Show Lecture.UsingNamespaceStd as a slide show.

CS253 Using Namespace Std

Namespaces

• #include <iostream> allows you to use symbols such as cin and cout.

#include <iostream>

using namespace std;

int main() {
    cout << 1/8.1;
    return 0;
}

0.123457

Namespaces

• However, the full names of these symbols are really std::cin and std::cout.
• That’s why we need using namespace std;

#include <iostream>

int main() {
    cout << 1/8.1;  // 🦡
    return 0;
}

c.cc:4: error: ‘cout’ was not declared in this scope; did you mean 
   ‘std::cout’?

• In these slides, if #include is not present, then sufficient #includes and using namespace std; are added.

Namespaces

• You could, if you wished, just use the full name.
• It’s not pretty.

#include <iostream>

int main() {
    std::cout << 1/8.1;
    return 0;
}

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How it works

#include <iostream>

constexpr auto pi = 3.14159;
void cout() {
    std::cout << "π=" << ::pi << std::endl;
}

int main() {
    ::cout();
}

π=3.14159

• No collisions! Library symbols don’t interfere with your symbols.
  • Library symbols (cout, vector, endl, exit()) go into the std:: namespace.
  • User symbols (main(), other functions, global vars) go into the global namespace.
• Local variables (in a function) aren’t in any namespace.
• Class methods & data don’t go into any namespace—they’re sort of in the namespace of the class, though we don’t say it that way.