“ opt”不能出现在常量表达式中

Question

In the following C++ program:

int opt;
in(opt);
switch(opt)
 case(opt == 1):
 //and so on…

where in(opt); is the procedure used to get the value of the integer opt.

I get the error here: case(opt == 1):

Basically the point is I want to make a way for the user to decide what feature of the program is going to be used. I also tried with a char but had no luck with it as well. I just can't figure out what's going on.

Answer 1

The case labels in a switch block need to be compile time evaluable constant expressions (and integral types).

Since opt == 1 is only known at run-time, compilation of case (opt == 1) will fail.

Did you mean simply case 1: ?

Answer 2

switch(opt) {
    case 1:
        break;
    case 2:
        break;
}

The switch part says that you're looking at the value of opt ; each case statement gives a possible matching value. The value in a case statement has to be a compile-time constant.