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Oct-file error message construction and octave_value conversions

From: Andrew Janke
Subject: Oct-file error message construction and octave_value conversions
Date: Tue, 29 Jan 2019 19:39:38 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

Hi, folks,

Thanks for bearing with me and all my questions on oct-files in IRC.

I'm trying to construct detailed error messages for some of my oct functions that incorporate size, type, and value info about their inputs, but having a hard time doing so. Here's what I'm trying:

#include <cmath> #include <iostream> #include <octave/oct.h> DEFUN_DLD 
(s01_02_error_msg_string_construction, args, nargout,
           "Error message construction using argument type")
  octave_value x = args(0);

  std::string x_type_name = x.type_name ();
  std::string msg =std::string ("Type of arg 1 is: ")+ x_type_name;
  error (msg.c_str ());

  builtin_type_t x_type = x.builtin_type ();
  std::string msg2 =std::string ("Builtin type id of arg 1 is: ")+ x_type;
  error (msg2.c_str ());

  octave_idx_type n_elems = x.numel ();
  std::string msg3 =std::string ("Numel in arg 1: ")+ n_elems;
  error (msg3.c_str ());

The first "msg = ..." statement works. But each of those
other "msgX = ..." lines is giving me a compiler error. It's complaining
that "no viable conversion from 'octave_value' to 'std::string'". Which is 
because builtin_type_t and octave_idx_type are not octave_values, are they?
builtin_type_t is an enum, and octave_idx_type is a typedef to an integer type.
Why does the compiler think an octave_value is involved here?

(BTW, yes, I know I should be using type_name() and not builtin_type(); I'm just
including it here to help track down the conversion issue I'm not 

I'd like to stick with constructing std::string messages instead of using the 
printf style
controls that error accepts (like 'error ("Numel in arg 1: %ld", (long) 
because I want to use these with generic templated functions that will use the 
types in the error message construction; "std::string + x" is polymorphic, but 
placeholders are not.


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