> Here, there's a conflict between the lexical scope of the function (main) and the macro (SWAP).

It might interest to some to know that GCC has a macro called __COUNTER__ (apparently also implemented by clang and MS), which auto-increments its value. You can then create something like a gensym to avoid conflict.

Here's an implementation of the "defer" keyword in C++:

    template <typename F>
    struct privDefer {
     F f;
     privDefer(F f) : f(f) {}
     ~privDefer() { f(); }
    };

    template <typename F>
    privDefer<F> defer_func(F f) {
     return privDefer<F>(f);
    }

    #define DEFER_1(x, y) x##y
    #define DEFER_2(x, y) DEFER_1(x, y)
    #define DEFER_3(x)    DEFER_2(x, __COUNTER__)
    #define defer(code)   auto DEFER_3(_defer_) =     defer_func([&](){code;})

It was written by gingerBill, author of the Odin programming language.