namespaces_using.md

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Using namespaces

Namespaces

• Namespaces
• Using using

📺 Watch the related YouTube video!

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Prerequisites:

• Hello world dissection lecture
• Variables of fundamental types

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Special symbols used in slides

• 🎨 - Style recommendation
• 🎓 - Software design recommendation
• 😱 - Not a good practice! Avoid in real life!
• ✅ - Good practice!
• ❌ - Whatever is marked with this is wrong
• 🚨 - Alert! Important information!
• 💡 - Hint or a useful exercise
• 🔼1️⃣7️⃣ - Holds for this version of C++(here, 17) and above
• 🔽1️⃣1️⃣ - Holds for versions until this one C++(here, 11)

Style (🎨) and software design (🎓) recommendations mostly come from Google Style Sheet and the CppCoreGuidelines

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Namespaces

• Variables (and other stuff) can live in namespaces
• Namespaces are defined with the keyword namespace:

  namespace cpp_course {
  // Variables, functions etc.
  }  // namespace cpp_course

• Namespaces can live within other namespaces:

  namespace foo {
  namespace bar {
  // Variables, functions etc.
  }  // namespace bar
  }  // namespace foo

• 🎨 End with comment: // namespace <name> (clang_format will take care of this)
• 🎨 Name them like variables in snake_case (source)
• 🎨 Do not indent the code inside the namespace (source)

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Namespaces have special scopes

• Variables in namespaces don't die at the end of their namespace scope
• They still die with the global scope
• Namespaces allow to "partition" the global scope
• This avoids clashes between variable names

#include <cstdio>
namespace foo {
constexpr auto kConstant{42};
}  // namespace foo
namespace bar {
constexpr auto kConstant{23};
}  // namespace bar
int main() {
  std::printf("%d\n", foo::kConstant);
  std::printf("%d\n", bar::kConstant);
  return 0;
}

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Access variables in namespaces

• Access variables within a namespace normally
• Namespaces can repeat
• Access variables from outside of a namespace using the ::

  namespace cpp_course {
  constexpr auto kNumber{42};
  }  // namespace cpp_course
  
  namespace cpp_course {
  constexpr auto kAnotherNumber{kNumber};
  }  // namespace cpp_course
  
  int main() {
    constexpr auto number{cpp_course::kNumber};
    constexpr auto another_number{cpp_course::kAnotherNumber};
    return 0;
  }

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Use unnamed namespaces!

• ✅ Use "unnamed" namespaces in source files

  namespace {
  constexpr auto kConstant{42};
  }  // namespace
  
  int main() {
    constexpr auto number{kConstant};
    return 0;
  }

• They are sometimes also called "anonymous" namespaces
• This generates a namespace with a unique name available only in this "translation unit" (aka source file)
• Also has "linkage" implications (stay tuned for when we talk about static)
• Only use them in .cpp, .cc files, never in .h, .hpp etc. (stay tuned for when we talk about headers)

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Use using when needed

• using lifts a variable from its scope into the current scope
• Can be used from any scope

namespace cpp_course {
constexpr auto kNumber{42};
constexpr auto kAnotherNumber{kNumber};
}  // namespace cpp_course

int main() {
  using cpp_course::kAnotherNumber;
  // Now kAnotherNumber is available in this scope!
  constexpr auto number{cpp_course::kNumber};
  constexpr auto another_number{kAnotherNumber};
  return 0;
}

• ❌ Don't use from global scope (unless in a cpp file)
• ❌ Never use using namespace foo; 😱 It's too permissive and pollutes the current namespace!

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