COMPILE_ASSERT

爱被打了一巴掌 2022-09-29 13:51 190阅读 0赞

最近在看chrome的代码，发现了有个叫COMPILE\_ASSERT的宏，用于编译时做判断。COMPILE\_ASSERT宏的实现在D:/project/chrome/src/src/base/basictypes.h ，注释部分有其原理的介绍，如下：

// The COMPILE_ASSERT macro can be used to verify that a compile time
    // expression is true. For example, you could use it to verify the
    // size of a static array:
    //
    //   COMPILE_ASSERT(ARRAYSIZE_UNSAFE(content_type_names) == CONTENT_NUM_TYPES,
    //                  content_type_names_incorrect_size);
    //
    // or to make sure a struct is smaller than a certain size:
    //
    //   COMPILE_ASSERT(sizeof(foo) < 128, foo_too_large);
    //
    // The second argument to the macro is the name of the variable. If
    // the expression is false, most compilers will issue a warning/error
    // containing the name of the variable.
    
    template <bool>
    struct CompileAssert {
    };
    
    #undef COMPILE_ASSERT
    #define COMPILE_ASSERT(expr, msg) /
      typedef CompileAssert<(bool(expr))> msg[bool(expr) ? 1 : -1]
    
    // Implementation details of COMPILE_ASSERT:
    //
    // - COMPILE_ASSERT works by defining an array type that has -1
    //   elements (and thus is invalid) when the expression is false.
    //
    // - The simpler definition
    //
    //     #define COMPILE_ASSERT(expr, msg) typedef char msg[(expr) ? 1 : -1]
    //
    //   does not work, as gcc supports variable-length arrays whose sizes
    //   are determined at run-time (this is gcc's extension and not part
    //   of the C++ standard).  As a result, gcc fails to reject the
    //   following code with the simple definition:
    //
    //     int foo;
    //     COMPILE_ASSERT(foo, msg); // not supposed to compile as foo is
    //                               // not a compile-time constant.
    //
    // - By using the type CompileAssert<(bool(expr))>, we ensures that
    //   expr is a compile-time constant.  (Template arguments must be
    //   determined at compile-time.)
    //
    // - The outter parentheses in CompileAssert<(bool(expr))> are necessary
    //   to work around a bug in gcc 3.4.4 and 4.0.1.  If we had written
    //
    //     CompileAssert
       
    //
    //   instead, these compilers will refuse to compile
    //
    //     COMPILE_ASSERT(5 > 0, some_message);
    //
    //   (They seem to think the ">" in "5 > 0" marks the end of the
    //   template argument list.)
    //
    // - The array size is (bool(expr) ? 1 : -1), instead of simply
    //
    //     ((expr) ? 1 : -1).
    //
    //   This is to avoid running into a bug in MS VC 7.1, which
    //   causes ((0.0) ? 1 : -1) to incorrectly evaluate to 1.

主要是用了数组的长度不能小于0的特性，例如int a\[-1\]会在编译时会报错。为了兼容各个编译器，使用了模板技术。

在msdn上，也有一篇文章来介绍COMPILE\_ASSERT：[http://blogs.msdn.com/b/abhinaba/archive/2008/10/27/c-c-compile-time-asserts.aspx][http_blogs.msdn.com_b_abhinaba_archive_2008_10_27_c-c-compile-time-asserts.aspx]

在boost里面，与COMPILE\_ASSERT相似有*BOOST\_STATIC\_ASSERT*，实现可以参考：[http://www.boost.org/doc/libs/1\_36\_0/boost/static\_assert.hpp][http_www.boost.org_doc_libs_1_36_0_boost_static_assert.hpp]

在搜索代码时，发现了一个库[mili][]（[http://code.google.com/p/mili/][mili]），这里面也有一个compile\_assert的实现（[http://code.google.com/p/mili/source/browse/trunk/include/compile\_assert.h][http_code.google.com_p_mili_source_browse_trunk_include_compile_assert.h]）。这个库还有其他一些实用的类和特性，例如Properties（[http://code.google.com/p/mili/wiki/Properties][http_code.google.com_p_mili_wiki_Properties]）。Properties的实现原理可以参考[http://xiaolingyao.spaces.live.com/blog/cns!6A8F02D95D2DDE46!201.entry?sa=991256254][http_xiaolingyao.spaces.live.com_blog_cns_6A8F02D95D2DDE46_201.entry_sa_991256254]。

[http_blogs.msdn.com_b_abhinaba_archive_2008_10_27_c-c-compile-time-asserts.aspx]: http://blogs.msdn.com/b/abhinaba/archive/2008/10/27/c-c-compile-time-asserts.aspx
[http_www.boost.org_doc_libs_1_36_0_boost_static_assert.hpp]: http://www.boost.org/doc/libs/1_36_0/boost/static_assert.hpp
[mili]: http://code.google.com/p/mili/
[http_code.google.com_p_mili_source_browse_trunk_include_compile_assert.h]: http://code.google.com/p/mili/source/browse/trunk/include/compile_assert.h
[http_code.google.com_p_mili_wiki_Properties]: http://code.google.com/p/mili/wiki/Properties
[http_xiaolingyao.spaces.live.com_blog_cns_6A8F02D95D2DDE46_201.entry_sa_991256254]: http://xiaolingyao.spaces.live.com/blog/cns!6A8F02D95D2DDE46!201.entry?sa=991256254

COMPILE_ASSERT

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