print variable name in a #define

Question

I am trying to print out to console using a macro the variable name for display members value while debugging ( logging ). How to do that? I tried the following but it doesnt' work.

#define MY_PRINT(x) std::cout << "'x'=" << x << std::endl;

int main(){
   int my_variable=3;
   MY_PRINT( my_variable );
   // I would like to print to console
   // 'my_variable'=3
}

Define "it doesn't work". I've had to say that to you so many times in the past, as well, which is a shame. — Lightness Races in Orbit, Oct 14 '11 at 10:24
Hi Tomalek, please see response below. It is enough to add '#' character — Abruzzo Forte e Gentile, Oct 14 '11 at 16:31
Yes, I know what the answer is myself, too. But I am trying to train you, a little late, into writing proper questions. — Lightness Races in Orbit, Oct 14 '11 at 16:40

score 25 · Accepted Answer · edited Oct 14 '11 at 10:24

25

Auch... I found the solution.

I should write the macro like this

 #define MY_PRINT(x) std::cout << #x"=" << x << std::endl

edited Oct 14 '11 at 10:24

Lightness Races in Orbit

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answered Oct 14 '11 at 10:09

Abruzzo Forte e Gentile

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shodanho · Answer 2 · 2013-10-20T13:00:15.823

For C++ I use this:

#define STR(x) #x << '=' << x

int main()
{
  int i = 1;
  std::string str("hello");
  std::vector<std::string> vec;
  my_class mc;

  ...

  std::cout << STR(i) << std::endl
            << STR(str) << std::endl
            << STR(vec) << std::endl
            << STR(mc) << std::endl;

  return 0;
}

This way the compiler chooses the streaming operator based on the data type, so you don't need to bother with different macros for each, and it can go to any std::ostream, not just std::cout. Just provide the appropriate streaming operator for your data:

std::ostream operator<<(std::ostream&, const T&);
std::ostream operator<<(std::ostream&, const std::vector<T>&);
etc

But I wish there was a template way to replace the macro, or at least the variable name provided by the #x.

print variable name in a #define

2 Answers2