source: downloads/boost_1_34_1/libs/static_assert/doc/static_assert.qbk @ 33

[library Boost.StaticAssert
    [copyright 2000 2005 Steve Cleary and John Maddock]
    [purpose Compile time diagnostics library]
    [license
        Distributed under the Boost Software License, Version 1.0.
        (See accompanying file LICENSE_1_0.txt or copy at
        <ulink url="http://www.boost.org/LICENSE_1_0.txt">
            http://www.boost.org/LICENSE_1_0.txt
        </ulink>)
    ]
    [authors [Maddock, John], [Cleary, Steve]]
    [category template]
    [category testing]
    [category generic]
    [last-revision $Date: 2007/05/09 17:20:56 $]
]

[section:intro Overview and Tutorial]

This documentation is 
[@http://boost-consulting.com/vault/index.php?action=downloadfile&filename=boost_static_assert-1.34.pdf&directory=PDF%20Documentation& 
also available in printer-friendly PDF format].

The header `<boost/static_assert.hpp>` supplies a single macro `BOOST_STATIC_ASSERT(x)`, 
which generates a compile time error message if the integral-constant-expression `x`
is not true. In other words it is the compile time equivalent of the assert macro; 
this is sometimes known as a "compile-time-assertion", but will be called a 
"static assertion" throughout these docs. Note that if the condition is `true`, 
then the macro will generate neither code nor data - and the macro can also 
be used at either namespace, class or function scope. When used in a template, 
the static assertion will be evaluated at the time the template is instantiated; 
this is particularly useful for validating template parameters. 

One of the aims of `BOOST_STATIC_ASSERT` is to generate readable error messages. 
These immediately tell the user that a library is being used in a manner that 
is not supported. While error messages obviously differ from compiler to compiler, 
but you should see something like: 

   Illegal use of STATIC_ASSERTION_FAILURE<false>

Which is intended to at least catch the eye!

You can use `BOOST_STATIC_ASSERT` at any place where you can place a declaration, 
that is at class, function or namespace scope, this is illustrated by the 
following examples:

[section:namespace Use at namespace scope.]

The macro can be used at namespace scope, if there is some requirement must 
always be true; generally this means some platform specific requirement. 
Suppose we require that `int` be at least a 32-bit integral type, and that `wchar_t` 
be an unsigned type. We can verify this at compile time as follows:

   #include <climits>
   #include <cwchar>
   #include <limits>
   #include <boost/static_assert.hpp>

   namespace my_conditions {

      BOOST_STATIC_ASSERT(std::numeric_limits<int>::digits >= 32);
      BOOST_STATIC_ASSERT(WCHAR_MIN >= 0);

   } // namespace my_conditions

The use of the namespace my_conditions here requires some comment. 
The macro `BOOST_STATIC_ASSERT` works by generating an typedef declaration, 
and since the typedef must have a name, the macro generates one automatically by 
mangling a stub name with the value of `__LINE__`. When `BOOST_STATIC_ASSERT` is 
used at either class or function scope then each use of `BOOST_STATIC_ASSERT` 
is guaranteed to produce a name unique to that scope (provided you only use 
the macro once on each line). However when used in a header at namespace 
scope, that namespace can be continued over multiple headers, each of which 
may have their own static assertions, and on the "same" lines, thereby generating 
duplicate declarations. In theory the compiler should silently ignore duplicate 
typedef declarations, however many do not do so (and even if they do they are 
entitled to emit warnings in such cases). To avoid potential problems, if you 
use `BOOST_STATIC_ASSERT` in a header and at namespace scope, then enclose 
them in a namespace unique to that header.

[endsect]

[section:function Use at function scope]

The macro is typically used at function scope inside template functions, 
when the template arguments need checking. Imagine that we have an 
iterator-based algorithm that requires random access iterators. 
If the algorithm is instantiated with iterators that do not meet our 
requirements then an error will be generated eventually, but this may 
be nested deep inside several templates, making it hard for the user to 
determine what went wrong. One option is to add a static assertion at 
the top level of the template, in that case if the condition is not met, 
then an error will be generated in a way that makes it reasonably obvious to 
the user that the template is being misused.

   #include <iterator>
   #include <boost/static_assert.hpp>
   #include <boost/type_traits.hpp>

   template <class RandomAccessIterator >
   RandomAccessIterator foo(RandomAccessIterator from, RandomAccessIterator to)
   {
      // this template can only be used with
      // random access iterators...
      typedef typename std::iterator_traits< RandomAccessIterator >::iterator_category cat;
      BOOST_STATIC_ASSERT((boost::is_convertible<cat, const std::random_access_iterator_tag&>::value));
      //
      // detail goes here...
      return from;
   }

A couple of footnotes are in order here: the extra set of parenthesis around the 
assert, is to prevent the comma inside the `is_convertible` template being 
interpreted by the preprocessor as a macro argument separator; the target type 
for `is_convertible` is a reference type, as some compilers have problems 
using `is_convertible` when the conversion is via a user defined constructor 
(in any case there is no guarantee that the iterator tag classes are 
copy-constructible).

