Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Contexts

Contexts are a facility provided by the Unit Test Framework in order to be able to trace the location of assertions better. To grasp the idea, consider the following example:

void test_operations(Processor& processor, int limit)
{
  for (int i = 0; i < limit; ++i) {
    BOOST_TEST(processor.op1(i));
    for (int j = 0; j < i; ++j) {
      BOOST_TEST(processor.op2(i, j));
    }
  }
}

In case of failure, in order to see in the logs at which point of the loops the failure occurred, we need some extra information in the assertion:

BOOST_TEST(processor.op1(i));

replaced by

BOOST_TEST_MESSAGE(processor.op1(i), "With parameter i = " << i);

We see in this trivial example that a context, which is the variable i in this case, should be acknowledged by the assertion BOOST_CHECK in a particular way. In the approach above, this is done by adding a message to the assertion itself.

What if the context is more complex than that? In case the complexity of the context increases, the fact that the assertion and the context is tightly coupled as in the approach above is difficult to maintain:

void test_operations(Processor& processor, int limit, int level)
{
  for (int i = 0; i < limit; ++i) {
    BOOST_TEST_MESSAGE(processor.op1(i),
                       "With optimization level " << level << ", With parameter i = " << i);
    for (int j = 0; j < i; ++j) {
      BOOST_TEST_MESSAGE(processor.op2(i, j),
                         "With optimization level " << level <<
                         ", With parameter i = " << i << ", With parameter j = " << j);
    }
  }
}

BOOST_AUTO_TEST_CASE(test1)
{
  Processor processor;

  for (int level = 0; level < 3; ++level) {
    processor.optimization_level(level);
    test_operations(processor, 2, level);
  }
}

Note the length of the messages, the repetition, and the fact, that we pass argument level to function test_operations only for the sake of generating an error message in case of a failure.

Therefore, loose coupling between the context of an assertion and the assertion point is a property that is desirable.

Assertion-bound context

Macro BOOST_TEST_INFO can be used to define an error message to be bound to the first following assertion. If (and only if) the assertion fails, the bound message will be displayed along:

Example: Assertion-bound context

Code

#define BOOST_TEST_MODULE example80
#include <boost/test/included/unit_test.hpp>

void test()
{
  BOOST_TEST(false);
}

BOOST_AUTO_TEST_CASE(test_case1)
{
  BOOST_TEST_INFO("Alpha");
  BOOST_TEST_INFO("Beta");
  BOOST_TEST(true);

  BOOST_TEST_INFO("Gamma");
  char a = 'a';
  BOOST_TEST_INFO("Delt" << a);
  test();
}

Output

> example
Running 1 test case...
test.cpp(14): error: in "test_case1": check false has failed
Failure occurred in a following context:
    Gamma
    Delta

*** 1 failures is detected in test module "example80"

Observe the following things. The information composed inside BOOST_TEST_INFO is bound only to the first assertion following the declaration. Thus bound information is only displayed if the assertion fails; otherwise the message is discarded. The BOOST_TEST_INFO declaration does not have to immediately precede the assertion, it is allowed to intertwine them with other instructions, they can even be declared in different scopes. Therefore it is also possible to bind more than one information to a given assertion.

With BOOST_TEST_INFO, we can improve our initial example as follows:

void test_operations(Processor& processor, int limit, int level)
{
  for (int i = 0; i < limit; ++i) {
    BOOST_TEST_INFO("With optimization level " << level);
    BOOST_TEST_INFO("With parameter i = " << i);
    BOOST_TEST(processor.op1(i));
    for (int j = 0; j < i; ++j) {
      BOOST_TEST_INFO("With optimization level " << level);
      BOOST_TEST_INFO("With parameter i = " << i);
      BOOST_TEST_INFO("With parameter j = " << j);
      BOOST_TEST(processor.op2(i, j));
    }
  }
}

BOOST_AUTO_TEST_CASE(test1)
{
  Processor processor;

  for (int level = 0; level < 3; ++level) {
    processor.optimization_level(level);
    test_operations(processor, 2, level);
  }
}

Scope-bound context

Macro BOOST_TEST_CONTEXT defines a diagnostic message and a scope. The message is bound to every assertion in the scope, and is displayed along with every failed assertion.

