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

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Wed, 03 Mar 2010 23:15:07 +0100
Message-ID:
<hmmn1e$7g4$1@news.eternal-september.org>
* Ian Collins:

DeMarcus wrote:

Ian Collins wrote:

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 );
}


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."


My example above was for a unit test harness used to confirm that
assertions trigger when expected. It was not a suggestion for
production code!

In a test harness, you could get the mock __assert function to set a
flag to indicate that it has been called, but that won't case the called
function to terminate. It will blunder on with invalid inputs and do
nasty things. In this instance, mapping assertions to exceptions is the
only practical solution.


I must disagree. I think it *can* be practical when you have good knowledge of
the code being tested. But for arbitrary code there might just be a catch(...)
somewhere.

So I think that in general it's better to just have the test harness (script,
likely) detect the failed execution.

And sometimes that failed execution is what's expected for a successful test.

Cheers,

- Alf

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