4/17/08

Understanding the Difference Between Postfix and Prefix Operators

The built-in ++ and—operators can appear on both sides of their operand:
int n=0;
++n; // prefix ++
n++; // postfix ++
You probably know that a prefix operator first changes its operand before taking its value. For example:
int n=0, m=0;
n = ++m; // first increment m, then assign its value to n
cout << n << m; // display 1 1
In this example, n equals 1 after the assignment because the increment operation took place before m's value was taken and assigned to n. By contrast,
int n=0, m=0;
n = m++; // first assign m's value to n, then increment m
cout << n << m; // display 0 1
In this example, n equals 0 after the assignment because the increment operation took place after m's original value was taken and assigned to n.
To understand the difference between postfix and prefix operators better, you can examine the disassembly code generated for these operations. Even if you're not familiar with assembly languages, you can immediately see the difference between the two—simply notice where the inc (increment) assembly directive appears:
// disassembly of the expression: m=n++;
mov ecx, [ebp-0x04] // store n's value in ecx register
mov [ebp-0x08], ecx // assign value in ecx to m
inc dword ptr [ebp-0x04] // increment n;

// disassembly of the expression: m=++n;
inc dword ptr [ebp-0x04] // increment n;
mov eax, [ebp-0x04] // store n's value in eax register
mov [ebp-0x08], eax // assign value in eax to m

1 comment:

Shabbir said...

Really nice information.

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