hjk41的日志

Name conflicts have long been a pain for C programers. When you try to include two header files which happen to have the name conflicts, it can really make your day an unpleasant one. Fortunately, C++ provides namespace, which is a cure to name conflicts.
Without namespace, you may write code like this:

const double sdmBOOK_VERSION = 2.0;      // in this library,
                                         // each symbol begins
class sdmHandle { ... };                 // with "sdm"

sdmHandle& sdmGetHandle();             // see Item 47 for why you
                                       // might want to declare
                                       // a function like this

while you may have it like this if you use namespace:

namespace sdm {
  const double BOOK_VERSION = 2.0;
  class Handle { ... };
  Handle& getHandle();
}

Here is how we use namespace:

void f1()
{
  using namespace sdm;           // make all symbols in sdm
                                 // available w/o qualification
                                 // in this scope
  cout << BOOK_VERSION;          // okay, resolves to
                                 // sdm::BOOK_VERSION
  ...
  Handle h = getHandle();        // okay, Handle resolves to
                                 // sdm::Handle, getHandle
  ...                            // resolves to sdm::getHandle
}
void f2()
{
  using sdm::BOOK_VERSION;        // make only BOOK_VERSION
                                  // available w/o qualification
                                  // in this scope
  cout << BOOK_VERSION;           // okay, resolves to
                                  // sdm::BOOK_VERSION
  ...
  Handle h = getHandle();         // error! neither Handle
                                  // nor getHandle were
  ...                             // imported into this scope
}
void f3()
{
  cout << sdm::BOOK_VERSION;      // okay, makes BOOK_VERSION
                                  // available for this one use
  ...                             // only
  double d = BOOK_VERSION;        // error! BOOK_VERSION is
                                  // not in scope
  Handle h = getHandle();         // error! neither Handle
                                  // nor getHandle were
  ...                             // imported into this scope
}