Question

I am using an application in c++ that uses a special dprintf function to print information, this is an example:

dprintf(verbose, 'The value is: %d', i); 

What is doing is when I define verbose for test purposes then I print the information and when I am working in normal execution I do not define it and I do not see useless information in the screen. My question is how can I do that function or implement the same idea?.

Answer 1

I try to avoid using var-arg c-style functions for two main reasons:

• They are not type-safe, can’t use operator<<
• They don’t recognize when too few or many arguments were provided

I’ve made a way that works using boost::fusion, which is given arguments in a type-safe way. It iterates over those arguments, printing out them when a % is encountered. If too few or too many arguments were given, an exception is thrown.

There is one problem still: Variadic macros are not yet standard in C++. So, i have made two versions. One that work with current C++. You have to invoke it using

dprintf('name: %, value: %\n', ('foo', 42)); 

Then. The other version, using variadic macros, can be used by defining a preprocessor symbol, which enables you to write

dprintf('name: %, value: %\n', 'foo', 42); 

Here is the code. The boost.fusion provides more details for this:

#include <boost/fusion/include/sequence.hpp> #include <boost/fusion/include/make_vector.hpp> #include <boost/fusion/include/next.hpp> #include <stdexcept> #include <iostream>  template<typename IterS, typename IterSeqE> void print_vec(IterS b, IterS e, IterSeqE, IterSeqE) {     while(b != e) {         if(*b == '%') {             if(++b != e && *b == '%') {                 std::cout << '%';             } else {                 throw std::invalid_argument('too many '%'');             }         } else {             std::cout << *b;         }         ++b;     } }  template<typename IterS, typename IterSeqB, typename IterSeqE> void print_vec(IterS b, IterS e, IterSeqB seqb, IterSeqE seqe) {     while(b != e) {         if(*b == '%') {             if(++b != e && *b == '%') {                 std::cout << '%';             } else {                 std::cout << *seqb;                 return print_vec(b, e, next(seqb), seqe);             }         } else {             std::cout << *b;         }         ++b;     }     throw std::invalid_argument('too few '%''); }  template<typename Seq> void print_vec(std::string const& msg, Seq const& seq) {     print_vec(msg.begin(), msg.end(), begin(seq), end(seq)); }  #ifdef USE_VARIADIC_MACRO #  ifdef DEBUG #    define dprintf(format, ...) \          print_vec(format, boost::fusion::make_vector(__VA_ARGS__)) #  else  #    define dprintf(format, ...) #  endif #else #  ifdef DEBUG #    define dprintf(format, args) \          print_vec(format, boost::fusion::make_vector args) #  else  #    define dprintf(format, args) #  endif #endif  // test, using the compatible version.  int main() {     dprintf('hello %, i'm % years old\n', ('litb', 22)); }