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To read a mapping owned by another deployed Solidity contract, call its public getter through a typed contract reference and pass the mapping key. You do not index the other contract’s storage directly. If a factory tracks multiple child contracts, first select the child, then pass that child’s mapping key.
Call the getter through a contract reference
Solidity generates a getter function for a state variable declared public. A different contract can call that function using the deployed contract’s address and a contract type or interface that declares the getter. For example, if the target declares mapping(address => uint256) public balances, a caller with a typed reference named a reads an entry with a.balances(account).
The expression is a function call on the target contract, not direct access to its mapping storage. Solidity documents the generated getter behavior in its Contracts documentation.
Declare only the interface you need
If the caller does not import the full implementation, it can use an interface containing the getter signature:
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interface IBalanceBook {
function balances(address account) external view returns (uint256);
}
contract Reader {
function readBalance(address book, address account)
external
view
returns (uint256)
{
return IBalanceBook(book).balances(account);
}
}Here, book must be the address of a deployed contract whose callable getter matches the interface. The interface describes how to call it; it does not create access to the target’s storage or guarantee that an arbitrary address implements the expected behavior.
Choose the correct contract and mapping key
When a factory creates and tracks multiple child contracts, a read can involve two independent lookups: select the child instance from the factory, then select an entry in that child’s mapping. A practical example uses this getter:
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function SfGet(uint256 _SimpleStorageDataID, uint256 _ID)
public view returns (string memory, address)
{
return ListOfSimpleStorageContracts[_SimpleStorageDataID]
.DataIdToData(_ID);
}_SimpleStorageDataID selects a deployed SimpleStorage instance in the factory’s list. _ID is passed to that instance’s public DataIdToData mapping getter and selects an entry within that child. Although both parameters are uint256, they mean different things. Using a valid entry ID with the wrong child can return a plausible but unintended result. The example and its factory context appear in Reading a Mapping That Lives on a Different Contract.
Return every field later logic needs
The example returns both a string and an owner address. If downstream logic uses ownership to decide whether an action is allowed, the getter must preserve and return the address as well as the data. A mapping read should not silently drop a field that later authorization depends on.
Reading mappings whose values are structs
A generated getter for a public mapping whose values are structs returns the accessible members as function outputs. The caller should declare and decode the getter’s actual output types and ordering; it does not receive a storage reference to a struct held by another contract. See the discussion of calling a mapping of structs from another contract.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What a mapping getter does—and does not—allow
It reads; it does not authorize writes
A public mapping exposes a getter, not a general-purpose way to change the mapping. To modify the target contract’s state, that contract must implement a state-changing function, and the function must enforce the authorization rules the application requires. A caller cannot gain write access merely by declaring an interface or calling a getter. The distinction is also discussed in this OpenZeppelin Forum example.
It cannot enumerate keys by itself
Mappings have no built-in length and do not expose their keys for iteration. If an application needs to list entries, the contract must maintain a separate key list or another indexing scheme. The Solidity Types documentation describes mapping behavior and its constraints.
A default value does not prove an entry was written
Mappings behave as though every possible key has a value initialized to the default for the mapped value type. A getter returning a default value therefore does not establish that the application explicitly stored that value for the key. If the distinction matters, track existence separately—for example, with an additional mapping or an indexed record.
Quick Recap
Choose the right read design
| Need | Design |
|---|---|
| The mapping key is known and the mapping is public | Call the generated getter through a reference or interface matching its signature. |
| The mapping is not public or the caller needs a tailored result | Implement an explicit view function on the target contract and expose the required output. |
| The caller needs to discover or list keys | Maintain a separate key list or other index; a mapping alone cannot enumerate entries. |
| The caller must change target state | Implement a state-changing function on the target and apply the intended access control there. |
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