C++ MACRO that will execute a block of code and a certain command after that block

Question

void main()
{
    int xyz = 123; // original value
    { // code block starts
        xyz++;
        if(xyz < 1000)
            xyz = 1;
    } // code block ends
    int original_value = xyz; // should be 123
}

void main()
{
    int xyz = 123; // original value
    MACRO_NAME(xyz = 123) // the macro takes the code code that should be executed at the end of the block.
    { // code block starts
        xyz++;
        if(xyz < 1000)
            xyz = 1;
    } // code block ends << how to make the macro execute the "xyz = 123" statement?
    int original_value = xyz; // should be 123
}

Only the first main() works.
I think the comments explain the issue.

It doesn't need to be a macro but to me it just sounds like a classical "macro-needed" case.

By the way, there's the BOOST_FOREACH macro/library and I think it does the exact same thing I'm trying to achieve but it's too complex for me to find the essence of what I need.
From its introductory manual page, an example:

#include <string>
#include <iostream>
#include <boost/foreach.hpp>

int main()
{
    std::string hello( "Hello, world!" );

    BOOST_FOREACH( char ch, hello )
    {
        std::cout << ch;
    }

    return 0;
}

Answer 1

You can't make a macro perform a command after a loop unless you put the loop in the macro. And seriously? It would be a much better idea just to make a scoped variable.

template<typename T> class CallFunctionOnScopeExit {
    T t;
public:
    CallFunctionOnScopeExit(T tt) : t(tt) {}
    ~CallFunctionOnScopeExit() { t(); }
};

Guaranteed in the cases of exception, etc, whereas the macro version most definitely isn't. I would prefer to use this pattern for the exception guarantees, and because it's more flexible than just copying the int.

Answer 2

The cleanest way to do this is probably to use an RAII container to reset the value:

// Assumes T's assignment does not throw
template <typename T> struct ResetValue
{
    ResetValue(T& o, T v) : object_(o), value_(v) { }
    ~ResetValue() { object_ = value_; }

    T& object_;
    T value_;
};

used as:

{
    ResetValue<int> resetter(xyz, 123);
    // ...
}

When the block ends, the destructor will be called, resetting the object to the specified value.

If you really want to use a macro, as long as it is a relatively simple expression, you can do this using a for-block:

for (bool b = false; b == false; b = true, (xyz = 123))
{
    // ...
}

which can be turned into a macro:

#define DO_AFTER_BLOCK(expr) \
    for (bool DO_AFTER_BLOCK_FLAG = false; \
         DO_AFTER_BLOCK_FLAG == false; \
         DO_AFTER_BLOCK_FLAG = true, (expr))

used as:

DO_AFTER_BLOCK(xyz = 123)
{
    // ...
}

I don't really think the macro approach is a good idea; I'd probably find it confusing were I to see this in production source code.

Answer 3

You don't absolutely need a macro - you could use inner scope variables:

#include <stdio.h>
int main(void)
{
    int xyz = 123;
    printf("xyz = %d\n", xyz);
    {
        int pqr = xyz;
        int xyz = pqr;
        printf("xyz = %d\n", xyz);
        xyz++;
        if (xyz < 1000)
            xyz = 1;
        printf("xyz = %d\n", xyz);
    }
    printf("xyz = %d\n", xyz);
    return(0);
}

This produces the output:

xyz = 123
xyz = 123
xyz = 1
xyz = 123

If you compile with GCC and -Wshadow you get a warning; otherwise, it compiles clean. You can't write int xyz = xyz; in the inner block reliably; once the '=' is parsed, the declaration is complete and so the initializer is the inner 'xyz', not the outer. The two step dance works, though.

The primary demerit of this is that it requires a modification in the code block.

If there are side-effects in the block - like the print statements above - you could call a function that contains the inner block. If there are no side-effects in the block, why are you executing it at all.

#include <stdio.h>
static void inner(int xyz)
{
    printf("xyz = %d\n", xyz);
    xyz++;
    if (xyz < 1000)
        xyz = 1;
    printf("xyz = %d\n", xyz);
}

int main(void)
{
    int xyz = 123;
    printf("xyz = %d\n", xyz);
    inner(xyz);
    printf("xyz = %d\n", xyz);
    return(0);
}