Analysis of auto-incrementing operators in C Language

The discussion about auto-increment operations such as a ++ and ++ A in C language is boring, but sometimes I have to discuss it in detail to cope with interviews.

Environment: win7, vs2010

Example:

Write and judge whether the four expressions of ABCD are correct. If yes, write the value of A in the expression (3 points)

Int A = 4;
(A) A ++ = (a ++); (B) a ++ = (++ A); (c) (a ++) ++ = A; (d) (++ A) ++ = (a ++ );

Q: A =?

A: C error. The left side is not a valid variable and cannot be assigned a value. You can change it to (++ A) + =;

After the change, the answers are 9, 10, 10, and 11 in sequence.

Analysis:

A option A + = (a ++ );

Int _ tmain (INT argc, _ tchar * argv []) {int A = 4; 00f7136e mov dword ptr [a], 4 A ++ = (a ++ ); 00f71375 mov eax, dword ptr [a] // Save the value of a to eax. In this case, eax = 400f71378 add eax, dword ptr [a] // Add a to the value of eax, in this case, eax = 800f7137b mov dword ptr [a] and eax // save eax to A, that is, a = 8. It can be seen that a + = (a ++) first, run a = a + A; 00f7137e mov ECx, dword ptr [a] // save a to ecx00f71381 add ECx, 1 // ECx + 1, then ECx = 9; 00f71384 mov dword ptr [a], ECx // ECx is stored back to A, that is, a = 9 return 0; 00f71387 XOR eax, eax}

B Option A + = (++ );

Int _ tmain (INT argc, _ tchar * argv []) {int A = 4; 0129136e mov dword ptr [a], 4 A ++ = (++ ); 01291375 mov eax, dword ptr [a] 01291378 add eax, 1 // Add 1 first, then eax = 50129137b mov dword ptr [a], eax // eax is stored back to, in this case, a = 50129137e mov ECx, dword ptr [a] 01291381 add ECx, dword ptr [a] // ECx = ECx + ECx. In this case, ECx = 1001291384 mov dword ptr [a], ECX // at this time a = 10 return 0; 01291387 XOR eax, eax}

C option: (A ++) + =;

Compilation error: Error c2106: '+ =': left operand must be L-value. (A ++) is an expression and cannot be assigned a value.

D option: (++ A) + = (a ++ );

Int _ tmain (INT argc, _ tchar * argv []) {int A = 4; 0129136e mov dword ptr [a], 4 (++) + = (a ++); 00d81375 mov eax, dword ptr [a] 00d81378 add eax, 1 00d8137b mov dword ptr [a], eax // complete ++, at this time, a = 5; 00d8137e mov ECx, dword ptr [a] 00d81381 add ECx, dword ptr [a] // complete a = a + A; then a = 10; 00d81384 mov dword ptr [a], ECx 00d81387 mov edX, dword ptr [a] 00d8138a add edX, 1 00d8138d mov dword ptr [a], EDX // complete a ++; at this time, a = 11 return 0; 01291387 XOR eax, eax}