$a places scalar $a on the stack (the variable $a, not its value 10).
$a-- places a scalar containing the original value of $a on the stack, so $a == $a-- compares $a with the original value of $a. They will never be equal.
--$a places scalar $a on the stack, so $a == --$a compares $a and $a. They will always be equal.
C behaves differently since it places values on the stack or in the registers. You can replicate the behaviour using pointers in C or references in C++.
#include <stdio.h>
#include <stdlib.h>
int& postdec(int& a) {
int* rv_ptr = (int*)malloc(sizeof(int));
*rv_ptr = a;
--a;
return *rv_ptr;
}
int& predec(int& a) {
--a;
return a;
}
bool& eq(int& a, int& b) {
bool* rv_ptr = (bool*)malloc(sizeof(bool));
*rv_ptr = a == b;
return *rv_ptr;
}
int main() {
int a = 10;
if (eq(a, postdec(a))) {
printf("T1\n");
}
a = 10;
if (eq(a, predec(a))) {
printf("T2\n");
}
}
Update: Removed overspecification identified by BrowserUk.
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