c++ Google test (gtest): how to create custom asserts and expects?

Question

I am using gtest to create unit tests to my c++ program. In my tests I have to write a lot of checks like this:

ASSERT_TRUE(myObject.IsValid());
EXPECT_EQ(myObject.GetSomeAttribute(), expectedValue);

I have to write both checks because if I omit the ASSERT_TRUE and myObject happened to be not valid, than myObject.GetSomeAttributre() call crashes. That's not good even in tests.

What I want is to write something like:

EXPECT_XXX_EQ(myObject.GetSomeAttribute(), expectedValue);

This line of code should do approximately the same as the original two lines (with optional bonus that if myObject is not valid, this will be reported, GetSomeAttribute() would not be called, but the test will continue running).

How can I write such custom assert/expect?

Answer 1

From the Advanced Guide, we can see that there are a couple ways we could do this.

The easiest way is by using assertions in a subroutine:

template<typename T>
void AssertAttributeEquals(MyObject const& obj, T value) {
    ASSERT_TRUE(obj.IsValid());
    // googletest has the assumption that you put the
    // expected value first
    EXPECT_EQ(value, obj.GetAttribute());
}

And you can call it like so:

AssertAttributeEquals(myObject, expectedValue);

Although you may want to use SCOPED_TRACE to get a better message on failure:

{
    SCOPED_TRACE("whatever message you want");
    AssertAttributeEquals(myObject, expectedValue);
}

---

Alternatively, you can use a function that returns an AssertionResult:

template<typename T>
::testing::AssertionResult AttributeEquals(MyObject const& obj, T value) {
    if (!obj.IsValid()) {
        // If MyObject is streamable, then we probably want to include it
        // in the error message.
        return ::testing::AssertionFailure() << obj << " is not valid";
    }
    auto attr = obj.GetAttribute();

    if (attr == value) {
        return ::testing::AssertionSuccess();
    } else {
        return ::testing::AssertionFailure() << attr << " not equal to " << value;
    }
}

This can be used like so:

EXPECT_TRUE(AttributeEquals(myObject, expectedValue));

This second technique has the benefit of producing nice error messages even if you don't use SCOPED_TRACE