Pythons 'with' statement in C++

with.cc

#include <type_traits>

template <template <typename...> class With, typename T> inline
decltype(auto)
make_with_guard (T& value) {
    return With<T>(value);
}

#define PP_CAT(A, B) PP_CAT_X(A, B)
#define PP_CAT_X(A, B) A ## B
#define PP_UNIQUE(BASE) PP_CAT(BASE, __LINE__)

#define WITH_IMPL(T, OBJ, GUARD)                                \
    struct PP_CAT(GUARD,_t) {                                   \
        T<std::remove_reference<decltype(OBJ)>::type> guard;    \
        operator bool() const noexcept { return false; }        \
    };                                                          \
    if (PP_CAT(GUARD,_t) GUARD = {make_with_guard<T>(OBJ)}) {   \
        (void) GUARD;                                           \
    } else

#define WITH(T,OBJ) WITH_IMPL(T, OBJ,                           \
                              PP_UNIQUE(with_guard_for_line_))

/*
 * Example usage
 */

#include <iostream>

template <typename T>
struct inc_dec {
    T& x;
    inc_dec(T& r): x(r) { ++x;  }
    ~inc_dec() { --x; }
};

int main() {
    int i = 0;
    std::cout << i << std::endl;
    WITH (inc_dec, i) {
        std::cout << i << std::endl;
    }
    std::cout << i << std::endl;
}