How do you implement mutual casts?

Question

Say you have two struct types, one with int members and one with float.

struct i { 
    int a, b; 
    i(int a, int b): a(a), b(b) {}
};

struct f { 
    float a, b;
    f(float a, float b): a(a), b(b) {}
};

We want to define two cast operators, from i to f and conversely. If we try to do it by operator overloading

struct i { 
    int a, b; 
    i(int a, int b): a(a), b(b) {}
    operator f() const { return f(a, b); };
};

struct f { 
    float a, b;
    f(float a, float b): a(a), b(b) {}
    operator i() const { return i(a, b); };
};

we run in a problem of order of declaration, because i needs to know f and f needs to know i. Furthermore, the cast operators must be declared inside the classes. A forward declaration of f doesn't work.

Is there a solution ?

Answer 1

A forward declaration works fine:

struct i;
struct f;

struct i
{
    int a, b; 
    i(int a, int b): a(a), b(b) {}
    operator f() const;
};

struct f
{
    float a, b;
    f(float a, float b): a(a), b(b) {}
    explicit operator i() const;
};

i::operator f() const { return f(a, b); }
f::operator i() const { return i(a, b); }