Update bit field restrictions to address #57 (#231)

Author: Colin Robertson

docs/cpp/cpp-bit-fields.md

@@ -12,74 +12,77 @@ ms.author: "mblome"
 ms.workload: ["cplusplus"]
 ---
 # C++ Bit Fields
-Classes and structures can contain members that occupy less storage than an integral type. These members are specified as bit fields. The syntax for bit-field *member-declarator* specification follows:  
-  
-## Syntax  
-  
-```  
-  
-declarator  : constant-expression  
-```  
-  
-## Remarks  
- The (optional) `declarator` is the name by which the member is accessed in the program. It must be an integral type (including enumerated types). The *constant-expression* specifies the number of bits the member occupies in the structure. Anonymous bit fields — that is, bit-field members with no identifier — can be used for padding.  
-  
+
+Classes and structures can contain members that occupy less storage than an integral type. These members are specified as bit fields. The syntax for bit-field *member-declarator* specification follows:
+
+## Syntax
+
+*declarator* **:** *constant-expression*
+
+## Remarks
+
+The (optional) *declarator* is the name by which the member is accessed in the program. It must be an integral type (including enumerated types). The *constant-expression* specifies the number of bits the member occupies in the structure. Anonymous bit fields — that is, bit-field members with no identifier — can be used for padding.
+
 > [!NOTE]
->  An unnamed bit field of width 0 forces alignment of the next bit field to the next `type` boundary, where `type` is the type of the member.  
-  
- The following example declares a structure that contains bit fields:  
-  
-```  
-// bit_fields1.cpp  
-// compile with: /LD  
-struct Date {  
-   unsigned short nWeekDay  : 3;    // 0..7   (3 bits)  
-   unsigned short nMonthDay : 6;    // 0..31  (6 bits)  
-   unsigned short nMonth    : 5;    // 0..12  (5 bits)  
-   unsigned short nYear     : 8;    // 0..100 (8 bits)  
-};  
-```  
-  
- The conceptual memory layout of an object of type `Date` is shown in the following figure.  
-  
- ![Memory layout of a date object](../cpp/media/vc38uq1.png "vc38UQ1")  
-Memory Layout of Date Object  
-  
- Note that `nYear` is 8 bits long and would overflow the word boundary of the declared type, **unsigned short**. Therefore, it is begun at the beginning of a new **unsigned short**. It is not necessary that all bit fields fit in one object of the underlying type; new units of storage are allocated, according to the number of bits requested in the declaration.  
-  
- **Microsoft Specific**  
-  
- The ordering of data declared as bit fields is from low to high bit, as shown in the figure above.  
-  
- **END Microsoft Specific**  
-  
- If the declaration of a structure includes an unnamed field of length 0, as shown in the following example,  
-  
-```  
-// bit_fields2.cpp  
-// compile with: /LD  
-struct Date {  
-   unsigned nWeekDay  : 3;    // 0..7   (3 bits)  
-   unsigned nMonthDay : 6;    // 0..31  (6 bits)  
-   unsigned           : 0;    // Force alignment to next boundary.  
-   unsigned nMonth    : 5;    // 0..12  (5 bits)  
-   unsigned nYear     : 8;    // 0..100 (8 bits)  
-};  
-```  
-  
- the memory layout is as shown in the following figure.  
-  
- ![Layout of Date object with zero-length bit field](../cpp/media/vc38uq2.png "vc38UQ2")  
-Layout of Date Object with Zero-Length Bit Field  
-  
- The underlying type of a bit field must be an integral type, as described in [Fundamental Types](../cpp/fundamental-types-cpp.md).  
-  
-## Restrictions on bit fields  
- The following list details erroneous operations on bit fields:  
-  
-1.  Taking the address of a bit field.  
-  
-2.  Initializing a reference with a bit field.  
-  
-## See Also  
- [Classes and Structs](../cpp/classes-and-structs-cpp.md)
+> An unnamed bit field of width 0 forces alignment of the next bit field to the next `type` boundary, where `type` is the type of the member.
+
+ The following example declares a structure that contains bit fields:
+
+```cpp
+// bit_fields1.cpp
+// compile with: /LD
+struct Date {
+   unsigned short nWeekDay  : 3;    // 0..7   (3 bits)
+   unsigned short nMonthDay : 6;    // 0..31  (6 bits)
+   unsigned short nMonth    : 5;    // 0..12  (5 bits)
+   unsigned short nYear     : 8;    // 0..100 (8 bits)
+};
+```
+
+The conceptual memory layout of an object of type `Date` is shown in the following figure.
+
+![Memory layout of a date object](../cpp/media/vc38uq1.png "vc38UQ1")
+Memory Layout of Date Object
+
+Note that `nYear` is 8 bits long and would overflow the word boundary of the declared type, **unsigned** **short**. Therefore, it is begun at the beginning of a new **unsigned** **short**. It is not necessary that all bit fields fit in one object of the underlying type; new units of storage are allocated, according to the number of bits requested in the declaration.
+
+**Microsoft Specific**
+
+The ordering of data declared as bit fields is from low to high bit, as shown in the figure above.
+
+**END Microsoft Specific**
+
+If the declaration of a structure includes an unnamed field of length 0, as shown in the following example,
+
+```cpp
+// bit_fields2.cpp
+// compile with: /LD
+struct Date {
+   unsigned nWeekDay  : 3;    // 0..7   (3 bits)
+   unsigned nMonthDay : 6;    // 0..31  (6 bits)
+   unsigned           : 0;    // Force alignment to next boundary.
+   unsigned nMonth    : 5;    // 0..12  (5 bits)
+   unsigned nYear     : 8;    // 0..100 (8 bits)
+};
+```
+
+then the memory layout is as shown in the following figure:
+
+![Layout of Date object with zero-length bit field](../cpp/media/vc38uq2.png "vc38UQ2")
+Layout of Date Object with Zero-Length Bit Field
+
+The underlying type of a bit field must be an integral type, as described in [Fundamental Types](../cpp/fundamental-types-cpp.md).
+
+If the initializer for a reference of type `const T&` is an lvalue that refers to a bit field of type `T`, the reference is not bound to the bit field directly. Instead, the reference is bound to a temporary initialized to hold the value of the bit field.
+
+## Restrictions on bit fields
+
+The following list details erroneous operations on bit fields:
+
+- Taking the address of a bit field.
+
+- Initializing a non-**const** reference with a bit field.
+
+## See also
+
+- [Classes and Structs](../cpp/classes-and-structs-cpp.md)