No header files included in file but can still use classes/functions from unincluded header files

Question

Here's some the code from the file in question, called Global.h, which is used in other header files and seems to compile just fine:

#pragma once

enum SType {null, lab, assignment, testPrep};
enum Code {none, 123, 222, 333, 432};

template<typename D>
bool validate(D task = string, D date = string) {

    bool result = true;

    if (task.size() < 3) {
        cout << "Task too simple, please elaborate." << endl;
        result = false;
    }
    else if (task.size() > 50) {
        cout << "Task too detailed. Only 30 chars allowed." << endl;
        task.empty();
        result = false;
    }

if (date == "02/20/93") {
    date.empty();
    date = "My birthday!";
}

    return result;

}

As you can see I'm able to use string and ostream objects without declaring the use of a namespace or particular file. Obviously this means that Global.h is drawing the information from somewhere else, but I'm curious as to where this information is coming from? I always thought that a header file will only reconize code from other files if they've been included with the #include directive in the file itself, so I'm not sure how this is happening and am curious to know what's going on.

Answer 1

No. It's just that whatever is including this Global.h header file must be already #includeing all the required headers.

Somewhat of a simplication: an #include statement gets replaced by logically inserting the contents of the #included file in place of the #include statement itself. When you compile a translation unit, all #include statements are processed in this manner. As if all #include statements get logically replaced by the contents of the referenced file. The final result is a single logical source file, a translation unit, that gets compiled from start to finish.

So, if after all #include statement are processed in this manner, as long as the requisite header files, <iostream>, and others, get logically inserted in the logical translation unit prior to the classes and other resources from <iostream> that are referenced from this header file, this translation unit will compile without issues. Whether it's the same header file that #includes the required header files, <iostream> and others; or some other header file that gets #included earlier, which #includes these header files; it doesn't matter.

It is true that good practice indicates that each file should explicitly #include its prerequisites. But if it doesn't, as long as some other header file has already been #included, the translation unit will still compile.