Test a specific exception type is thrown AND the exception has the right properties

Question

I want to test that MyException is thrown in a certain case. EXPECT_THROW is good here. But I also want to check the exception has a specific state

Answer 1

I recommend defining a new macro based on Mike Kinghan's approach.

#define ASSERT_EXCEPTION( TRY_BLOCK, EXCEPTION_TYPE, MESSAGE )        \
try                                                                   \
{                                                                     \
    TRY_BLOCK                                                         \
    FAIL() << "exception '" << MESSAGE << "' not thrown at all!";     \
}                                                                     \
catch( const EXCEPTION_TYPE& e )                                      \
{                                                                     \
    EXPECT_EQ( MESSAGE, e.what() )                                    \
        << " exception message is incorrect. Expected the following " \
           "message:\n\n"                                             \
        << MESSAGE << "\n";                                           \
}                                                                     \
catch( ... )                                                          \
{                                                                     \
    FAIL() << "exception '" << MESSAGE                                \
           << "' not thrown with expected type '" << #EXCEPTION_TYPE  \
           << "'!";                                                   \
}

Mike's TEST(foo_test,out_of_range) example would then be

TEST(foo_test,out_of_range)
{
    foo f;
    ASSERT_EXCEPTION( { f.bar(111); }, std::out_of_range, "Out of range" );
}

which I think ends up being much more readable.

Answer 2

A colleague came up with the solution by just re-throwing the exception.

The knack: no need of extra FAIL() statements, just the two EXPECT... calls that test the bits you actually want: the exception as such and its value.

TEST(Exception, HasCertainMessage )
{
    // this tests _that_ the expected exception is thrown
    EXPECT_THROW({
        try
        {
            thisShallThrow();
        }
        catch( const MyException& e )
        {
            // and this tests that it has the correct message
            EXPECT_STREQ( "Cucumber overflow", e.what() );
            throw;
        }
    }, MyException );
}

Answer 3

You can try Boost lightweight test:

#include <boost/detail/lightweight_test.hpp>
#include <stdexcept>

void function_that_would_throw(int x)
{
  if (x > 0) {
    throw std::runtime_error("throw!");
  }
}

int main() {
 BOOST_TEST_THROWS(function_that_would_throw(10), std::runtime_error);
 boost::report_errors();
}

Answer 4

Jeff Langr describes a good approach in his book, Modern C++ Programming with Test-Driven Development:

If your [testing] framework does not support a single-line declarative assert that ensures an exception is thrown, you can use the following structure in your test:

    TEST(ATweet, RequiresUserNameToStartWithAnAtSign) {
        string invalidUser("notStartingWith@");
        try {
            Tweet tweet("msg", invalidUser);
            FAIL();
        }
        catch(const InvalidUserException& expected) {}
    }

[...] You might also need to use the try-catch structure if you must verify any postconditions after the exception is thrown. For example, you may want to verify the text associated with the thrown exception object.

    TEST(ATweet, RequiresUserNameToStartWithAtSign) {
        string invalidUser("notStartingWith@");
        try {
            Tweet tweet("msg", invalidUser);
            FAIL();
        }
        catch(const InvalidUserException& expected) {
            ASSERT_STREQ("notStartingWith@", expected.what());
        }
    }

(p.95)

This is the approach I've used, and have seen in practice elsewhere.

Edit: As has been pointed out by @MikeKinghan, this doesn't quite match the functionality provided by EXPECT_THROW; the test doesn't fail if the wrong exception is thrown. An additional catch clause could be added to address this:

catch(...) {
    FAIL();
}

Answer 5

I mostly second Lilshieste's answer but would add that you also should verify that the wrong exception type is not thrown:

#include <stdexcept>
#include "gtest/gtest.h"

struct foo
{
    int bar(int i) {
        if (i > 100) {
            throw std::out_of_range("Out of range");
        }
        return i;
    }
};

TEST(foo_test,out_of_range)
{
    foo f;
    try {
        f.bar(111);
        FAIL() << "Expected std::out_of_range";
    }
    catch(std::out_of_range const & err) {
        EXPECT_EQ(err.what(),std::string("Out of range"));
    }
    catch(...) {
        FAIL() << "Expected std::out_of_range";
    }
}

int main(int argc, char **argv) {
  ::testing::InitGoogleTest(&argc, argv);
  return RUN_ALL_TESTS();
}

Answer 6

My version; it produces the same output as EXPECT_THROW and just adds the string test:

#define EXPECT_THROW_MSG(statement, expected_exception, expected_what)                    \
  try                                                                                     \
  {                                                                                       \
    statement;                                                                            \
    FAIL() << "Expected: " #statement " throws an exception of type " #expected_exception \
              ".\n"                                                                       \
              "  Actual: it throws nothing.";                                             \
  }                                                                                       \
  catch (const expected_exception& e)                                                     \
  {                                                                                       \
    EXPECT_EQ(expected_what, std::string{e.what()});                                      \
  }                                                                                       \
  catch (...)                                                                             \
  {                                                                                       \
    FAIL() << "Expected: " #statement " throws an exception of type " #expected_exception \
              ".\n"                                                                       \
              "  Actual: it throws a different type.";                                    \
  }