Thread: Where to put the "include" directive and "using namespace std" with multiple files?

I have two files that I want to compile:

main.cpp

Code:

#include <iostream>

using namespace std;

int ReadNumber();
void WriteAnswer();

int main()
{
    WriteAnswer();
    return 0;
}

and

io.cpp

Code:

int ReadNumber() {
    int n = 0;
    cin >> n;
    return n;
}
void WriteAnswer() {
    cout << ReadNumber() + ReadNumber() << endl;
}

The compiler complains:

io.cpp||In function 'int ReadNumber()':|
io.cpp|3|error: 'cin' was not declared in this scope|
io.cpp||In function 'void WriteAnswer()':|
io.cpp|7|error: 'cout' was not declared in this scope|
io.cpp|7|error: 'endl' was not declared in this scope|

In io.cpp file, should I put the two statements ("include <iostream>" and "using namespace std") at the top, outside of the functions?

Or should I put the two statements inside each of the functions?

Sorry for the noob question.

By the way, it is generally a bad idea to import such a big namespace wholesale.

Many beginner tutorials do it for simplicity, but you'll find that almost no serious code does that.

One of the problems with "using namespace std;" is that the std namespace is huge and always growing.

For example, it grew quite a bit in C++11 (the new C++ specification).

If you had older code that did "using namespace std;" and you had a function called "move()", it would have been correct and would have compiled.

Now if you upgrade your compiler (or compiler setting) to C++11 your code will suddenly break because now there's a std::move(). This is an actual problem I had to fix last week. It was someone else's code.

I would recommend not using "using namespace std;".

You can either write std:: all the time, which is what I do and I like it because it serves as a form of documentation, too, to make it immediately clear to the reader that I am referring to something in the standard library (could be hard to tell sometimes in projects with hundreds of source files and thousands of functions).

Another way is to import specific symbols, and only import the ones you need -

Code:

using std::cout;
using std::endl;
using std::cin;
...

And as already mentioned, never ever do "using namespace std;" in header files.