REQUIRE fails on floating point assertion but the value is correct

[Ruben Di Battista]

I'm validating a floating point method that should return 0 for a particular case

```

    SECTION("Test 1-ring gauss curvature") {

        REQUIRE(std::abs(point0.G()) == Approx(0));

    }

```

but when I run the catch test executable, It fails the assert, with:

```

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

test_HG is a Catch v2.3.0 host application.

Run with -? for options

-------------------------------------------------------------------------------

Test on a simple roof-shaped PolyData

  Test 1-ring gauss curvature

-------------------------------------------------------------------------------

/PhD/HG/tests/test_roof.cxx:99

...............................................................................

/PhD/HG/tests/test_roof.cxx:100: FAILED:

  REQUIRE( std::abs(point0.G()) == Approx(0.0) )

with expansion:

  0.0 == Approx( 0.0 )

===============================================================================

test cases:  1 |  0 passed | 1 failed

assertions: 13 | 12 passed | 1 failed

```

I stepped in the execution with GDB, and indeed `point0.G()` is something in the order of magnitude of `1e-14`.

How can I further investigate the cause for the failure?

[Martin Hořeňovský]

The simple answer is that the function didn't return exactly 0, but something close to 0. By default, Approx has a little bit of relative tolerance and no absolute tolerance, and relative tolerance breaks down at 0, for 2 simple causes

1) 0 times anything is still a 0

2) Anything +/- times less than 1 cannot become 0.

What you need to do is to decide if you have some absolute tolerance for your results, and if yes, set Approx::margin appropriately.

Martin Hořeňovský

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[Ruben Di Battista]

Dear Martin, 

thank you. `Approx::margin` was exactly what I was looking for! 

Thanks!

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