C++ for 循环更新表达式中的赋值表达式

Question

According to cppreference.com the update expression , or iteration expression , of a for cycle in C++ language can be

any expression, which is executed after every iteration of the loop and before re-evaluating condition.

Hence, I thought that the following for cycle was correct:

//initializations

Point cursor;
cursor.y = minR * pixelSpacing.y + origin.y;
for (int r = minR; r <= maxR; r++, cursor.y += pixelSpacing.y, cursor.x = minC * pixelSpacing.x + origin.x)
{
    //loop statements
}

However, the results I obtain at the end of the cycle are different from the ones obtained with the following code:

//initializations

Point cursor;
cursor.y = minR * pixelSpacing.y + origin.y;
for (int r = minR; r <= maxR; r++, cursor.y += pixelSpacing.y)
{
    cursor.x = minC * pixelSpacing.x + origin.x;

    //loop statements
}

Is it therefore not valid to use an assignment expression in an update expression of a for loop?

Answer 1

This

for (int r=minR; r<=maxR; r++, cursor.y += pixelSpacing.y, cursor.x = minC * pixelSpacing.x + origin.x)

is equivalent to this:

for (int r=minR; r<=maxR; /* nothing here */)
{
  /* loop body here */

  r++;
  cursor.y += pixelSpacing.y; 
  cursor.x = minC * pixelSpacing.x + origin.x;
}

So the only difference is the order in which you execute the various statements. Note that the comma operator evaluates left-to-right, where the left-most operand is guaranteed to be executed first.