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Why should I include the <iostream> header file after using the std namespace?

Since the std namespace already has C ++ libraries containing function definitions (if I'm right), why do we include header files on top of this? Since the std namespace includes the standard C ++ libraries, I see no reason to include its declarations separately.

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When you execute #include <iostream>, this leads to the inclusion of a set of classes and other things in the source file. For iostream and most standard library headers, they put these objects in a namespace with a name std.

, #include <iostream> :

namespace std { 
    class cin  { ... };
    class cout { ... };
    class cerr { ... };
    class clog { ... };
    ...
}

, , :

#include <iostream>

int main() {
    std::cout << "hello\n";
    return 0;
}

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#include <iostream>
using namespace std;

int main() {
    cout << "hello\n";
    return 0;
}

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#include <iostream>
using std::cout;

int main() {
    cout << "hello\n";
    return 0;
}

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Using the #include preprocessor directive is aging like C ++. And he doesn’t leave earlier. The C ++ namespace does not import anything into your program; it just defines the scope of your specific header file function. Therefore, both are required. Click here to understand why use the namespace.

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