cin erratic behaviour

Question

I'm a newbie to C++. Small code sample follows:

int main(int argc, char* argv[]) {    

    char ch1;
    int int1;

    cin >> ch1;
    cin >> int1;

    cout << ch1 << '\n';
    cout << int1 << '\n';

    return 0;
}

When I run the program and input the following:

az

I get as output:

a 32767

I understand the 'a' but why the integer value of 32767? I just want to test and see what happen if instead of a numeric value assigned to int1 i used a 'z'.

I try inputting:

ax

...and I also get same results.

Now if instead of int int1 I use short int1 and run the program with input:

az

I get the output:

a 0

P.S.

sizeof(int) = 4
sizeof(short) = 2

I am using a 64-bit machine.

Answer 1

When an input stream fails to read valid data, it doesn't change the value you passed it. 'z' is not a valid number, so int1 is being left unchanged. int1 wasn't initialized, so it happened to have the value of 32767. Check the value of cin.fail() or cin.good() after reading your data to make sure that everything worked the way you expect it to.

Answer 2

cin >> int1; means "read an integer and put it in int1." So you feed it z, which is not a valid character in an integer, and it simply aborts the read and leaves whatever in int1.

Test this by initializing int1 to something and seeing what happens.

Answer 3

The c++ cin stream is doing input validation for the program.

When streaming from cin into an int cin will only accept valid number didgits, -0123456789, and also only values between INT_MIN and INT_MAX.

If the numerical value for z (122) is required I would recommend using the c getchar function rather than the cin stream.

int main(int argc, char* argv[]) {

    cout << "function main() .." << '\n';

    char ch1 = getchar();
    int int1 = getchar();
    cout << ch1 << '\n';
    cout << int1 << '\n';

    return 0;

}

When az is input this will output

a
122

Answer 4

Using cin directly is, personally, i.e., for me, a bad idea for reading data in non-trivial programs. I suggest you read another answer I gave for a similar question:

C++ character to int