Why can't i use std:cin as an argument

Question

learning c++ and can't understand why I can't use "std::cin" as an argument.

#include <iostream>
#include "stdafx.h"
int doubleNumber(int a)
{
    return 2 * a;
}

int main()
{
    int x;
    std::cout << doubleNumber(std::cin >> x);
    return 0;
}

Petr Skocik · Accepted Answer · 2016-05-21T22:44:34.660

std::cin >> x returns a reference to cin, which isn't implicitly convertible to int.

You can use the , operator like so:

(std::cin >> x, x)

to first run std::cin >> x and then evaluate that expression to x.

#include <iostream>
int doubleNumber(int a)
{
    return 2 * a;
}

int main()
{
    int x;
    std::cout << doubleNumber( (std::cin >> x, x) );
    return 0;
}

Splitting it will probably into two lines will probably make it more readable, though.

In any case std::cin >> x can be used as an expression. E.g., it's common to have streams implicitly converted to booleans to check whether they're in a succeeding (good) state. (E.g., if(std::cin >> x){ //...)

thank you so much, that was exactly what i was looking for :) — Rodrigo Lopes de Melo Dinis da, May 21 '16 at 22:48

score 2 · Answer 2 · answered May 21 '16 at 22:35

2

You presumably want to pass x. But the result of cin >> x is cin, not x.

The solution is easy

 std::cin >> x;
 std::cout << doubleNumber(x);

You can't actually pass cin if you did want to because it's a stream, not an int.

And the return type of >> is the way it is in order to allow things like std::cin >> x >> y >> z; to work.

answered May 21 '16 at 22:35

Alan Stokes

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so there is no way to pass the cin >> x as an argument, you will always need to to ask before using it as an argument? – Rodrigo Lopes de Melo Dinis da May 21 '16 at 22:38
1

Can not the function take a parameter of stream type? (just askin' :). – Martin James May 21 '16 at 22:43
@MartinJames Devils advocate, eh' ;-P – πάντα ῥεῖ May 21 '16 at 22:44
1

@MartinJames You can pass anything if the argument type matches. – Carey Gregory May 21 '16 at 22:45
@MartinJames Of course. (By reference, normally.) But that would require changing the function definition, which did not seem to me to be the sense of the question. – Alan Stokes May 21 '16 at 22:46

score 1 · Answer 3 · answered May 21 '16 at 22:38

std::cin is a global object and operator >>, which you are calling is a method that returns std::cin again, so you can write things like:

std::cin >> x >> y;

What I am trying to say is that the output of std::cin >> x is not the value you just typed in, as you seem to expect, but std::cin itself.

See http://en.cppreference.com/w/cpp/io/basic_istream/operator_gtgt for further details.

Why can't i use std:cin as an argument

3 Answers3