C++: Should I use 'typedef' or 'using namespace'?

Question

I am writing a library with mutiple dependent modules. When I include a file from a different module, should I resolve the namespace with:

using namespace project1::namespace1;
class1 obj;

or

typedef project1::namespace1::class1 class1;
class1 obj;

What are the pros/cons of each approach? I read somewhere that we should use typedef in .H files and using in .C files, is this advisable?

One problem I have encountered with 'typedef' is it leads to namespace ambiguity if I include both original class and the class with 'typedef' in a third module.

Answer 1

The two options you state are not equivalent. This one:

using namespace project1::namespace1;

pulls in everything from the namespace, giving you little control and making clashes likely. I see only cons, and no pros here.

But you don't need to use a typedef to bring in a single symbol, you can use

using project1::namespace1::class1;

Whether you use this or the typedef doesn't make too much of a difference. But bear in mind that typedef is limited to types and enumerations, whereas using can refer to values, functions, etc:

namespace X {
  const int x{42};
  enum Fruit{Apple, Pear};
}

using X::x; // OK
typedef X::x xx; // Error! 'x' in namespace 'X' does not name a type

so the two expressions are not completely equivalent.