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Is there any real world example of nested mappings of solidity? The previous day I was working with mapping functions and I found literally many real world applications for simple mapping but not for nested mappings. Please help enter image description here

Is there any real world example of nested mappings of solidity? The previous day I was working with mapping functions and I found literally many real world applications for simple mapping but not for nested mappings. Please help enter image description here

• javascript
• blockchain
• solidity

Share Improve this question Follow asked Mar 18, 2022 at 5:01 Coder and DeveloperCoder and Developer 3111 silver badge22 bronze badges

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A very mon case is the approval mechanism on ERC-20 and other token standards.

mapping (address => mapping (address => uint256)) approvals;

This structure is used to keep track of token approvals. Example: "Alice (1st address) approves Bob (2nd address) to spend 100 (uint) of her tokens".

A more mon case than "approvals between people" is usually an approval from a person to a DApp. For example: "Alice approves Uniswap to pull 100 USDT from her wallet." And Uniswap is programed to take her USDT only at the moment when she's buying some other tokens against USDT.

It's also used for the same reason in the OpenZeppelin ERC20 implementation.

Yes, today I had an example of that: In a NFT contract I wanted to create a mapping of metadata linked to addresses. So I tried it:

//create a mapping
mapping(string => address) public metadataUri;

//set a mapping
metadataUri[_uri] = _address;

But in that case, if I have one more address with the same string, the first one will override all next addresses with the same string. So in that case I should have something like this:

//create a mapping
mapping(string => mapping(address => uint)) public metadataUri;

//set a mapping
metadataUri[_stringURI][_address] = _uint;

Yes The approval mechanisms in the token contracts use nested mappings. The ERC1155 contracts use nested mappings to record the balances of an address. Line number 24 at https://github./OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC1155/ERC1155.sol

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