Editing Assertion (software development) (section)

=== Static assertions ===

Assertions that are checked at compile time are called static assertions.

Static assertions are particularly useful in compile time [[template metaprogramming]], but can also be used in low-level languages like C by introducing illegal code if (and only if) the assertion fails. [[C1X|C11]] and [[C++11]] support static assertions directly through <code>static_assert</code>. In earlier C versions,  a static assertion can be implemented, for example, like this:

<syntaxhighlight lang="c">
#define SASSERT(pred) switch(0){case 0:case pred:;}

SASSERT( BOOLEAN CONDITION );
</syntaxhighlight>

If the <code>(BOOLEAN CONDITION)</code> part evaluates to false then the above code will not compile because the compiler will not allow two [[Switch statement#C and languages with C-like syntax|case labels]] with the same constant. The boolean expression must be a compile-time constant value, for example <code>([[sizeof]](int)==4)</code> would be a valid expression in that context.  This construct does not work at file scope (i.e. not inside a function), and so it must be wrapped inside a function.

Another popular<ref>Jon Jagger, ''[http://www.jaggersoft.com/pubs/CVu11_3.html Compile Time Assertions in C]'', 1999.</ref> way of implementing assertions in C is:

<syntaxhighlight lang="c">
static char const static_assertion[ (BOOLEAN CONDITION)
                                    ? 1 : -1
                                  ] = {'!'};
</syntaxhighlight>

If the <code>(BOOLEAN CONDITION)</code> part evaluates to false then the above code will not compile because arrays may not have a negative length. If in fact the compiler allows a negative length then the initialization byte (the <code>'!'</code> part) should cause even such over-lenient compilers to complain. The boolean expression must be a compile-time constant value, for example <code>(sizeof(int) == 4)</code> would be a valid expression in that context.

Both of these methods require a method of constructing unique names.  Modern compilers support a <code>__COUNTER__</code> preprocessor define that facilitates the construction of unique names, by returning monotonically increasing numbers for each compilation unit.<ref>[https://gcc.gnu.org/gcc-4.3/changes.html GNU, "GCC 4.3 Release Series&nbsp;— Changes, New Features, and Fixes"]</ref>

[[D (programming language)|D]] provides static assertions through the use of <code>static assert</code>.<ref>{{cite web | url = http://dlang.org/version.html#StaticAssert | work = D Language Reference | title = Static Assertions | publisher = The D Language Foundation | accessdate = 2022-03-16}}</ref>