Do all Smart Contracts use the eosio namespace?

Question

I'm new to creating Smart Contracts on EOS and I've followed a few tutorials (for example the official Hello World guide).

Would someone be able explain why all of the examples seem to use the namespace eosio?

Is it possible to use a different namespace?

#include <eosiolib/eosio.hpp>
#include <eosiolib/print.hpp>
using namespace eosio;

class hello : public eosio::contract {
  public:
      using contract::contract;

      /// @abi action 
      void hi( account_name user ) {
         print( "Hello, ", name{user} );
      }
};

EOSIO_ABI( hello, (hi) )

If it is possible to use a different namespace, what are some example situations in which you would use one?

If it isn't possible to use a different namespace, is there any reason why this needs to be in the code and couldn't be left for the compiler to add?

Answer 1

eosiolib uses the eosio namespace to avoid collisions when imported in a different scope--this is standard programming practice and is not specific to EOSIO. The using namespace <name> directive only brings that namespace in the current scope in order to avoid overusing the scope-resolution operator ::. How you actually use it in the end is just a styling distinction. You can write the same contract in many ways:

No using

#include <eosiolib/eosio.hpp>

class hello : public eosio::contract {
  public:
      using eosio::contract::contract;

      /// @abi action 
      void hi( account_name user ) {
         eosio::print( "Hello, ", eosio::name{user} );
      }
};

EOSIO_ABI( hello, (hi) )

With using namespace

#include <eosiolib/eosio.hpp>
using namespace eosio;

class hello : public contract {
  public:
      using contract::contract;

      /// @abi action 
      void hi( account_name user ) {
         print( "Hello, ", name{user} );
      }
};

EOSIO_ABI( hello, (hi) )

With using eosio::<name>

#include <eosiolib/eosio.hpp>

using eosio::contract;
using eosio::print;
using eosio::name;

class hello : public contract {
  public:
      using contract::contract;

      /// @abi action 
      void hi( account_name user ) {
         print( "Hello, ", name{user} );
      }
};

EOSIO_ABI( hello, (hi) )

I personally find the last one most readable as it is clear where every name is coming from, but it requires more typing than the second example.

Answer 2

I don't think you have to use the eosio namespace, but given that your contract will inherit from eosio::contract, it would be standard practice to also use the eosio namespace.