Test Anything Protocol

Testing with C++

Testing using libtap++


#include <tap++/tap++.h>
#include <string>

using namespace TAP;

int foo() {
  return 1;

std::string bar() {
  return "a string";

int main() {
  ok(true, "This test passes");
  is(foo(), 1, "foo() should be 1");
  is(bar(), "a string", "bar() should be \"a string\"");
  return exit_status();

Remember to link with libtap++. For example: g++ -ltap++ tap-synopsis.o -o tap-synopsis


libtap++ is a TAP producer for C++ programs. It tries to follow the interface of Test::More as much as possible, while at the same time making use of the full extent of C++’s features.

Getting libtap++

libtap++ is currently part of libperl– hosted on github. It can be downloaded at https://github.com/Leont/libperl–.


All functions and variables are defined in the TAP namespace.

I love it when a plan comes together

Before anything else, you need a testing plan. This basically declares how many tests your script is going to run to protect against premature failure.


void plan(int number_of_tests);
void plan(skip_all, const std::string& reason="");
void plan(no_plan);

The function plan is used to indicate the plan of your test run. Usually you will just give it the number of tests as argument.

Alternatively, you can give it the skip_all or no_plan constants as arguments. The first means you will not run the tests at all, the second means you will run an unknown number of tests (the latter is not recommended).


void done_testing();
void done_testing(int number_of_tests);

If you don’t know how many tests you’re going to run, you can issue the plan when you’re done running tests. number_of_tests is the same as plan(), it’s the number of tests you expected to run. You can omit this, in which case the number of tests you ran doesn’t matter, just the fact that your tests ran to conclusion. This is safer than and replaces the “no_plan” plan.

Test names

By convention, each test is assigned a number in order. This is largely done automatically for you. However, it’s often very useful to assign a name to each test. Which would you rather see:

ok 4
not ok 5
ok 6


ok 4 - basic multi-variable
not ok 5 - simple exponential
ok 6 - force == mass * acceleration

The later gives you some idea of what failed. It also makes it easier to find the test in your script, simply search for “simple exponential”. All test functions take a name argument. It’s optional, but highly suggested that you use it.

I’m ok, you’re not ok.

The basic purpose of this module is to print out either “ok #” or “not ok #” depending on if a given test succeeded or failed. Everything else is just gravy. All of the following print “ok” or “not ok” depending on if the test succeeded or failed. They all also return true or false, respectively.


bool ok(bool condition, const std::string& test_name = "");

ok is the basic test expression in TAP. It simply evaluates any expression, for example, got == expected, taking a true value to mean that the test passed and a false value to mean that the test failed. test_name is a very short description of the test that will be printed out. It makes it very easy to find a test in your script when it fails and gives others an idea of your intentions. test_name is optional, but we very strongly encourage its use.

is(); isnt()

template<typename T, typename U> bool is(
  const T& got, const U& expected, std::string& test_name = "");
template<typename T, typename U> bool isnt(
  const T& got, const U& expected, std::string& test_name = "");

Similar to ok(), is() and isnt() compare their two arguments with == and != respectively and use the result of that to determine if the test succeeded or failed. So these:

# Is the ultimate answer 42?
is(ultimate_answer(), 42, "Meaning of Life");

# foo isn't empty
isnt(foo, "", "Got some foo");

are similar to these:

ok(ultimate_answer() == 42, "Meaning of Life");
ok(foo != "", "Got some foo");

(Mnemonic: “This is that.” “This isn’t that.”) So why use these? They produce better diagnostics on failure. ok() cannot know what you are testing for (beyond the name), but is() and isnt() know what the test was and why it failed. For example this test:

std::string foo("waffle"), bar("yarblokos");
is(foo, bar, "Is foo the same as bar?");

Will produce something like this:

not ok 17 - Is foo the same as bar?
#   Failed test 'Is foo the same as bar?'
#          got: 'waffle'
#     expected: 'yarblokos'

pass(); fail()

bool pass(const std::string& test_name = "");
bool fail(const std::string& test_name = "");

Sometimes you just want to say that the tests have passed. Usually the case is you’ve got some complicated condition that is difficult to wedge into an ok(). In this case, you can simply use pass() (to declare the test ok) or fail (for not ok). They are synonyms for ok(true, test_name) and ok(false, test_name). Use these very, very, very sparingly.

Conditional testing


void skip(int number, const std::string& reason = "");

skip tells the TAP harness that you’re skipping a number of tests for the given reason. Note that you have to do the skipping yourself.


If you pick the right test function, you’ll usually get a good idea of what went wrong when it failed. But sometimes it doesn’t work out that way. So here we have ways for you to write your own diagnostic messages which are safer than just print STDERR.



Prints a diagnostic message which is guaranteed not to interfere with test output. The arguments are simply concatenated together. Returns false, so as to preserve failure. Handy for this sort of thing:

ok(has_user("foo"), "There's a foo user") or
diag("Since there's no foo, check that /etc/bar is set up right");

which would produce:

not ok 42 - There's a foo user
#   Failed test 'There's a foo user'
# Since there's no foo, check that /etc/bar is set up right.

You might remember ok() or diag() with the mnemonic open() or die().

NOTE The exact formatting of the diagnostic output is still changing, but it is guaranteed that whatever you throw at it it won’t interfere with the test.



Like diag(), except the message will not be seen when the test is run in a harness. It will only be visible in the verbose TAP stream. Handy for putting in notes which might be useful for debugging, but don’t indicate a problem.

note("Tempfile is ", tempfile);

diag simply catenates its arguments to the error output, while note prints diagnostics to the TAP stream.

set_output(); set_error()

void set_output(std::ofstream& new_output);
void set_error(std::ofstream& new_error);

These set the filehandle of the TAP stream and the error stream. They default to std::cout and std::cerr, respectively. These can only be set before any output is written to them.

Ending a test run


If all your tests passed, Test::Builder will exit with zero (which is normal). If anything failed it will exit with how many failed. If you run less (or more) tests than you planned, the missing (or extras) will be considered failures. If the test died, even after having successfully completed all its tests, it will still be considered a failure and will exit with 255. So the exit codes are…

0                   all tests successful
255                 test died or all passed but wrong # of tests run
any other number    how many failed (including missing or extras)

If you fail more than 254 tests, it will be reported as 254.


int exit_status();
void bail_out(const std::string& reason);

bail_out terminates the current test program with exit code 255, indicating to the test harness that all subsequent testing should halt. Typically this is used to indicate that testing cannot continue at all.