thibaud-d
11/23/2015 - 7:31 AM

Demonstrates the use of the do{ ... }while(0) hack to secure the use of multi-line macros in conditionals. Note: this code does not compile.

Demonstrates the use of the do{ ... }while(0) hack to secure the use of multi-line macros in conditionals. Note: this code does not compile.

do_while_hack.cpp
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#include <iostream>

#define SAFE_LOG_FIRST_N(arg,n) \
  do{ \
    static int LOG_OCCURRENCES = 0; \
    if (LOG_OCCURRENCES <= n) ++LOG_OCCURRENCES; \
    if (LOG_OCCURRENCES <= n) std::cout<<arg; \
  } while(0)

#define LOG_FIRST_N(arg,n) \
  { \
    static int LOG_OCCURRENCES = 0; \
    if (LOG_OCCURRENCES <= n) ++LOG_OCCURRENCES; \
    if (LOG_OCCURRENCES <= n) std::cout<<arg; \
  }

int main(int argc, const char * argv[]) {
    if(true)
        SAFE_LOG_FIRST_N("Om mane padme om", 1);
    else
        exit(5);
    if(true)
        LOG_FIRST_N("Om mane padme om", 1);
    else // fails here with "expected expression"
        exit(5);
    return 0;
}