例如:
struct a {
uint32_t foreColor_ : 32;
uint32_t backColor_ : 32;
uint16_t lfHeight_ : 16;
uint16_t flags_: 4;
bool lfBold_: 1;
bool lfItalic_: 1;
bool lfUnderLine_: 1;
bool lfDashLine_: 1;
bool lfStrike_: 1;
bool lfSubscript_: 1;
bool lfSuperscript_: 1;
};
是 16 个字节但是
struct a {
uint32_t foreColor_ : 32;
uint32_t backColor_ : 32;
uint16_t lfHeight_ : 16;
uint8_t flags_: 4;
bool lfBold_: 1;
bool lfItalic_: 1;
bool lfUnderLine_: 1;
bool lfDashLine_: 1; //for ime
bool lfStrike_: 1;
bool lfSubscript_: 1;
bool lfSuperscript_: 1;
};
是 12 个字节长。
我以为 flags_ 应该有相同的长度,但似乎不是。
为什么?
最佳答案
标准(the working draft 的 9.6)是这样说的:
specifies a bit-field; its length is set off from the bit-field name by a colon. The bit-field attribute is not part of the type of the class member. The constant-expression shall be an integral constant-expression with a value greater than or equal to zero. The constant-expression may be larger than the number of bits in the object representation ( 3.9 ) of the bit-field’s type; in such cases the extra bits are used as padding bits and do not participate in the value representation ( 3.9 ) of the bit-field. Allocation of bit-fields within a class object is implementation-defined. Alignment of bit-fields is implementation-defined. Bit-fields are packed into some addressable allocation unit. [ Note: bit-fields straddle allocation units on some machines and not on others. Bit-fields are assigned right-to-left on some machines, left-to-right on others. —end note]
(我的重点)
因此这将取决于您的编译器。在你的情况下似乎发生了什么 - 我会描述为相当正常的行为 - 它只是组合相同类型的位域然后将结构打包到 4 字节边界,所以在第一种情况下我们有:
struct a {
uint32_t foreColor_ : 32; // 4 bytes (total)
uint32_t backColor_ : 32; // 8 bytes
uint16_t lfHeight_ : 16; // 10 bytes
uint16_t flags_: 4; // 12 bytes
bool lfBold_: 1; // 13 bytes
bool lfItalic_: 1;
bool lfUnderLine_: 1;
bool lfDashLine_: 1;
bool lfStrike_: 1;
bool lfSubscript_: 1;
bool lfSuperscript_: 1; // still 13 bytes
};
然后将其填充为 16 个字节,在第二个中我们有:
struct a {
uint32_t foreColor_ : 32; // 4 bytes (total)
uint32_t backColor_ : 32; // 8 bytes
uint16_t lfHeight_ : 16; // 10 bytes
uint8_t flags_: 4; // 11 bytes
bool lfBold_: 1; // 12 bytes
bool lfItalic_: 1;
bool lfUnderLine_: 1;
bool lfDashLine_: 1;
bool lfStrike_: 1;
bool lfSubscript_: 1;
bool lfSuperscript_: 1; // still 12 bytes
};
不需要填充并保持在 12 字节。
关于c++ - 编译器如何确定位域结构的大小?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/18611253/