Step 5 of 7

Namespaces and using

Large programs combine code from many libraries, and two libraries might both define a function called load or a type called Config. C++ avoids these clashes with namespaces: named scopes that group related names.

#include <iostream>
#include <string>

namespace audio {
    int volume = 7;
    std::string name() { return "audio"; }
}

namespace video {
    int volume = 3;
    std::string name() { return "video"; }
}

using std::cout;

int main() {
    cout << audio::name() << " " << audio::volume << "\n";
    cout << video::name() << " " << video::volume << "\n";
}

audio 7
video 3

• namespace audio { ... } puts everything inside into the audio namespace.
• :: is the scope resolution operator: audio::volume means "the volume inside audio". Both namespaces have a volume and there's no conflict.
• The standard library lives in std, which is why you write std::cout and std::string.

Shortening names

• using std::cout; (a using-declaration) brings one name into scope, so you can write cout.
• using namespace std; brings in every name from std. You'll see it in tutorials, and it's fine in tiny programs, but it's a bad habit in real code: std contains hundreds of names, and pulling them all in invites clashes (with your own count, max, distance, ...). Never put it in a header, where it would affect every file that includes it.

Most professional code simply writes std:: everywhere; it quickly becomes invisible.

Your turn: fill in the scope operators and the using-declaration.

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