include and using namespace in C++

Question

for using cout, I need to specify both:

#include

and

using namespace std;

Answer 1

If your C++ implementation uses C style header files (many do) then there is a file that contains something similar to:

#include ... // bunches of other things included

namespace std {

... // various things

extern istream cin;
extern ostream cout;
extern ostream cerr;

... // various other things

}

std is the namespace that the C++ standard says most of the standard things should reside in. This is to keep from overpopulating the global namespace, which could cause you difficulty in coming up with names for your own classes, variables, and functions which aren't already used as names for standard things.

By saying

using namespace std;

you are telling the compiler that you want it to search in the namespace std in addition to the global namespace when looking up names. If the compiler sees the source line:

return foo();

somewhere after the using namespace std; line it will look for foo in various different namespaces (similar to scopes) until it finds a foo that meets the requirements of that line. It searches namespaces in a certain order. First it looks in the local scope (which is really an unnamed namespace), then the next most local scope until over and over until outside of a function, then at the enclosing object's named things (methods, in this case), and then at global names (functions, in this case unless you've been silly and overloaded () which I'm ignoring), and then at the std namespace if you've used the using namespace std; line. I may have the last two in the wrong order (std may be searched before global), but you should avoid writing code that depends on that.