是否值得用常量替换开关中的参数？

Question

In my code I have

switch (cd->op)
{
...
}

and I'm wondering whether I should do

CalcWizConsts::eqOps thisOp = cd->op;
switch (thisOp)
{
...
}

Answer 1

The argument to switch will only be evaluated once, so there is no need to store it in a temporary first. There is no performance difference, and you don't have to worry about changing the value in one of the case clauses either. The only reason I can think of for assigning to a variable first is to make the code more readable, if the expression is lengthy.