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

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Closed 9 years ago.

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.

Answer 2

You should never use using or typedef in a header file just for the sake of making names easier to type.

In the source file, it's up to you. It seems to be considered good practice to write out the whole name, as it makes it very clear what you meant. If your namespace is too long, you can use a namespace alias to reduce clutter, but still keep the meaning clear: namespace ns = project1::namespace1;

Either way, if you're going to import symbols into the global namespace, use using, not typedef. typedef is used mainly when you want to call the type by a different name, many times because it is a template - for example, my_map instead of std::map<std::string, my_type>, which is still clear, but much nicer to type.

Also, see this question: Why is "using namespace std" considered bad practice?

Answer 3

The most clear way is not use any of those methods - just write new project1::namespace1::class1().

Answer 4

use using project1::namespace1::class1

Or, you can limit your using namespce in a local scope, both for your convenience and not polute the global namespace.

void function()
{
    using namespace project1::namespace1;

    class1 obj;

    ...
}