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Show Lecture.ProxyObjects as a slide show.

CS253 Proxy Objects

The problem

• Consider pi, the ratio of the circumference to the diameter of a circle.
• Is it “π”, or 3.14159+?
• They’re both valid representations.
• I want a function that returns one or the other, depending on context.
  • If I’m asking for a string, give me “π”.
  • If I’m asking for a number, give me 3.14159.

Overloading Functions

This doesn’t work:

double pi() { return 3.14159; }
string pi() { return "π"; }

int main() {
    double d = pi();
    string s = pi();
    cout << d << ' ' << s << '\n';
}

c.cc:2: error: ambiguating new declaration of 'std::__cxx11::string pi()'

The failure is at line 2, not at the point of use. You can’t overload just on the return type.

Overloading Methods

This doesn’t work, either:

class Foo {
  public:
    double pi() { return 3.14159; }
    string pi() { return "π"; }
};

c.cc:4: error: 'std::__cxx11::string main()::Foo::pi()' cannot be overloaded 
   with 'double main()::Foo::pi()'

Same problem.

Pouting

• But I waaaaaaaant it to work! 😭
• How can I accomplish this?
• I simply cannot overload a function by its return type.
• Sure, I could have .pi_double() and .pi_string(), but I would then plunge into depression over the sheer inelegance.

Solution

class Dual {
  public:
    operator double() const { return 3.14159; }
    operator string() const { return "π"; }
};

Dual pi() { return Dual(); }

int main() {
    double d = pi();
    string s = pi();
    cout << d << ' ' << s << '\n';
}

3.14159 π

pi() doesn’t return a number or a string. It returns a proxy object, which will produce either value, based on context.

More General Solution

class Dual {
    const double d;
    const string s;
  public:
    Dual(double dd, const string &ss) : d(dd), s(ss) { }
    operator double() const { return d; }
    operator string() const { return s; }
};

Dual pi() { return Dual(3.14159, "π"); }

int main() {
    double d = pi();
    string s = pi();
    cout << d << ' ' << s << '\n';
}

3.14159 π

Even More General Solution

template <typename T, typename U>
class Dual {
    const T t;
    const U u;
  public:
    Dual(const T &tt, const U &uu) : t(tt), u(uu) { }
    operator T() const { return t; }
    operator U() const { return u; }
};

Dual<double, string> pi() { return Dual<double, string>(3.14159, "π"); }

int main() {
    double d = pi();
    string s = pi();
    cout << d << ' ' << s << '\n';
}

3.14159 π

Slightly Prettier Solution

template <typename T, typename U>
class Dual {
    const T t;
    const U u;
  public:
    Dual(const T &tt, const U &uu) : t(tt), u(uu) { }
    operator T() const { return t; }
    operator U() const { return u; }
};

Dual<double, string> pi() { return {3.14159, "π"}; }

int main() {
    double d = pi();
    string s = pi();
    cout << d << ' ' << s << '\n';
}

3.14159 π

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