Re: Emulating 'swizzling' in C++

From:
"Francesco S. Carta" <entuland@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Sat, 19 Sep 2009 07:27:43 -0700 (PDT)
Message-ID:
<554a39e2-5488-4886-b6ba-13be0f108312@q14g2000vbi.googlegroups.com>
On 18 Set, 17:17, Alan Woodland <aj...@aber.ac.uk> wrote:

Hi,

A number of 'C-style' languages define a 'swizzle' operator which allows
the permutation of the elements of vector datatypes. This is typically
done with an extra meaning to the . operator.

In C++ we can't overload operator. for fairly good reasons! Is there a
tidier way of emulating this syntax than the following:

#include <iostream>
#define zyxw swizzle(3,2,1,0)
struct ivec4 {
        int data[4];
        ivec4 swizzle(unsigned char p1, unsigned char p2, unsigne=

d char

p3, unsigned char p4) {
                ivec4 ret;
                ret.data[0] = data[p1];
                ret.data[1] = data[p2];
                ret.data[2] = data[p3];
                ret.data[3] = data[p4];
                return ret;
        }

};

int main() {
        ivec4 f1, f2;
        f1.data[0] = -666;

        f2 = f1.zyxw;
        std::cout << f2.data[3] << std::endl;
        return 0;

}

This has some pretty serious problems, because of the macro use (and
generating a macro like this for all possible permutations of larger
vectors is quite sizeable!)

Can anyone suggest a cleaner way to achieve this 'syntax' within the
existing (including proposed C++0x) language?


See if you like this ugly beast too:

-------
#include <iostream>
#include <string>
#include <sstream>
#include <vector>

using namespace std;

template<bool> struct CompileTimeAssert;
template<> struct CompileTimeAssert<true> {};
#define STATIC_ASSERT(expr) CompileTimeAssert<(expr)>()

string tostring(int i) {
  ostringstream oss;
  oss << i;
  return oss.str();
}

struct vec_error {
  const string msg;
  vec_error(const string& s) : msg(s) {}
};

template<class T>
class vec_base {
public:
  virtual size_t size() const = 0;
  virtual T& operator[](int) throw(vec_error) = 0;
  virtual T operator[](int) const throw(vec_error) = 0;
  virtual ~vec_base() {};
};

template<char first_char, char last_char, class T>
class vec : public vec_base<T> {
  vector<T> v_data;
  static const int v_size = last_char - first_char + 1;
  T& checked(int vi) {
    if (0 <= vi && vi < v_size) {
      return v_data[vi];
    } else {
      throw vec_error(string("index out of range: ") + tostring(vi));
    }
  }
  const T& checked(int vi) const {
    if (0 <= vi && vi < v_size) {
      return v_data[vi];
    } else {
      throw vec_error(string("index out of range: ") + tostring(vi));
    }
  }
public:
  vec() {
    STATIC_ASSERT(v_size > 0);
    v_data.resize(v_size);
  }
  size_t size() const {
    return v_size;
  }
  vec operator()(const string& order) const throw(vec_error) {
    if (order.size() != v_size) throw vec_error("string size error");
    vec result;
    for (int i = 0; i < v_size; ++i) {
      try {
        result.v_data[i] = checked(order[i] - first_char);
      } catch (vec_error) {
        throw vec_error(string("char out of range: ") + order[i]);
      }
    }
    return result;
  }
  T& operator()(char ch) throw(vec_error) {
    try {
      return checked(ch - first_char);
    } catch (vec_error) {
      throw vec_error(string("char out of range: ") + ch);
    }
  }
  T operator()(char ch) const throw(vec_error) {
    try {
      return checked(ch - first_char);
    } catch (vec_error) {
      throw vec_error(string("char out of range: ") + ch);
    }
  }
  T& operator[](int vi) throw(vec_error) {
    return checked(vi);
  }
  T operator[](int vi) const throw(vec_error) {
    return checked(vi);
  }
  void detail_to(ostream& os) const {
    char ch = first_char;
    os << "{" << endl;
    for (int i = 0; i < v_size-1; ++i) {
      os << " '" << ch++ << "' == " << v_data[i] << endl;
    }
    os << " '" << ch << "' == " << v_data[v_size-1] << endl;
    os << "}" << endl;
  }
};

template<class T>
ostream& operator<<(ostream& os, const vec_base<T>& v) {
  os << "{";
  for (int i = 0, e = v.size()-1; i < e; ++i) {
    os << v[i] << ", ";
  }
  os << v[v.size()-1] << "}";
  return os;
}

void test() {
  vec<'x', 'z', string> xz1, xz2;

  xz1('x') = "roll";
  xz1('y') = "pitch";
  xz1('z') = "yaw";

  xz2 = xz1("zxy");

  cout << "xz1 == " << xz1 << endl;
  cout << "xz2 == " << xz2 << endl;

  vec<'a', 'f', int> af1, af2;

  for (int i = 0, e = af1.size(); i < e; ++i) {
    af1[i] = (i+1) * 10;
  }

  af2 = af1("fedcba");

  cout << "af1 details:" << endl;
  af1.detail_to(cout);
  cout << "af2 details:" << endl;
  af2.detail_to(cout);

}

int main() {
  try {
    test();
  } catch (vec_error m) {
    cout << "vec_error! " << m.msg << endl;
  }
}

-------

Output:

-------
xz1 == {roll, pitch, yaw}
xz2 == {yaw, roll, pitch}
af1 details:
{
  'a' == 10
  'b' == 20
  'c' == 30
  'd' == 40
  'e' == 50
  'f' == 60
}
af2 details:
{
  'a' == 60
  'b' == 50
  'c' == 40
  'd' == 30
  'e' == 20
  'f' == 10
}
-------

Cheers,
Francesco
--
 Francesco S. Carta, hobbyist
 http://fscode.altervista.org

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