (very shortend).

Ofcourse, the + operator is misleading (the leading empty StackArg<>()
clarifies a lot) but otherwise it's pretty convinient.

template<class T>
class StackArg
{ public:
    StackArg *link;
    T *data;
    StackArg() link(0), data(0) {}
    StackArg(StackArg *prev, T *t) :link(prev), data(t) { }
    StackArg operator + (T &t) { return StackArg(this, &t); }
    iterator begin();
    iterator end();
};

The first time when begin() or end() is called, the list is reversed because
since operator+() is evaluated from left to right, the last element becomes
the container handle (s in code below).

// usage example:

void print(StackArg<const int> s)  // outputs 345
{
    StackArg<const int>::iterator i= s.begin(), e=end();
    while(i!=e) cout << *i++;
}

void test()
{
    print( StackArg<const int>() + 3 + 4 +5 );
}