Interesting C++ problem

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Hello CodeForces community!

Recently I've recalled one interesting C++ problem. In short: is it possible to write functions action and cond in such a way, so that the following code snippets work differently:

// Snippet #1:
action();
while (cond()) {
    action();
}

// Snippet #2:
do {
    action();
} while (cond());

More formally: suppose you're given the following three files:

common.h

#include <iostream>

bool cond() {}

void action() {
    std::cout << "Action" << std::endl;
}

main_while.cpp

#include "common.h"

int main() {
    action();
    while (cond()) {
        action();
    }
    return 0;
}

main_do_while.cpp

#include "common.h"

int main() {
    do {
        action();
    } while (cond());
    return 0;
}

You're only allowed to modify common.h. You have to do that in such a way, so that Action word is printed different amount of times by execution of main_while.cpp and main_do_while.cpp.

One of the possible solutions (try to think yourself before opening the spoiler!):

Dirty hack

#include <iostream>

bool cond() {
    #define do if(true)
    static int attempt = 0;
    attempt++;
    return attempt % 2 != 0;
}

void action() {
    std::cout << "Action" << std::endl;
}

This soluion has obvious disadvantages. It "hacks" the language and doesn't have any interesting point. Actually, trying define was my first idea.

Share your solutions in the comments and don't forget to hide them under the spoilers! It would be interesting to see something smarter (e.g. solution which will work in any language with analogues of while and do-while)

Comments (2)

Write comment?

slycelote

8 years ago, # |

+26

Solution without macros:

#include <iostream>

bool cond() {
   static int cnt = 1;
   return cnt-- > 0;
}

constexpr int flag (int);

template<class Tag>
struct writer {
  friend constexpr int flag (Tag) {
    return 0;
  }
};

template<bool B, class Tag = int>
struct dependent_writer : writer<Tag> { };

template<
  bool B = noexcept (flag (0)),
  int    =   sizeof (dependent_writer<B>)
>
void action() {
  if (B) {
    std::cout << "Action" << std::endl;
  }
}

It uses this magic, which is legal as of C++ 14. The link also contains workarounds if your compiler doesn't support this code.

→ Reply

md5_84

Spoiler

An alternative solution (which doesn't pollute the namespace with the do replacement):

#include <cstdio>
#define action() puts("action") 
#define A() if(1
#define cond 0); A

A different idea:

#include <cstdio>
#define action() printf("%d\n", __LINE__)
#define cond() 0

A shorter solution (the two snippets "work" differently: the second one compiles, whereas the first one doesn't):

#define A()
#define action a:A
#define cond() 0

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