What is the difference between std::string and eosio::string?

Question

Why should I use eosio::string over std::string? What are the use cases where it would make a difference?

Answer 1

According to the release notes:

A new type to act as a more efficient replacement to std::string has been added (#459). eosio::string should be more efficient in memory usage and help to reduce the bloat to smart contract WASMs that std::string imposes.

Where PR 459 references the following string bloat problem:

Given a minimal smart contract (con.cpp) with a no-op action:

#include <eosio/eosio.hpp>

class [[eosio::contract]] con : public eosio::contract {
public:
    using contract::contract;
    [[eosio::action]] void act()
    { }
};

EOSIO_DISPATCH(con, (act))

The size of the said contract comes out out to be:

~/con $ ll
total 1
-rwxrwxr-x   1 i              i        743 2019-03-06 13:37 a.out
-rw-rw-r--   1 i              i        202 2019-03-06 13:37 con.cpp

Given a minimal smart contract whilst introducing a std::string:

#include <eosio/eosio.hpp>

class [[eosio::contract]] con : public eosio::contract {
public:
    using contract::contract;
    [[eosio::action]] void act(const std::string& s)
    { }
};

EOSIO_DISPATCH(con, (act))

The size of the contract is now:

~/con $ ll
total 4
-rwxrwxr-x   1 i              i        3531 2019-03-06 13:46 a.out
-rw-rw-r--   1 i              i         222 2019-03-06 13:46 con.cpp

As is shown, this comes out to an extra 2788 bytes of code-bloat.

This poses to be a problem if a developer were to have many trivial/small smart contracts which utilize a std::string. But if the smart contract is already larger in nature, then this effect is minimal.

Therefore a new string type shall be developed for size optimization. This new string class will implement the small string optimization and provide the normal interface that is provided by std::string but at a fraction of the memory footprint.