BOOST_STATIC_ASSERT without boost

Since boost is forbidden in a company I work for I need to implement its functionality in pure C++. I've looked into boost sources but they seem to be too complex to understand, at least for me. I know there is something called static_assert() in the C++0x standart, but I'd like not to use any C++0x features.

Solution 1:

One other trick (which can be used in C) is to try to build an array with a negative size if the assert fail:

#define ASSERT(cond) int foo[(cond) ? 1 : -1]

as a bonus, you may use a typedef instead of an object, so that it is usable in more contexts and doesn't takes place when it succeed:

#define ASSERT(cond) typedef int foo[(cond) ? 1 : -1]

finally, build a name with less chance of name clash (and reusable at least in different lines):

#define CAT_(a, b) a ## b
#define CAT(a, b) CAT_(a, b)
#define ASSERT(cond) typedef int CAT(AsSeRt, __LINE__)[(cond) ? 1 : -1]

Solution 2:

template<bool> struct StaticAssert;
template<> struct StaticAssert<true> {};

int main() {
   StaticAssert< (4>3) >(); //OK
   StaticAssert< (2+2==5) >(); //ERROR
}

Solution 3:

Here is my own implementation of static assertions extracted from my code base: Pre-C++11 Static Assertions Without Boost.

Usage:

STATIC_ASSERT(expression, message);

When the static assertion test fails, a compiler error message that somehow contains the STATIC_ASSERTION_FAILED_AT_LINE_xxx_message is generated.

message has to be a valid C++ identifier, like no_you_cant_have_a_pony which will produce a compiler error containing:

STATIC_ASSERTION_FAILED_AT_LINE_1337_no_you_cant_have_a_pony :)

#define CONCATENATE(arg1, arg2)   CONCATENATE1(arg1, arg2)
#define CONCATENATE1(arg1, arg2)  CONCATENATE2(arg1, arg2)
#define CONCATENATE2(arg1, arg2)  arg1##arg2

/**
 * Usage:
 *
 * <code>STATIC_ASSERT(expression, message)</code>
 *
 * When the static assertion test fails, a compiler error message that somehow
 * contains the "STATIC_ASSERTION_FAILED_AT_LINE_xxx_message" is generated.
 *
 * /!\ message has to be a valid C++ identifier, that is to say it must not
 * contain space characters, cannot start with a digit, etc.
 *
 * STATIC_ASSERT(true, this_message_will_never_be_displayed);
 */

#define STATIC_ASSERT(expression, message)\
  struct CONCATENATE(__static_assertion_at_line_, __LINE__)\
  {\
    implementation::StaticAssertion<static_cast<bool>((expression))> CONCATENATE(CONCATENATE(CONCATENATE(STATIC_ASSERTION_FAILED_AT_LINE_, __LINE__), _), message);\
  };\
  typedef implementation::StaticAssertionTest<sizeof(CONCATENATE(__static_assertion_at_line_, __LINE__))> CONCATENATE(__static_assertion_test_at_line_, __LINE__)

  // note that we wrap the non existing type inside a struct to avoid warning
  // messages about unused variables when static assertions are used at function
  // scope
  // the use of sizeof makes sure the assertion error is not ignored by SFINAE

namespace implementation {

  template <bool>
  struct StaticAssertion;

  template <>
  struct StaticAssertion<true>
  {
  }; // StaticAssertion<true>

  template<int i>
  struct StaticAssertionTest
  {
  }; // StaticAssertionTest<int>

} // namespace implementation


STATIC_ASSERT(true, ok);
STATIC_ASSERT(false, ko);

int main()
{
  return 0;
}