cout is not a member of std

I'm practicing using mulitple files and header files etc. So I have this project which takes two numbers and then adds them. Pretty simple.

Here are my files:

main.cpp

#include <iostream>
#include "add.h"

int main()
{
    int x = readNumber();
    int y = readNumber();

    writeAnswer(x + y);

    return(0);
}

io.cpp

int readNumber()
{
    int x;

    std::cout << "Number: ";
    std::cin >> x;

    return x;
}

void writeAnswer(int x)
{
    std::cout << "Answer: ";
    std::cout << x;
}

add.h

#ifndef ADD_H_INCLUDED
#define ADD_H_INCLUDED

int readNumber();
void writeAnswer(int x);

#endif // #ifndef ADD_H_INCLUDED

The error is showing up in io.cpp. The exact errors are:

Does anyone have any idea why this may be happening? Thanks.

EDIT: I made a small project yesterday with the same amount of files (2 .cpp and 1.h) and I didn't include the iostream header in the other .cpp and it still compiled and ran fine.

add #include <iostream> to the start of io.cpp too.

Also remember that it must be:

#include "stdafx.h"
#include <iostream>

and not the other way around

#include <iostream>
#include "stdafx.h"

I had a similar issue and it turned out that i had to add an extra entry in cmake to include the files.

Since i was also using the zmq library I had to add this to the included libraries as well.