std::cin , unwanted behaviour. and how can I fix this?

Question

I tried to play with some code to test overloading functions. The overloading part went well, however, I learned something about std::cin that made me feel stupid for not noticing it before!

#include <iostream>
void read (int *var){
    std::cout<<std::endl<<" input :";
    std::cin>>*var;
}
void read (float *var){
    std::cout<<std::endl<<" input :";
    std::cin>>*var;
}
void read (char *var){
    std::cout<<std::endl<<" input :";
    std::cin>>*var;
}
void read (bool *var){
    std::cout<<std::endl<<" input :";
    std::cin>>*var;
}
int main(){
    int a;
    float x;
    char c;
    bool s;
//  std::cin>>a;
//  std::cin>>x;
//  std::cin>>c;
//  std::cin>>s;
    read (&a);
    read (&x);
    read (&c);
    read (&s);
    std::cout<<a<<std::endl<<x<<std::endl<<c<<std::endl<<s<<std::endl;
    return (0);
}

When compiled with g++ this is what happens:

$ g++ test1.cpp -o test
$ ./test

 input :1.2

 input :
 input :a

 input :0
1
0.2
a
0

I already tried many values and added some instructions in between, but I still get the same behaviour, which is annoying if I try to read multiple values and more so if they were in different types.

The commented text does basically the same as the 'read' functions below and has the same behaviour. I am using a function because I just wanted to do so :D

`1.2` is not a valid integer. The `1` is read and the `.2` is left over for the next cin — drescherjm, Jan 15 '22 at 21:58
"**std::cin function**", `std::cin` is NOT a function. It's a global object: [std::cin, std::wcin](https://en.cppreference.com/w/cpp/io/cin) — digito_evo, Jan 15 '22 at 21:58
The commented code does not write any prompts to `std::cout`. But it does read from `std::cin` in the same way. Your overloaded versions of `read()` work because I/O stream objects (like `std::cin`) have overloads of `operator>>()`. — Peter, Jan 16 '22 at 01:41

score 3 · Accepted Answer · answered Jan 15 '22 at 22:04

3

This is not a strange behaviour what really happens is when this line is executed in the read int function:

 std::cin>>*var;

it expects an integer from your keyboard buffer and when you enter this as input:

1.2

the cin object reads the first digit until the decimal point because it is the integer part and leave the remaining characters inside the buffer so variable a will have the value 1 and the characters .2 will be lifted over in the buffer.

So when the read float function executes it doesn't wait for your input because there is already a floating point number inside the buffer so it reads it and stores it in the variable so it's value becomes 0.2.

answered Jan 15 '22 at 22:04

Bemwa Malak

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18

I kind of knew it was storing the part that it didn't read somewhere, but I wanted to delete it so I don't get any errors later on. – med amine riahi Jan 15 '22 at 22:18
You can do so using `cin.ignore(numeric_limits::max())` but you have to include `#include ` – Bemwa Malak Jan 15 '22 at 22:20
1

`numeric_limits::max() sets the maximum number of characters to ignore. Since this is the upper limit on the size of a stream, you are effectively telling cin that there is no limit to the number of characters to ignore.` – Bemwa Malak Jan 15 '22 at 22:21
this simply ignores the leftover part. – Bemwa Malak Jan 15 '22 at 22:22

std::cin , unwanted behaviour. and how can I fix this?

1 Answers1