.gitignore

fluid-let

fluid-let.cc

#include <stdio.h>

#include "fluid-let.h"

int x = 1;
struct pair {
    int a, b;
};

void print_x(void) {
    printf("x = %d\n", x);
}

int main(void) {
    print_x();
    {
        fluid_let(x);
        x = 2;
        print_x();
        x = 3;
        print_x();
    }
    print_x();

    struct pair p = {0,1};
    {
        fluid_let(p.a) = 4;
        fluid_let(p.b) = 7;
        printf("p.a = %d\n", p.a);
        printf("p.b = %d\n", p.b);
    }
    printf("p.a = %d\n", p.a);
}

fluid-let.h

template <class T>
class dynamic_binding {
public:
    dynamic_binding(T& ref) : ref_(ref), orig(ref) {
    }
    ~dynamic_binding() {
        ref_ = orig;
    }
    const T& operator=(const T& rhs) {
        ref_ = rhs;
        return ref_;
    }
private:
    T& ref_;
    T orig;
};

#define UNIQ__(token, lno)   token##lno
#define UNIQ_(token, lno)    UNIQ__(token, lno)
#define UNIQ(token)          UNIQ_(token, __LINE__)

#define fluid_let(x) dynamic_binding<typeof(x)> UNIQ(_dynamic)(x); UNIQ(_dynamic)