gistfile1.cpp

#include <iostream>
#include <cmath>

struct NormProxy
{
    operator float() const { return sqrtf(x * x + y * y); }

private:
    struct { float x, y; };
};

struct vec2
{
    vec2(float x, float y) : x(x), y(y) {}

    union
    {
        struct { float x, y; };

        NormProxy const norm;
    };
};

int main(void)
{
    vec2 f(3.f, 4.f);

    std::cout << f.norm << std::endl;
}