other way to let contract account provide for cpu/bandwidth than delegatew/undelegatew?

Question

as far as i can tell it is somehow inconvenient to let the user of an contract "pay" for the cpu time. I thought that the one who manages the contract, has to stake for cpu/bandwidth, so user can use his contract. But now after i used some contracts on the blockchain, i was always the one who paid + i had to pay for the ram.

• delegate/undelegate EOS to user is an option (but you have to wait 3 Days till you can use them again).
• I read also somewhere to use inline transaction but these were also paid from the user (see example code):

test code for inline transactions:

#include "testeoscpuio.hpp"

using namespace eosio;
//@abi action
void testeoscpuio::doggos(){
    auto itr_dog = tb_dogs1.begin();


    if(itr_dog == tb_dogs1.end()){
        tb_dogs1.emplace(_self, [&](auto& d) {
                d.id = 2;
                d.number = 1;
        }); 
    }else{
        tb_dogs1.modify(itr_dog, _self, [&](auto& d) {
            d.number = 1;
        });
    }

    uint16_t b = 0;
    for(uint16_t i = 0; i < 20000; i++){
        b = b +i;
    }

}


//@abi action
void testeoscpuio::calc(){
    action(permission_level{ _self, N(active) },
            N(testeoscpuio), N(doggos), std::make_tuple()).send();

    for(uint16_t i = 0; i < 200000; i++){
        b = b +i;
    }
}

Answer 1

Another way would be to use deferred transactions as the contract can set the payer to _self in the second argument for transaction::send(), but the contract still needs CPU/NET for the initial action that triggers the deferred transaction.

contracts/eosiolib/transaction.hpp

 void send(const uint128_t& sender_id, account_name payer, bool replace_existing = false) const {
    auto serialize = pack(*this);
    send_deferred(sender_id, payer, serialize.data(), serialize.size(), replace_existing);
}

Otherwise, one may design second layer services that abstract the blockchain accounts and execute actions through an admin account while handling authentication off-chain.