CODEWITHSUNDEEP
c++error-handlingcin
user1668903
user1668903

Reputation: 407

What does cin do when there is an error

#include<iostream>;

int main()
{
    int a = 1;
    int b = 2;
    std::cin >> a >> b;
    std::cout << a << "+" << b << "=" << a+b << std::endl;
    return 0;
}

when I enter 3 4 as input,the output will be 3+4=7,well,it's strange; But when I enter a b,the output is 0+0=0(Why it is 0 and 0?); The most confusing,a 4,it will be 0+0=0(Why not '0+4=4'?????); Then i write another prog.

#include<iostream>;

int main()
{
    int a = 1;
    int b = 2;
    std::cin >> a;
    std::cin.clear();
    std::cin >> b;
    std::cout << a << "+" << b << "=" << a+b << std::endl;
    return 0;
}

When i enter a 4,why is it still 0+0=0?Shouldn't it be 0+4=4?

Thanks to all the warm-hearted!!

I write prog3,to test what will happen when i don't write int a=1;int b=2;

2

#include <iostream>
using namespace std;
int main()
{  
    int a,b;
    cin >> a  ;
    cin >> b;
    cout<< a << "+"<< b <<"="<< a+b << endl;
    return 0;
}

When a bagain,it outputs 0+-1218170892=-1218170892(Why isn't 0+0=0??)

Upvotes: 9

Views: 2550

Answers (3)

Sogger
Sogger

Reputation: 16122

The value is set to zero on an error as per C++11: If extraction fails, zero is written to value and failbit is set.

On the 'a 4' example, both values are 0 because the buffer has not been flush/cleared, so the second cin read is still reading the error, and also receives a value of 0.

Upvotes: 3

Drise
Drise

Reputation: 4388

Like all istreams, std::cin has error bits. These bits are set when errors occur. For example, you can find the values of the error bits with functions like good(), bad(), eof(), etc. If you read bad input (fail() returns true), use clear() to clear the flags. You will also likely need an ignore(1); to remove the offending character.

See the State functions section for more information. http://en.cppreference.com/w/cpp/io/basic_ios

Upvotes: 4

CashCow
CashCow

Reputation: 31435

std::cin is an istream instance and thus it maintains its error state when it reads something invalid.

In order to "cure" it you must both clear its flag 

std::cin.clear();

and flush its buffer. 

std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

What is more surprising though is that it doesn't return 1 + 2 = 3 when you input invalid characters, as I would expect a failing cin stream to have no side effects on what it is trying to update.

Upvotes: 1

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