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Using reference data in C++ tests

The Testing Utilities (testutils) module provides (among other things) utilities to write Google Test tests that compare their results against stored reference data. This can either be used for

The current reference data functionality is quite basic, but it can be extended if/when more control over, e.g., comparison tolerances is needed.

Reference data organization

Conceptually, the reference data consists of a tree-like structure of nodes. Each leaf node checks a single primitive value (an integer, a floating-point value, a string etc.), and each inner node acts as a compound value that helps organizing the data. Within each compound node (including the root of the tree), child nodes are identified by an id string. Each node within a single compound must have a unique id, and it is possible to compare multiple values produced by the test against this single node (naturally, the test only passes if the test produces the same value in all such cases).

Each node also has a type (a string). For leaf nodes, the type is from a predetermined set of strings, and identifies the type of the value stored in the node. For compound nodes, the type is just a string provided by the test. In all cases, the type in the reference data must match the type provided by the test. This provides additional safety when changing the test to detect mismatches between the test and the reference data. The intention is that compound nodes whose contents have the same structure would have the same type; this will simplify using XSLT for viewing the reference data (see below).

Some compound types are predefined, e.g., for simple sequences, but more complicated compounds can be defined ad-hoc in tests that need them. See below for how to use them in the code.

As a special case, the id can be empty (NULL). This is intended for cases where one is checking for a sequence of items, and the only thing distinguishing the items is their position in this sequence. Using an empty id removes the need to generate unique identifiers for the items, and makes textual diffs of the reference data files easier to read. Only a single sequence of nodes with an empty id is supported within one parent node: if you first check some nodes with an empty id, followed by a non-empty id, the next check for an empty id will again match the first node in the sequence. For clarity, all the nodes that have an empty id should be of the same type, but this is not enforced.

Using reference data in code

To use reference data in a test, the test should first create exactly one instance of gmx::test::TestReferenceData. It can do so as a local variable in the test, as a member variable in its test fixture, or by subclassing a test fixture that already contains such a variable (e.g., gmx::test::StringTestBase or gmx::test::CommandLineTestBase). Only use the default constructor! The other constructor is intended for self-testing utility code used in other tests (including self-testing the reference data implementation itself), and behaves differently from what is described here.

To access the root node of the data, gmx::test::TestReferenceData::rootChecker() needs to be called. This returns a gmx::test::TestReferenceChecker that provides various check*() methods that can be used to check values against top-level nodes. gmx::test::TestReferenceChecker::checkCompound() can be called to create custom compound types: it returns another gmx::test::TestReferenceChecker that can be used to check values against child nodes of the created compound.

Whenever a gmx::test::TestReferenceChecker method detects a mismatch against reference data, it will generate a non-fatal Google Test failure in the current test. The test can naturally also use its own test assertions for additional checks, but any mismatch will automatically also fail the test.

It is also possible to read values of the reference data items using gmx::test::TestReferenceChecker, so that they can be used programmatically. For this to work, those items should first be written in the same test. This supports tests that want to both check data against a reference, and use that reference as a persistence layer for storing information. This is useful at least for serialization tests. This is currently not supported for all use cases, but with some caveats, it is possible to use this for testing.

When using floating-point values in reference data, the tolerance for the comparison can be influenced with gmx::test::TestReferenceChecker::setDefaultTolerance(). Per-comparison tolerances would be possible to implement if necessary, but currently you can either change the default tolerance whenever you need to, or create copies of the gmx::test::TestReferenceChecker object and set different tolerances in the different instances. Note that there is an implicit assumption that a mixed- and a double-precision build will produce the same results (within the given tolerance). This means that some things cannot be tested with the reference data (e.g., multiple steps of MD integration), and that reference data for such tests needs to be always generated in double precision (unless the results are nice, exact binary floating-point numbers).

Just creating a gmx::test::TestReferenceData instance does not enforce using reference data in the test; the data is loaded/used only when gmx::test::TestReferenceData::rootChecker() is first called. If the test never calls this method, the gmx::test::TestReferenceData object does nothing. This allows using the same test fixture (e.g., CommandLineTestBase) also in tests that do not need the reference data, but benefit from other features of the fixture.

Running tests that use reference data

To run a test that uses the reference data, you just execute the test binary as you would otherwise. However, when you first add a test, the reference data does not exist, and the test will fail with an assertion message saying that the reference data could not be found. To generate the reference data, you need to run the test binary with a -ref-data create command-line option (it is also possible to use any of the update options below to generate the reference data).

If you change a test (or the tested code) such that the reference data needs to be changed, you need to run the test binary with -ref-data update-all or -ref-data update-changed. The first will recreate the reference data from scratch. The latter will retain old reference values if they are still valid. In other words, floating-point reference values that are within the test tolerance will be kept at their old values. Only values that are outside the tolerance (or otherwise do not match or do not exist) are updated. This is useful (at least) for tests that contain floating-point data, where it is not expected that those floating-point values would actually need to change. This allows you to update other parts of the reference data without doing a double-precision build, and also makes it easier to avoid spurious changes in the last bits of other reference data values when just a single output value is expected to change.

To create or update reference data, the test needs to pass when run with the corresponding flag. All comparisons against reference data will pass in these modes, but you need to ensure that other assertions in the test also pass, and that the test does not throw exceptions. Note that if your test does multiple comparisons against the same id node, reference data comparison can still fail during create/update if the test does not produce the same results for each comparison.

With all the operations that create or update the reference data, you can use the --gtest_filter=<...> command-line option provided by Google Test to select the tests whose reference data you want to influence.


The reference data is stored in XML files under src/gromacs/module/tests/refdata/ in the source tree. This part of the framework depends on tinyxml2, which is bundled in src/external. One file is produced per test that uses reference data. If you rename tests or otherwise change the reference data, you currently need to manually manage the files with git.

For inspecting the reference data in a browser, there are XSLT stylesheets that transform the XML files into HTML. Such custom transformations need to be written for each type of test if the output is not easy to check otherwise. Because of security features in browsers, the transformations may not work for all browsers. For the same reason, the XSLT files must be in the same folder as the XML files. For cases where the XSLT files are shared between multiple modules, src/testutils/ takes care to synchronize the files after a master copy is edited.