Default getter and setter for mapping in solidity

Question

In my smart contract, I have a public mapping as mapping (uint=>User) public users where User is a Struct. Is there any way to get the values of this mapping using a default getter?

For Example : If we have public uint count, we can simply call it by using Object.count. Is it possible with mapping as well?

Please help!

Answer 1

First of all, there is no such thing as default setter (mentioned in title of your question).

Now about default getter. Solidity documentation gives the following example:

struct Data {
    uint a;
    bytes3 b;
    mapping (uint => uint) map;
}
mapping (uint => mapping(bool => Data[])) public data;

will generate the following getter:

function data(uint arg1, bool arg2, uint arg3) public returns (uint a, bytes3 b) {
    a = data[arg1][arg2][arg3].a;
    b = data[arg1][arg2][arg3].b;
}

So, default getter returns not the structure, but rather tuple of all atomic structure fields.

In your case getter will look like:

function users (uint id) public returns (/* all atomic fields of User structure */)

Answer 2

You get a function that takes an index argument. It returns the scalar values in the struct, so everything that isn't itself a mapping or an array.

For instance:

pragma solidity 0.5.1;

contract StuctsGetter {

    struct MyStruct {
        uint a;
        bytes32 b;
        mapping(address => bool) active;
        bytes32[] keys;
    }

    mapping(uint => MyStruct) public myStructs;

    function setMyStruct(uint index, uint a, bytes32 b) public {
        MyStruct storage m = myStructs[index];
        m.a = a;
        m.b = b;
    }

}

You can call instance.myStructs(uint) to fetch the mapped struct at a certain index. It will return a and b but not active or keys.

Avoid a head-scratcher: If there isn't at least one scalar value (i.e. the struct is made entirely of mappings and/or arrays) then the compiler won't know how to construct the "free" getter. It will fail in a confusing way.

To set/get the indexed values in a mapped struct, write your own functions.

function appendKeyToMyStruct(uint _myStruct, address key) public ... {
  myStructs[_myStruct].keys.push(key);
}

function getMyStructKey(uint _myStruct, uint row) public ... {
  return myStructs[_myStruct].keys[row];
}

These patterns may give you some ideas: Are there well-solved and simple storage patterns for Solidity?

A little info about the storage pointer used in the example: https://blog.b9lab.com/storage-pointers-in-solidity-7dcfaa536089

Hope it helps.