Google Testing Blog: TotT: EXPECT vs. ASSERT

TotT: EXPECT vs. ASSERT

Thursday, July 10, 2008

Because the Google C++ Testing Framework was opensourced last week, there will be episodes focusing on it published here in the future. Watch for them. I've reshuffled the schedule to get one out this week.

Google C++ Testing Framework supports two families of assertions with the same interface:

ASSERT: Fails fast, aborting the current function.

EXPECT: Continues after the failure.

As Moka the code monkey has shown Uni the testing robot, EXPECT is often more appropriate because it: reveals more failures in a single run, and allows more to be fixed in a single edit/compile/run-tests cycle. Example:
TEST(WordStemmerTest, StemsPluralSuffixes) {
  EXPECT_STREQ("jump", stemmer->Stem("jumps"));
  EXPECT_STREQ("pixi", stemmer->Stem("pixies"));
  EXPECT_STREQ("prioriti", stemmer->Stem("priorities"));
  // etc ...
}
Use ASSERT when it doesn't make sense to continue. Example:TEST(FileGeneratorTest, CreatesFileContainingSequenceOfChars) {
  ASSERT_TRUE(fp = fopen(path, "r"))
        << "Failed to open " << path;
  ASSERT_EQ(10, fread(buffer, 1, 10, fp))
        << "File " << path << " is too small";
  buffer[10] = '\0';
  EXPECT_STREQ("123456789", buffer)
        << "File " << path << " is corrupted";
}
Remember to download this episode of Testing on the Toilet and post it in your office.

4 comments :

JohnJuly 12, 2008 at 8:26:00 AM PDT
I would argue that there is no need for EXPECT (or at least, this is not a good demonstration of the need). If your method does not work for *any* test it should fail, and that defect should be fixed and tested before you fix another, otherwise you might be chasing your tail trying to figure out which change to the code might have introduced another defect. One fix at a time. It insures boolean accuracy when fixing defects, and supports agile development.
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pschwennOctober 18, 2009 at 11:42:00 AM PDT
I have a fundamental, probably naive, question: in "ASSERT: Fails fast, aborting the current function", what exactly is "the current function"?

Perhaps you could give an explicit example.

For instance in your ASSERT sample, what is the function aborted in the assert:

ASSERT_TRUE(fp = fopen(path, "r"))
<< "Failed to open " << path;

Is it really the Test "FileGeneratorTest" that is aborted and not some "function"?

Peter Schwenn
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TotT: EXPECT vs. ASSERT

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