strongtype.cpp

#include <iostream>
#include <vector>

struct Feet {
    double f;
    Feet(double f)
        : f(f)
    {
    }
    explicit constexpr operator double() const { return f; }
};

struct Meters {
    double m;

    constexpr Meters(double m)
        : m(m)
    {
    }

    constexpr Meters(const Feet& f)
        : m(0.3048 * f.f)
    {
    }

    constexpr Meters& operator+=(const Meters& rhs)
    {
        m += rhs.m;
        return *this;
    }

    explicit constexpr operator double() const { return m; }
};

Meters foo(Meters m1) { return Meters{ 2 * m1.m }; }

int main(int argc, char* argv[])
{
    size_t N = 10000000;

    std::vector<Feet> feet;
    feet.reserve(N);
    for (int i = 0; i < N; i++) {
        feet.push_back(i);
    }

    Meters sum = 0.0;
    for (int i = 0; i < N; i++) {
        sum += foo(feet[i]);
    }

    std::cout << double(sum);
}

weaktype.cpp

#include <iostream>
#include <vector>

typedef double Meters;
typedef double Feet;

Meters foo(Meters m1) { return Meters{ 2 * m1 }; }

int main(int argc, char* argv[])
{
    size_t N = 10000000;

    std::vector<Feet> feet;
    feet.reserve(N);
    for (int i = 0; i < N; i++) {
        feet.push_back(i);
    }

    Meters sum = 0.0;
    for (int i = 0; i < N; i++) {
        sum += foo(0.3048 * feet[i]);
    }

    std::cout << sum;
}