Unit Tests - D Programming Language
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Keywords: D programming language

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# Unit Tests

Contents

1. Attributed Unittests
2. Documented Unittests

    UnitTest:
        unittest BlockStatement

Unit tests are a builtin framework of test cases applied to a module to determine if it is working properly. A D program can be run with unit tests enabled or disabled.

Unit tests are a special function defined like:

    unittest
    {
        ...test code...
    }

Individual tests are specified in the unit test using AssertExpressions. Unlike AssertExpressions used elsewhere, the assert is not assumed to hold, and upon assert failure the program is still in a defined state.

There can be any number of unit test functions in a module, including within struct, union and class declarations. They are executed in lexical order.

Unit tests, when enabled, are run after all static initialization is complete and before the main() function is called.

For example, given a class Sum that is used to add two values, a unit test can be given:

    class Sum
    {
        int add(int x, int y) { return x + y; }

        unittest
        {
            Sum sum = new Sum;
            assert(sum.add(3,4) == 7);
            assert(sum.add(-2,0) == -2);
        }
    }

When unit tests are enabled, the version identifier unittest is predefined.

## Attributed Unittests

A unittest may be attributed with any of the global function attributes. Such unittests are useful in verifying the given attribute(s) on a template function:

    void myFunc(T)(T[] data)
    {
        if (data.length > 2)
            data[0] = data[1];
    }

    @safe nothrow unittest
    {
        auto arr = [1,2,3];
        myFunc(arr);
        assert(arr == [2,2,3]);
    }

This unittest verifies that myFunc contains only @safe, nothrow code. Although this can also be accomplished by attaching these attributes to myFunc itself, that would prevent myFunc from being instantiated with types T that have @system or throwing code in their opAssign method, or other methods that myFunc may call. The above idiom allows myFunc to be instantiated with such types, yet at the same time verify that the @system and throwing behavior is not introduced by the code within myFunc itself.

Implementation Defined:

1. If unit tests are not enabled, the implementation is not required to check the UnitTest for syntactic or semantic correctness. This is to reduce the compile time impact of larger unit test sections. The tokens must still be valid, and the implementation can merely count { and } tokens to find the end of the UnitTest's BlockStatement.
2. The presentation of unit test results to the user.
3. The method used to enable or disable the unit tests. Use of a compiler switch such as -unittest to enable them is suggested.
4. The order in which modules are called to run their unit tests.
5. Whether the program stops on the first unit test failure, or continues running the unit tests.

Best Practices:

1. Using unit tests in conjunction with coverage testing (such as -cov) is effective.
2. A unit test for a function should appear immediately following it.

## Documented Unittests

Documented unittests allow the developer to deliver code examples to the user, while at the same time automatically verifying that the examples are valid. This avoids the frequent problem of having outdated documentation for some piece of code.

If a declaration is followed by a documented unittest, the code in the unittest will be inserted in the example section of the declaration:

    /// Math class
    class Math
    {
        /// add function
        static int add(int x, int y) { return x + y; }

        ///
        unittest
        {
            assert(add(2, 2) == 4);
        }
    }

    ///
    unittest
    {
        auto math = new Math();
        auto result = math.add(2, 2);
    }

The above will generate the following documentation:

class Math;
  Math class
  Example:

      auto math = new Math;
      auto result = math.add(2, 2);

  int add(int x, int y);
    add function
    Example:

        assert(add(2, 2) == 4);

A unittest which is not documented, or is marked as private will not be used to generate code samples.

There can be multiple documented unittests and they can appear in any order. They will be attached to the last non-unittest declaration:

    /// add function
    int add(int x, int y) { return x + y; }

    /// code sample generated
    unittest
    {
        assert(add(1, 1) == 2);
    }

    /// code sample not generated because the unittest is private
    private unittest
    {
        assert(add(2, 2) == 4);
    }

    unittest
    {
        /// code sample not generated because the unittest isn't documented
        assert(add(3, 3) == 6);
    }

    /// code sample generated, even if it only includes comments (or is empty)
    unittest
    {
        /** assert(add(4, 4) == 8); */
    }

The above will generate the following documentation:

int add(int x, int y);
  add function
  Examples:
  code sample generated

      assert(add(1, 1) == 2);

  Examples:
  code sample generated, even if it is empty or only includes comments

      /** assert(add(4, 4) == 8); */

Error Handling
Garbage Collection
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