Pretty printing C++ types with dbx

Since Lawrence doesn’t work at Sun any more,I’ll swipe a blog entry of his to make sure it stays available.

A tip from Lawrence Crowl:

One of the problems with debugging C++ programs is that they have many user-defined types. The debugger typically does not know anything about those types,and so cannot provide any high-level printing of those types. Instead,the debugger prints the types’representations.

For example,consider a simple C++ standard string.

  #include <string> #include <iostream> int main(){ std::string variable( "Hello!" ); std::cout <<variable <<std::endl; }

In dbx,the result of printing variable is:

  (dbx) print variable  variable ={ __data_  ={ __data_ = 0x41260 "Hello!"  }  npos = 0  __nullref = 0  }

Not nice.

The Sun dbx debugger provides a helpful facility for up-leveling the printout. This facility is called pretty printing.

We can help dbx by defining a pretty printing call-back function. In essence,we write a function that converts from our high-level type into a char*. Dbx will look for pretty printing functions with the name db_pretty_print and a first parameter that is a pointer to the high-level type. In our example,the function is:

  char* db_pretty_print( std::string* var_addr,int,char* ){ return const_cast<char* >( var_addr->data() ); }

(The second and third parameters are not needed in this example.)

Now,with the -p flag on the dbx print command line,dbx calls the pretty-printing function and uses the result for its output.

  (dbx) print -p variable  variable = Hello!

You can make pretty printing the default by executing

  dbxenv output_pretty_print on

either in the debugger or in your ~/.dbxrc. Once pretty printing is the default,you can print the type’s representation with the +p flag.

  (dbx) print +p variable  variable ={ __data_  ={ __data_ = 0x41260 "Hello!"  }  npos = 0  __nullref = 0  }

For more information,type “help prettyprint”within dbx.