Re: What is the best way to check a function's postcondition?

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 24 Feb 2011 05:51:14 CST

Message-ID:

<5ee58ca5-dff1-4bea-9903-514387a4e7d0@r4g2000vbq.googlegroups.com>

On Feb 22, 10:18 pm, DeMarcus <use_my_alias_h...@hotmail.com> wrote:

On 2011-02-22 20:16, Howard Hinnant wrote:

Given a function:

Y f(X a)
{
    Y y(a);
    // do some computations
    return y;
}

What is the best way to introduce a postcondition to ensure, at least
in debug builds, that the correct value of y is returned?

I typically see:

Y f(X a)
{
    Y y(a);
    // do some computations
    assert(y has correct properties);
    return y;
}

But more recently I've seen:

class Checker
{
    Y& y_;
public:
    Checker(Y& y);
    ~Checker()
    {
          assert(y_ has correct properties);
    }
};

Y f(X a)
{
    Y y(a);
    Checker c(y);
    // do some computations
    return y;
}

Which idiom should be preferred and why? Which idiom is currently
used in practice?

I see two advantages with the Checker:

1. It wraps assert so you can change it easier to something else.

Just use your own assert macro. (In practice, this is what I've
usually seen; the reader should be able to clearly see that he's
dealing with a post-condition, and not just any assert.)

2. The Checker will also catch the value of Y even if the
computations throw an exception.

Which is a very strong argument against using Checker. If you
get an exception, the usual post-conditions don't necessarily
apply.

--
James Kanze

--
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