switch-case error | the value of is not usable in a constant expression

Question

I wrote a simple example to solve the problems I faced when writing his program.

During program execution I get values of input1 and input2 when returning values from functions, which are then never change. Then a little later after various computations in the process of the program I get a result which is also no longer changeable.

I'm trying to compare them using switch-case, but I get one error "the value of ‘input1’ is not usable in a constant expression".

#include <iostream>

using namespace std;

char getChar()
{
    char c;
    cin >> c;
    return c;
}

int main()
{
    // it doesn't work
    const char input1 = getChar();
    const char input2 = getChar();

    // it it works
    //const char input1 = 'R';
    //const char input2 = 'X';

    char result = getChar();
    switch(result)
    {
        case input1:
            cout << "input1" << endl;
            break;
        case input2:
            cout << "input2" << endl;
            break;
    }
    return 0;
}

Answer 1

You have to have your case statements known at compile time. I.e.

switch(result)
    {
        case 1:
            cout << "input1" << endl;
            break;
        case 2:
            cout << "input2" << endl;
            break;
    }

These lines, whilst const only really mean read-only and are not initialised at compile time.

// it doesn't work
const char input1 = getChar();
const char input2 = getChar();

The reason the following two lines would work is because the compiler just substitues in X & R into your switch statement before your code even runs

// it it works
//const char input1 = 'R';
//const char input2 = 'X';

I would suggest changing your switch to an if statement

if(input1)
{}
else if(intput2)
{}

The following code should work:

#include <iostream>

using namespace std;

char getChar()
{
    char c;
    cin >> c;
    return c;
}

int main()
{
    // it doesn't work
    const char input1 = getChar();
    const char input2 = getChar();

    // it it works
    //const char input1 = 'R';
    //const char input2 = 'X';

    char result = getChar();
    if(result == input1){
            cout << "input1" << endl;
    }
    else if(result == input2){
            cout << "input2" << endl;
    }

    return 0;
}