使用（。）运算符输出结构成员的值会得到与 - ＆gt;不同的结果。运营商，为什么？

Question

当我尝试使用（。）运算符输出结构成员的值时会出现奇怪的结果，而不是 - ＆gt;运算符，这是代码

typedef struct MonitoredDns
{
int16               dnLength;
char                setDn[26];
tDeviceId           setId;
tDeviceId           acquiredSetId;
tCallId             activeCallId;
tCallId             selectedCallId;
} MonitoredDns;

MonitoredDns* pMonitoredDn = (MonitoredDns*) malloc(sizeof(MonitoredDns));
MonitoredDns dnStruct = *pMonitoredDn;
mbMonitorDeviceBegin(dn, &dnStruct.setId);

//Now print setId value set by the mbMonitorDeviceBegin function using the two following methods
//%hu is for unsigned short as I think, setId is a tDevice which is uInt16
printf("before(1) AddDnToMonitoredDns SetID = %hu\n", pMonitoredDn->setId);
printf("before(2) AddDnToMonitoredDns SetID = %hu\n", dnStruct.setId);

Answer 1

MonitoredDns dnStruct = *pMonitoredDn;

执行此行后，dnStruct与*pMonitoredDn不同，它是*pMonitoredDn的副本。然后，您调用修改副本的方法，原始malloc版本仍未更改。

请尝试此操作，此版本将显示.和->具有相同的结果。但是，这个版本与你的版本不同，因为pMonitoredDn指向的内存的生命周期现在只有方法持续时间，而malloc - 内存将持续直到free - 编辑

MonitoredDns dnStruct;
MonitoredDns* pMonitoredDn = &dnStruct;
mbMonitorDeviceBegin(dn, &dnStruct.setId);

//Now print setId value set by the mbMonitorDeviceBegin function using the two following methods
//%hu is for unsigned short as I think, setId is a tDevice which is uInt16
printf("before(1) AddDnToMonitoredDns SetID = %hu\n", pMonitoredDn->setId);
printf("before(2) AddDnToMonitoredDns SetID = %hu\n", dnStruct.setId);

Answer 2

首先复制一下空结构。

然后你调用一个修改副本的函数。

然后你想知道为什么副本和原版不一样。