ERC-721 implementation in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - ERC-721 implementation

Contract Start

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Define ERC-721 Interface

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Implement Metadata Functions

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Manage Token Ownership Mapping

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Implement Transfer Functions

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Mint New Tokens

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Burn Tokens

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Contract End

This flow shows the main steps in creating an ERC-721 contract: starting the contract, defining interfaces, managing ownership, enabling transfers, minting, and burning tokens.

Execution Sample

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleERC721 {
    mapping(uint256 => address) private _owners;
    function ownerOf(uint256 tokenId) public view returns (address) {
        return _owners[tokenId];
    }
}

This code defines a simple ERC-721 contract with a mapping to track token owners and a function to get the owner of a token.

Execution Table

Step	Action	Variable/Function	Input	State Change	Output
1	Contract deployed	_owners	empty	mapping initialized empty	none
2	Mint token	mint	tokenId=1, to=0xABC	_owners[1] = 0xABC	none
3	Call ownerOf	ownerOf	tokenId=1	no change	0xABC
4	Mint token	mint	tokenId=2, to=0xDEF	_owners[2] = 0xDEF	none
5	Call ownerOf	ownerOf	tokenId=2	no change	0xDEF
6	Transfer token	transferFrom	from=0xABC, to=0x123, tokenId=1	_owners[1] = 0x123	none
7	Call ownerOf	ownerOf	tokenId=1	no change	0x123
8	Burn token	burn	tokenId=2	delete _owners[2]	none
9	Call ownerOf	ownerOf	tokenId=2	no change	address(0) (no owner)
10	End	N/A	N/A	N/A	Execution complete

💡 Execution stops after all steps complete; tokens minted, transferred, and burned as shown.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	After Step 8	Final
_owners[1]	empty	0xABC	0xABC	0x123	0x123	0x123
_owners[2]	empty	empty	0xDEF	0xDEF	empty	empty

Key Moments - 3 Insights

Why does ownerOf return address(0) after burning a token?

Because burning deletes the token's owner from the _owners mapping, so ownerOf returns the default zero address as shown in step 9.

How does transferFrom update ownership?

In step 6, transferFrom changes _owners[tokenId] from the old owner to the new owner, updating the mapping accordingly.

What happens if you call ownerOf for a token that was never minted?

It returns address(0) because the token ID is not in the _owners mapping, similar to a burned token as shown in step 9.

Visual Quiz - 3 Questions

Look at the execution table, what is the owner of tokenId 1 after step 6?

A 0x123

B 0xABC

C 0xDEF

D address(0)

💡 Hint

Check the 'State Change' column at step 6 where _owners[1] is updated to 0x123.

At which step does the owner of tokenId 2 become address(0)?

A Step 4

B Step 8

C Step 9

D Step 6

💡 Hint

Look at step 9 where ownerOf returns address(0) after the burn in step 8.

If we mint tokenId 3 to 0x999 at step 11, what will _owners[3] be after that?

A empty

B address(0)

C 0x999

D 0x123

💡 Hint

Minting sets _owners[tokenId] to the new owner address, as shown in steps 2 and 4.

Concept Snapshot

ERC-721 is a standard for unique tokens.
Key parts: owner mapping, transfer functions, mint and burn.
ownerOf(tokenId) returns owner address or zero if none.
transferFrom changes ownership.
mint assigns new tokens.
burn deletes tokens.

Full Transcript

This visual execution traces a simple ERC-721 contract in Solidity. It starts with an empty ownership mapping. Minting tokens assigns owners to token IDs. ownerOf returns the current owner or zero address if none. Transferring tokens updates the owner mapping. Burning tokens removes ownership, making ownerOf return zero address. The execution table shows each step's action and state changes. Variable tracking highlights how ownership changes over time. Key moments clarify common confusions about ownership after burning and transfers. The quiz tests understanding of ownership at different steps. This helps beginners see how ERC-721 token ownership is managed step-by-step.