Why can't I have a variable in switch-case statement? [duplicate] - c++

Question

This question already has answers here:
Case expression not constant
(5 answers)
Closed 5 years ago.
Here is my code:
bool Character::keyPress(char c)
{
switch(c)
{
case up_key:
move(0, -1);
break;
case down_key:
move(0, 1);
break;
case left_key:
move(-1, 0);
break;
case right_key:
move(1,0);
break;
default:
return false;
}
return true;
}
And the compiler complains:
error C2051: case expression not constant
error C2051: case expression not constant
error C2051: case expression not constant
error C2051: case expression not constant
In my header file I have:
protected:
char up_key;
char down_key;
char right_key;
char left_key;
I am using Visual C++ 2008.

Answer 1

As the error message states, the case expressions must be constant. The compiler builds this as a very fast look-up table at compile time and it can't do that if there is a possibility that the values could change as the program runs.
If you do need them to be variable, not constant, your best bet is to use if/else statements instead.

Answer 2

Replace this long clumsy code,
switch(c)
{
case up_key:
move(0, -1);
break;
case down_key:
move(0, 1);
break;
case left_key:
move(-1, 0);
break;
case right_key:
move(1,0);
break;
default:
return false;
}
with something like this:
move( (c==right_key) - (c==left_key) , (c==down_key) - (c==up_key) );
You can litterly replace that 17 lines long of code with that much more neat single line of code.

Answer 3

You can't because the language doesn't work that way. For example, what would happen if up_key, down_key, right_key, and left_key were all equal?

Answer 4

Because the switch statement can take only constants, you know when reading the code that the things you're comparing against are all constants. On the other hand, you would use if statements (or some other structure) to compare against variables:
if (c == up_key) {
move(0, -1);
} else if (c == down_key) {
move(0, 1);
} else ...
This provides a distinct difference in structure which can greatly aid those who come after you in reading your code. Imagine if you had to look up every case label to see whether it was a variable or not?

Answer 5

I believe it's because the compiler generates a jump table, with the values hardcoded in, although I may be wrong. The way the tables are generated just doesn't allow for it.

Answer 6

Since other answers have covered why you are getting an error, here is a way to move in one of the four directions in response to a key press: use lookup tables instead of the conditionals/switches.
Setup portion:
std::map<char,pair<int,int> > moves;
moves[up_key] = make_pair(0, -1);
moves[down_key] = make_pair(0, 1);
moves[left_key] = make_pair(-1, 0);
moves[right_key] = make_pair(1, 0);
The function:
bool Character::keyPress(char c) {
if (moves.count(c)) {
pair<int,int> dir = moves[c];
move(dir.first, dir.second);
return true;
} else {
return false;
}
}

Answer 7

//here is the full functional code snippet which can be compiled and run with most of C++
//compiler/link ...console app was demoed but you can apply the code/logic to win32 app...
//if you have any problem, send me email to Samuel_Ni#yahoo.com
#include <iostream.h>
#include <map>
#include <conio.h>
class CkbdHanler{
private:
map<char,pair<int,int> > moves;
protected:
char up_key;
char down_key;
char right_key;
char left_key;
public:
CkbdHanler(char a,char b,char c,char d):up_key(a),
down_key(b),
right_key(c),
left_key(d)
{
moves[up_key] = make_pair(0, -1);
moves[down_key] = make_pair(0, 1);
moves[left_key] = make_pair(-1, 0);
moves[right_key] = make_pair(1, 0);
}
bool keyPress(char c){
if (moves.count(c)) {
pair<int,int> dir = moves[c];
move(dir.first, dir.second);
return true;
} else return false;
}
void move(int i,int j){
cout<<"(i,j)=("<<i<<","<<j<<")"<<endl;
}
};
int main(int argc, char* argv[])
{
CkbdHanler CmyKbdH('u','d','l','r');
cout << "Hello C++... here is a demo of Map to replace switch-case" << endl;
CmyKbdH.keyPress('d');
cout << endl << "Press any key to continue...";
getch();
return 0;
}