Re: Unit testing of expected failures -- what do you use?

From:
DeMarcus <use_my_alias_here@hotmail.com>
Newsgroups:
comp.lang.c++
Date:
Wed, 03 Mar 2010 19:02:12 +0100
Message-ID:
<4b8ea42b$0$280$14726298@news.sunsite.dk>
Ian Collins wrote:

Alf P. Steinbach wrote:

OK, this displays my ignorance of what's out there (it's been a long
time since I developed for a living), and also my laziness not
googling. :-)

However.

I want to unit-test some library code I'm sort of extracting from some
old code I have.

The things that should work without error are easy to test, and it's
currently not so much code that I've considered a testing framework,
although the code size increases. I'm thinking that perhaps the
popular frameworks don't support my needs: there are cases where the
code /should/ assert at run time. And worse, there are cases where the
could should assert at compile time...


I used to use cppUnit (and still do for older projects) but now I use
gtest (http://code.google.com/p/googletest/). Compile time asserts are
beyond the scope of a unit testing framework; if it won't compile, there
isn't a unit to test!

How do you deal with this kind of testing, testing that things fail as
they should (at compile time and at run time)?


For run time asserts, I interpose whatever function the system's assert
macro calls (__assert on Solaris) and have __assert throw an exception
with the file, line and expression passed by assert.

A boiled down (no framework) example:

#include <iostream>
#include <exception>
#include <assert.h>

struct AssertionException : std::runtime_error
{
  const char* expression;
  const char* file;
  int line;

  AssertionException(const char* expression, const char* file, int line)
    : std::runtime_error( expression ),
      expression(expression), file(file), line(line) {}
};

void __assert(const char* expression, const char* file, int line)
{
  throw AssertionException( expression, file, line );
}

void fut( const char* p )
{
  assert( NULL != p );
}

int main()
{
  try
  {
    fut( NULL );
    std::cerr << "Oops" << std::endl;
  }
  catch( const AssertionException& e )
  {
    std::cerr << "Caught" << ": " << e.what() << std::endl;
  }
}


I'm not a complete expert, nor am I a sulky person, but before writing
assert exceptions one might consider following that I found in the book
C++ Coding Standards by Sutter & Alexandrescu, Item 68 - Assert
liberally to document internal assumptions and invariants.

Quote:
"It is not recommended to throw an exception instead of asserting, even
though the standard std::logic_error exception class was originally
designed for this purpose. The primary disadvantage of using an
exception to report a programming error is that you don't really want
stack unwinding to occur - you want the debugger to launch on the exact
line where the violation was detected, with the line's state intact."

/Daniel

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