C++11新特性 - static_assert 的替代实现 (vs2010 error 出错）

  如字面含义, 静态(编译时)断言, 更多解释见:

• http://en.cppreference.com/w/cpp/language/static_assert
• http://msdn.microsoft.com/en-us/library/dd293588.aspx

    不过, IDE未必支持这个C++11的新特性, 因此就会有一些替代的实现方法.

替代实现

    若编译器不支持static_assert, 我们应该怎么自行实现该特性? 在Eigen中, 可以找到答案:    利用模板特例化和编译器编译时   "undeclared identifier"的   报错.  代码如下 :

namespace Eigen {

    namespace internal {

    template<bool condition>
    struct static_assertion {};

    template<>
    struct static_assertion<true>
    {
      enum {
        YOU_TRIED_CALLING_A_VECTOR_METHOD_ON_A_MATRIX,
        YOU_MIXED_VECTORS_OF_DIFFERENT_SIZES,
        YOU_MIXED_MATRICES_OF_DIFFERENT_SIZES,
        THIS_METHOD_IS_ONLY_FOR_VECTORS_OF_A_SPECIFIC_SIZE,
        THIS_METHOD_IS_ONLY_FOR_MATRICES_OF_A_SPECIFIC_SIZE,
        THIS_METHOD_IS_ONLY_FOR_OBJECTS_OF_A_SPECIFIC_SIZE,
        YOU_MADE_A_PROGRAMMING_MISTAKE,
        EIGEN_INTERNAL_ERROR_PLEASE_FILE_A_BUG_REPORT,
        EIGEN_INTERNAL_COMPILATION_ERROR_OR_YOU_MADE_A_PROGRAMMING_MISTAKE,
        YOU_CALLED_A_FIXED_SIZE_METHOD_ON_A_DYNAMIC_SIZE_MATRIX_OR_VECTOR,
        YOU_CALLED_A_DYNAMIC_SIZE_METHOD_ON_A_FIXED_SIZE_MATRIX_OR_VECTOR,
        UNALIGNED_LOAD_AND_STORE_OPERATIONS_UNIMPLEMENTED_ON_ALTIVEC,
        THIS_FUNCTION_IS_NOT_FOR_INTEGER_NUMERIC_TYPES,
        FLOATING_POINT_ARGUMENT_PASSED__INTEGER_WAS_EXPECTED,
        NUMERIC_TYPE_MUST_BE_REAL,
        COEFFICIENT_WRITE_ACCESS_TO_SELFADJOINT_NOT_SUPPORTED,
        WRITING_TO_TRIANGULAR_PART_WITH_UNIT_DIAGONAL_IS_NOT_SUPPORTED,
        THIS_METHOD_IS_ONLY_FOR_FIXED_SIZE,
        INVALID_MATRIX_PRODUCT,
        INVALID_VECTOR_VECTOR_PRODUCT__IF_YOU_WANTED_A_DOT_OR_COEFF_WISE_PRODUCT_YOU_MUST_USE_THE_EXPLICIT_FUNCTIONS,
        INVALID_MATRIX_PRODUCT__IF_YOU_WANTED_A_COEFF_WISE_PRODUCT_YOU_MUST_USE_THE_EXPLICIT_FUNCTION,
        YOU_MIXED_DIFFERENT_NUMERIC_TYPES__YOU_NEED_TO_USE_THE_CAST_METHOD_OF_MATRIXBASE_TO_CAST_NUMERIC_TYPES_EXPLICITLY,
        THIS_METHOD_IS_ONLY_FOR_COLUMN_MAJOR_MATRICES,
        THIS_METHOD_IS_ONLY_FOR_ROW_MAJOR_MATRICES,
        INVALID_MATRIX_TEMPLATE_PARAMETERS,
        INVALID_MATRIXBASE_TEMPLATE_PARAMETERS,
        BOTH_MATRICES_MUST_HAVE_THE_SAME_STORAGE_ORDER,
        THIS_METHOD_IS_ONLY_FOR_DIAGONAL_MATRIX,
        THE_MATRIX_OR_EXPRESSION_THAT_YOU_PASSED_DOES_NOT_HAVE_THE_EXPECTED_TYPE,
        THIS_METHOD_IS_ONLY_FOR_EXPRESSIONS_WITH_DIRECT_MEMORY_ACCESS_SUCH_AS_MAP_OR_PLAIN_MATRICES,
        YOU_ALREADY_SPECIFIED_THIS_STRIDE,
        INVALID_STORAGE_ORDER_FOR_THIS_VECTOR_EXPRESSION,
        THE_BRACKET_OPERATOR_IS_ONLY_FOR_VECTORS__USE_THE_PARENTHESIS_OPERATOR_INSTEAD,
        PACKET_ACCESS_REQUIRES_TO_HAVE_INNER_STRIDE_FIXED_TO_1,
        THIS_METHOD_IS_ONLY_FOR_SPECIFIC_TRANSFORMATIONS,
        YOU_CANNOT_MIX_ARRAYS_AND_MATRICES,
        YOU_PERFORMED_AN_INVALID_TRANSFORMATION_CONVERSION,
        THIS_EXPRESSION_IS_NOT_A_LVALUE__IT_IS_READ_ONLY,
        YOU_ARE_TRYING_TO_USE_AN_INDEX_BASED_ACCESSOR_ON_AN_EXPRESSION_THAT_DOES_NOT_SUPPORT_THAT,
        THIS_METHOD_IS_ONLY_FOR_1x1_EXPRESSIONS,
        THIS_METHOD_IS_ONLY_FOR_EXPRESSIONS_OF_BOOL,
        THIS_METHOD_IS_ONLY_FOR_ARRAYS_NOT_MATRICES,
        YOU_PASSED_A_ROW_VECTOR_BUT_A_COLUMN_VECTOR_WAS_EXPECTED,
        YOU_PASSED_A_COLUMN_VECTOR_BUT_A_ROW_VECTOR_WAS_EXPECTED,
        THE_INDEX_TYPE_MUST_BE_A_SIGNED_TYPE,
        THE_STORAGE_ORDER_OF_BOTH_SIDES_MUST_MATCH,
        OBJECT_ALLOCATED_ON_STACK_IS_TOO_BIG
      };
    };

    } // end namespace internal

    } // end namespace Eigen

#define EIGEN_STATIC_ASSERT(CONDITION,MSG) \
        {Eigen::internal::static_assertion<bool(CONDITION)>::MSG;}

SEE ALSO: Eigen\src\Core\util\StaticAssert.h

从上面的代码可以看出, 如果 EIGEN_STATIC_ASSERT 中的 CONDITION=false 时, 就会调用 Eigen::internal::static_assertion<false>::MSG , 再看看static_assertion定义, 它只特例化了static_assertion<true>, 而condition=false的模板参数则会调用默认的方法, 因此, 一旦 CONDITION=false 时, 编译器就必然会找不到MSG的定义, 也就是我们看到的"undeclared identifier".

参考资料

• Eigen - http://eigen.tuxfamily.org
• Boost - http://www.boost.org/