c++ - How to determine if two variables are equal

Question

1. Pointer variable PVOID lpbuffer points to a memory address,

2. Variable GUID g = {0x25a207b9,0xddf3,0x4660,{0x8e,0xe9,0x76,0xe5,0x8c,0x74,0x06,0x3e}};

How to determine whether the content pointed to by the pointer lpbuffer is equal to the GUID?

Answer 1

According to the definition of GUID structure on MSDN

typedef struct _GUID {
  DWORD Data1;
  WORD  Data2;
  WORD  Data3;
  BYTE  Data4[8];
} GUID;

GUID is a structure that does not have an overloaded == operator, so if you want to compare two GUIDs, you must either implement the == operator or honestly compare the member variables one by one.

// PVOID lpbuffer;
// GUID g = {0x25a207b9,0xddf3,0x4660,{0x8e,0xe9,0x76,0xe5,0x8c,0x74,0x06,0x3e}};
GUID *p = (GUID*)lpbuffer;
BOOL flag = FALSE;
if ((*p->Data1 == g.Data1)
    && (*p->Data2 == g.Data2)
    && (*p->Data3 == g.Data3)){
    flag = TRUE;
    for(int i; i < 8; i++){
       if (*p->Data4[i] != g.Data4[i]){
           flag = FALSE;
       }
    }
}

// flag变量为比较结果

Answer 2

There are two situations:

The addresses are different, assuming that the GUID type overloads the == operator

if (lpbuffer != &g && *(*GUID)lpbuffer == g) {}  // *为取值符，判断指针指向地址存放的值是否等于变量g

The address is the same and the number of bytes is the same

if (lpbuffer == &g && sizeof(*lpbuffer) == sizeof(g)) {} // &为取地址符，判断指针是否指向变量g的地址

As for the GUID type that does not overload the == operator but knows its internal structure, you can refer to the answer of other respondent @一代Key客

The GUID type has not overloaded the == operator and does not know its internal structure. I think it can be compared byte by byte

    if (sizeof(*lpbuffer) != sizeof(g)) return false;
    size_t size = sizeof(*lpbuffer);
    size_t i = 0;
    char* p1 = (char*)lpbuffer;
    char* p2 = &g;
    while ( i != size ) {
        if (*p1 != *p2) return false;
        p1++;
        p2++;
        i++;
    }
    return true;

Answer 3

if (memcmp(lpbuffer, GUID, 16) == 0) {
    ...
}