Can we use default parameters in EOS smart contract?

Question

I have a action in my contract :

// @abi action
void ContractClass::submittxn(string memo,string memo2,uint64_t a = 61)
{
    ...
}

I try to set a as default parameter but when I push the transaction, it shows the following error:

Error 3015014: Pack data exception
Error Details:
Missing 'a' in variant object

I push the transaction as :

cleos push action contract_name submittxn'{"memo":"qwerty","memo2":"asdfgh"}' -p account_name

Is this possible in EOS or I have to find some other way? It works fine if I send this with full params.

Answer 1

Not recommended, but you can mimic default value feature with eosio::binary_extension.

source:

#include <eosio/eosio.hpp>
#include <eosio/binary_extension.hpp>

using namespace eosio;
using namespace std;

class [[eosio::contract]] test : public contract {
public:
   using contract::contract;

   [[eosio::action]]
   void submittxn(string memo, string memo2, binary_extension<uint64_t> a) {
       // if a is passed, _a has same value of a
       // if a is NOT passed, _a becomes 61 (default value)
      uint64_t _a = a.value_or(uint64_t(61));

      // use _a instead of a
      print(_a);
   }
};

result:

$ cleos push action test submittxn '["eosio", "token"]' -p test@active
>> 61

$ cleos push action test submittxn '["eosio", "token", 10]' -p test@active
>> 10

Answer 2

You cannot use default parameters in actions. You could, however, check the value of the parameters and decide how to handle null or empty strings.

ACTION democontract::defaulttest( uint16_t i ) {
  if (i == 0) {
    i = 61;
  }
  printf("i is %u", i);
}

Another option would be to declare a vector of types to send into the action and only use what is required.

However you decide to proceed, you have to send in the same parameters that are declared in the ABI. A future version of EOSIO may add Variant actions, however they are not currently supported.

Answer 3

I guess, You use default and then save "a" with default to table database if it doesn't set.

If yes, You can use this way when declare table data

     struct account {
        string       memo;
        string       memo2;
        uint64_t     a = 61;
     };