Underlying mechanism of pointer

Raise questions ：

• Why do pointers have types ？

C++ Code

int gi;
int* p;
int main()
{
    
    p = &gi;
    *p = 12;
}

assembly

 mov  dword ptr ds:[006A02ECh],6A02E8h
 mov  eax,dword ptr ds:[006A02ECh]  
 mov  dword ptr [eax],0Ch

Machine code

C7 05 EC 02 6A 00 E8 02 6A 00
A1 EC 02 6A 00
C7 00 0C 00 00 00

Find the answer to the problem ：

• Pointer Type information Determines the assignment / Write on read / read How many bytes ;

Conclusion ：

> C The pointer type of language contains two aspects of information ：
1. Address , Stored in pointer variables ;
2. Type information , About the length of reading and writing , Not stored in pointer variables , It is placed at the time of reading and writing the pointer mov In the instruction , Different read and write lengths correspond to mov The instructions are different ;

Raise questions ：

• Why? Different types Of Pointer variable assignment Error will be reported in compilation ？ answer ： Compilers are designed to help programmers Avoid cross-border errors , This leads to compilation errors
• Why can we assign values after forced conversion ？ What happened to cast ？

C++ Code

int i;
int* pI;
short* pS;
char* pC;
int main()
{
    
    pI = &i;
    pS = (short*)pI;
    pC = (char*)pI;
    *pI = 0x12345678;
    *pS = 0x1234;
    *pC = 0x12;
}

assembly

 mov   dword ptr ds:[00360610h],36060Ch  
 mov   eax,dword ptr ds:[00360610h]  
 mov   dword ptr ds:[00360614h],eax  
 mov   eax,dword ptr ds:[00360610h]  
 mov   dword ptr ds:[00360618h],eax  
 mov   eax,dword ptr ds:[00360610h]  
 mov   dword ptr [eax],12345678h  
 mov   eax,1234h  
 mov   ecx,dword ptr ds:[00360614h]  
 mov   word ptr [ecx],ax  
 mov   eax,dword ptr ds:[00360618h]  
 mov   byte ptr [eax],12h

Machine code

C7 05 10 06 36 00 0C 06 36 00
A1 10 06 36 00
A3 14 06 36 00
A1 10 06 36 00
A3 18 06 36 00
A1 10 06 36 00
C7 00 78 56 34 12
B8 34 12 00 00
8B 0D 14 06 36 00
66 89 01
A1 18 06 36 00
C6 00 12

The answer to the question ：

• Pointer cast After the impact is not in When you switch happen , It's using Converted identity Go to Access memory Time is embodied in the instruction

you Really? Do you have it ？

int* pI;
short sI = 12;
pI = (int*)&sI;
print("%d, %x,",*pI, *pI);