[endsect]

[section:class Use at class scope]

The macro is typically used inside classes that are templates. 
Suppose we have a template-class that requires an unsigned integral type with 
at least 16-bits of precision as a template argument, we can achieve this 
using something like this:

   #include <climits>
   #include <boost/static_assert.hpp>

   template <class UnsignedInt>
   class myclass
   {
   private:
      BOOST_STATIC_ASSERT((std::numeric_limits<UnsignedInt>::digits >= 16)
                           && std::numeric_limits<UnsignedInt>::is_specialized
                           && std::numeric_limits<UnsignedInt>::is_integer
                           && !std::numeric_limits<UnsignedInt>::is_signed);
   public:
      /* details here */
   };

[endsect]

[section:templates Use in templates]

Normally static assertions when used inside a class or function template, 
will not be instantiated until the template in which it is used is instantiated.  
However, there is one potential problem to watch out for: if the static assertion 
is not dependent upon one or more template parameters, then the compiler is 
permitted to evaluate the static assertion at the point it is first seen, 
irrespective of whether the template is ever instantiated, for example:

   template <class T>
   struct must_not_be_instantiated
   {   
      BOOST_STATIC_ASSERT(false);
   };

Will produce a compiler error with some compilers (for example Intel 8.1 
or gcc 3.4), regardless of whether the template is ever instantiated.  A 
workaround in cases like this is to force the assertion to be dependent 
upon a template parameter:

   template <class T>
   struct must_not_be_instantiated
   {   
      // this will be triggered if this type is instantiated
      BOOST_STATIC_ASSERT(sizeof(T) == 0); 
   };


[endsect]

[endsect]

[section:how How it works]

`BOOST_STATIC_ASSERT` works as follows. There is class `STATIC_ASSERTION_FAILURE`
 which is defined as:

   namespace boost{

   template <bool> struct STATIC_ASSERTION_FAILURE;

   template <> struct STATIC_ASSERTION_FAILURE<true>{};

   }

The key feature is that the error message triggered by the undefined 
expression `sizeof(STATIC_ASSERTION_FAILURE<0>)`, tends to be consistent 
across a wide variety of compilers. The rest of the machinery of 
`BOOST_STATIC_ASSERT` is just a way to feed the `sizeof` expression into a `typedef`. 
The use of a macro here is somewhat ugly; however boost members have spent 
considerable effort trying to invent a static assert that avoided macros, 
all to no avail. The general conclusion was that the good of a static assert 
working at namespace, function, and class scope outweighed the ugliness of a macro.

[endsect]

[section:test Test Programs]

[table Test programs provided with static_assert
[[Test Program][Expected to Compile][Description]]

[[[@../../libs/static_assert/static_assert_test.cpp static_assert_test.cpp]] [Yes] [Illustrates usage, and should always compile, really just tests compiler compatibility.]] 
[[[@../../libs/static_assert/static_assert_example_1.cpp static_assert_example_1.cpp]] [Platform dependent.] [Namespace scope test program, may compile depending upon the platform. ]]
[[[@../../libs/static_assert/static_assert_example_2.cpp static_assert_example_2.cpp]] [Yes] [Function scope test program. ]]
[[[@../../libs/static_assert/static_assert_example_3.cpp static_assert_example_3.cpp]] [Yes] [Class scope test program. ]]
[[[@../../libs/static_assert/static_assert_test_fail_1.cpp static_assert_test_fail_1.cpp]] [No] [Illustrates failure at namespace scope. ]]
[[[@../../libs/static_assert/static_assert_test_fail_2.cpp static_assert_test_fail_2.cpp]] [No] [Illustrates failure at non-template function scope. ]]
[[[@../../libs/static_assert/static_assert_test_fail_3.cpp static_assert_test_fail_3.cpp]] [No] [Illustrates failure at non-template class scope. ]]
[[[@../../libs/static_assert/static_assert_test_fail_4.cpp static_assert_test_fail_4.cpp]] [No] [Illustrates failure at non-template class scope. ]]
[[[@../../libs/static_assert/static_assert_test_fail_5.cpp static_assert_test_fail_5.cpp]] [No] [Illustrates failure at template class scope. ]]
[[[@../../libs/static_assert/static_assert_test_fail_6.cpp static_assert_test_fail_6.cpp]] [No] [Illustrates failure at template class member function scope. ]]
[[[@../../libs/static_assert/static_assert_test_fail_7.cpp static_assert_test_fail_7.cpp]] [No] [Illustrates failure of class scope example. ]]
[[[@../../libs/static_assert/static_assert_test_fail_8.cpp static_assert_test_fail_8.cpp]] [No] [Illustrates failure of function scope example. ]]
[[[@../../libs/static_assert/static_assert_test_fail_9.cpp static_assert_test_fail_9.cpp]] [No] [Illustrates failure of function scope example (part 2). ]]

]

[endsect]