How to pass reference to a vector as variadic argument? - c++

Question

class Hex
{
short type;
Hex(short typE)
{
type = typE;
}
}
vector< vector<Hex> > hexes;
void HexRings(sf::Vector2i pos, void operation(sf::Vector2i, va_list), ...)
{
va_list args;
va_start(args, operation);
//block of irrelevant changes of pos
operation(pos, args);
//another block of irrelevant changes of pos
va_end(args);
}
void MakeHexTypeX(sf::Vector2i pos, va_list args)
{
hexes[pos.x][pos.y].type = va_arg(args, int);
}
void AddHexes(sf::Vector2i pos, va_list args)
{
vector<sf::Vector2i> returnThat;
returnThat = va_arg(args, vector<sf::Vector2i>());// how to make this line actually give back a reference to the original vector (visited)
list.push_back(sf::Vector2i(pos.x, pos.y));
}
int main()
{
vector<sf::Vector2i> visited;
for(int y=0; y<10; ++y)
{
hexes.push_back(vector<Hex>());
for(int x=0; x<10; ++x) hexes[x].push_back(Hex(0));
}
HexRings(sf::Vector2i(5, 5), AddHexesOfTypeX, visited);
HexRings(sf::Vector2i(5, 5), MakeHexTypeX, 1);
}
I have a function (HexRings), that iterates in a specific pattern on a two-dimensional vector. I want to have an ability to pass to this function a pointer to another function (like MakeHexTypeX or AddHexes), that performs a certain action on every visited tile. Everything is fine, if this function is changing the map, but I don't know what to do, if this function has to report back. For example, if I want to get all hexes of type X, that HexRings has encountered, the vector would have to be passed as reference, which I don't even know how to write.
TL;DR: How to pass reference to a vector as variadic argument?

Answer 1

Instead of passing the operation function into HexRings, I would pass a reference to a functor. Functors can store state and thus hold the vector you wish. Here we go:
struct processor {
// you will be overriding this for every different operation you want to perform
virtual void operator()(sf::Vector2i, va_list) = 0;
};
struct make_hex_type_x
: public processor {
void operator()(sf::Vector2i pos, va_list args) override {
hexes[pos.x][pos.y].type = va_arg(args, int);
}
};
struct add_hexes
: public processor {
private:
// this is our state
vector<sf::Vector2i> returnThat;
public:
void operator()(sf::Vector2i pos, va_list args) override {
returnThat = va_arg(args, vector<sf::Vector2i>());
list.push_back(sf::Vector2i(pos.x, pos.y));
}
// get access to your return vector
vector<sf::Vector2i>& get() {
return returnThat;
}
};
//now change HexRings so that it accepts the functor instead of a function
void HexRings(sf::Vector2i pos, processor& pr, ...)
{
va_list args;
va_start(args, operation);
//block of irrelevant changes of pos
pr(pos, args);
//another block of irrelevant changes of pos
va_end(args);
}
// and finally call as follows
int main()
{
vector<sf::Vector2i> visited;
for(int y=0; y<10; ++y)
{
hexes.push_back(vector<Hex>());
for(int x=0; x<10; ++x) hexes[x].push_back(Hex(0));
}
make_hex_type_x p1;
add_hexes p2;
HexRings(sf::Vector2i(5, 5), p2, visited);
HexRings(sf::Vector2i(5, 5), p1, 1);
// now extract your vector
auto& vec = p2.get();
}