C + = c -- | ++ B;

Everything starts from here.

A freshman sent me a C ++ question via QQ:

int c = 8, b = 3;c += c-- | ++b;

Ask the value of C. After computation, we can see that C is 19, and a C # console project is created at will. It is a tragedy... C # outputs 20. Re-calculate again or 19 ah, and quickly re-build a C ++ console project ran out of the result of 19. Why is C ++ different from C?

Verification 1

I learned from the online query about the C # order of value. What is the specific situation? Let's disassemble:

    .method private hidebysig static void  Main(string[] args) cil managed    {      .entrypoint      // Code size       33 (0x21)      .maxstack  4      .locals init ([0] int32 c,               [1] int32 b)      IL_0000:  nop      IL_0001:  ldc.i4.8      IL_0002:  stloc.0      IL_0003:  ldc.i4.3      IL_0004:  stloc.1      IL_0005:  ldloc.0      IL_0006:  ldloc.0      IL_0007:  dup      IL_0008:  ldc.i4.1      IL_0009:  sub      IL_000a:  stloc.0      IL_000b:  ldloc.1      IL_000c:  ldc.i4.1      IL_000d:  add      IL_000e:  dup      IL_000f:  stloc.1      IL_0010:  or      IL_0011:  add      IL_0012:  stloc.0      IL_0013:  ldloc.0      IL_0014:  call       void [mscorlib]System.Console::WriteLine(int32)      IL_0019:  nop      IL_001a:  call       valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()      IL_001f:  pop      IL_0020:  ret    } // end of method Program::Main

Through the Il code, we can clearly see its computing process:

• 0 ~ The four rows assign values to the variables. In the call stack, 0 indicates that the C value of the variable is 8, and 1 indicates that the B value of the variable is 3.
• The value of C is cached in five rows and stored in the evaluation stack. The cached value is 8.
• A c value is cached in six rows, and the cached value is 8.
• 7 ~ Row A executes c = C-1, and the value of C is 7.
• B ~ Row F executes B = B + 1 and leaves a copy. The value is 4.
• 8 and B ~ The value of 4 or left in Row F is 12.
• Result 12 in row 11 and row 10 plus 8 cached in Row 5 is 20
• 12 rows are stored in variable C

C + = c -- | ++ B; equivalent to C = C + (c -- | ++ B). Through disassembly, we can see that:

1. The order in which C # is evaluated from left to right is not changed because of the operation order. The value of the first C on the right of the equal sign is cached from the beginning.
2. C -- after evaluationSettled immediatelyThe value of the C variable changes to 7 at this time, but the value of the C variable does not affect the final result of the formula. The reason is as follows: 1.
3. Convert the formulaC = C + (-- c | ++ B );The obtained value is 15 (8 + (7 | 4). After disassembly, we observe that the conclusions are the same as the first one. c -- and -- C only affect the calculation results in parentheses.

According to the above conclusion, we changed the formulaC = (c -- | ++ B) + C;And the result is 19.

Proof 2

So how is C ++ executed? Continue, decompilation:

(The disassembly program is based on the C ++ debug compilation result of vs2013. The disassembly code of GCC 4.6.1 is slightly different, and the execution process is consistent. The conclusion is only limited to the two compiling environments) c ++ execution process:

1. Use the eax register for B = B + 1
2. Use the ECX register for C = 8 + (8 | 4) (B = 4)
3. Use the edX register for C = C-1

We can see from the disassembly:

• C ++ does not cache values, and C ++ does not specify the order of values.
• C ++ takes the value only during calculation, so the result of c = (c -- | ++ B) + C; is 19.
• C ++ is C at the end of the entire formula -- that is to say, the result is 19, not 20, not because the or in the brackets causes C to be 7 in the final addition,It is because c -- settlement is performed only after the formula value assignment is complete.. (Teachers, are you right ?)

Finally, verify that the formula changedC = C * 2 + (c -- | ++ B );The output result of C ++ is 27,8*2 + 12 = 28-1 = 27. The disassembly can also see the sub executed at the end.

Last

• If you are learning C ++, do not use C # To verify the answers to your assignment questions...
• Let '+ +' only appear in the for statement...