Example: Scope-bound context

Code

#define BOOST_TEST_MODULE example81
#include <boost/test/included/unit_test.hpp>

void test()
{
  BOOST_TEST(2 != 2);
}

BOOST_AUTO_TEST_CASE(test_case1)
{
  BOOST_TEST_CONTEXT("Alpha") {
    BOOST_TEST(1 != 1);
    test();

    BOOST_TEST_CONTEXT("Be" << "ta")
      BOOST_TEST(3 != 3);

    BOOST_TEST(4 == 4);
  }

  BOOST_TEST(5 != 5);
}

Output

> example
Running 1 test case...
test.cpp(20): error: in "test_case1": check 1 != 1 has failed [1 == 1]
Failure occurred in a following context:
    Alpha
test.cpp(14): error: in "test_case1": check 2 != 2 has failed [2 == 2]
Failure occurred in a following context:
    Alpha
test.cpp(24): error: in "test_case1": check 3 != 3 has failed [3 == 3]
Failure occurred in a following context:
    Alpha
    Beta
test.cpp(29): error: in "test_case1": check 5 != 5 has failed [5 == 5]

*** 4 failures are detected in test module "example81"

Observe the following things. After the BOOST_TEST_CONTEXT macro we have a pair of braces: they define the scope in which the diagnostic message is in effect. If there is no braces, the scope applies only to the following statement. BOOST_TEST_CONTEXT declarations can nest.

With BOOST_TEST_CONTEXT, we can further improve our initial example, by putting variable level into a scope-level context and not pass it as function parameter:

void test_operations(Processor& processor, int limit)
{
  for (int i = 0; i < limit; ++i) {
    BOOST_TEST_INFO("With parameter i = " << i);
    BOOST_TEST(processor.op1(i));
    for (int j = 0; j < i; ++j) {
      BOOST_TEST_INFO("With parameter i = " << i);
      BOOST_TEST_INFO("With parameter j = " << j);
      BOOST_TEST(processor.op2(i, j));
    }
  }
}

BOOST_AUTO_TEST_CASE(test1)
{
  Processor processor;

  for (int level = 0; level < 3; ++level) {
    BOOST_TEST_CONTEXT("With optimization level " << level) {
      processor.optimization_level(level);
      test_operations(processor, 2);
    }
  }
}

If we observe that variable i also applies in a certain scope, we can improve our example further still.

Example: Using contexts

Code

#define BOOST_TEST_MODULE example82
#include <boost/test/included/unit_test.hpp>

struct Processor
{
  int level;
  void optimization_level(int l) { level = l; }
  bool op1(int) { return level < 2; }
  bool op2(int, int) { return level < 1; }
};

void test_operations(Processor& processor, int limit)
{
  for (int i = 0; i < limit; ++i) {
    BOOST_TEST_CONTEXT("With parameter i = " << i) {
      BOOST_TEST(processor.op1(i));
      for (int j = 0; j < i; ++j) {
        BOOST_TEST_INFO("With parameter j = " << j);
        BOOST_TEST(processor.op2(i, j));
      }
    }
  }
}

BOOST_AUTO_TEST_CASE(test1)
{
  Processor processor;

  for (int level = 0; level < 3; ++level) {
    BOOST_TEST_CONTEXT("With optimization level " << level) {
      processor.optimization_level(level);
      test_operations(processor, 2);
    }
  }
}

Output

> example
Running 1 test case...
test.cpp(27): error: in "test1": check processor.op2(i, j) has failed
Failure occurred in a following context:
    With optimization level 1
    With parameter i = 1
    With parameter j = 0
test.cpp(24): error: in "test1": check processor.op1(i) has failed
Failure occurred in a following context:
    With optimization level 2
    With parameter i = 0
test.cpp(24): error: in "test1": check processor.op1(i) has failed
Failure occurred in a following context:
    With optimization level 2
    With parameter i = 1
test.cpp(27): error: in "test1": check processor.op2(i, j) has failed
Failure occurred in a following context:
    With optimization level 2
    With parameter i = 1
    With parameter j = 0

*** 4 failures are detected in the test module "example82"

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