ERC-721 NFT standard in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - ERC-721 NFT standard

Start: Deploy ERC-721 Contract

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Mint NFT Token

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Assign Token ID to Owner

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Check Ownership & Metadata

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Transfer Token to Another Owner

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Update Ownership Records

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Query Token Info or Owner

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End

This flow shows how an ERC-721 contract is deployed, NFTs are minted and assigned unique IDs, ownership is tracked, tokens can be transferred, and metadata can be queried.

Execution Sample

Blockchain / Solidity

contract SimpleNFT is ERC721 {
    uint256 public nextTokenId;
    constructor() ERC721("SimpleNFT", "SNFT") {}
    function mint() external {
        _safeMint(msg.sender, nextTokenId);
        nextTokenId++;
    }
}

This contract lets users mint unique NFTs with increasing token IDs assigned to their address.

Execution Table

Step	Action	Token ID	Owner	nextTokenId	Output
1	Deploy contract	-	-	0	Contract ready
2	User calls mint()	0	UserAddress	0	Mint token 0 to UserAddress
3	Increment nextTokenId	-	-	1	nextTokenId updated to 1
4	User calls mint() again	1	UserAddress	1	Mint token 1 to UserAddress
5	Increment nextTokenId	-	-	2	nextTokenId updated to 2
6	Transfer token 0 to AnotherAddress	0	AnotherAddress	2	Ownership of token 0 updated
7	Query owner of token 1	1	UserAddress	2	Returns UserAddress
8	Query owner of token 0	0	AnotherAddress	2	Returns AnotherAddress
9	No more actions	-	-	2	End of execution

💡 No more mint or transfer calls, execution ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	Final
nextTokenId	0	1	1	2	2	2	2
Owner of token 0	None	UserAddress	UserAddress	UserAddress	UserAddress	AnotherAddress	AnotherAddress
Owner of token 1	None	None	None	UserAddress	UserAddress	UserAddress	UserAddress

Key Moments - 3 Insights

Why does nextTokenId increment after minting?

Because each NFT must have a unique token ID, nextTokenId increments after minting to prepare the next unique ID (see steps 2 and 3 in execution_table).

What happens to ownership when a token is transferred?

Ownership changes from the current owner to the new owner, updating the record (see step 6 in execution_table where token 0 ownership changes).

Can two tokens have the same ID?

No, ERC-721 requires unique token IDs. The contract ensures this by incrementing nextTokenId after each mint (see variable_tracker for nextTokenId).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, who owns token 1?

A AnotherAddress

B UserAddress

C No owner yet

D Contract itself

💡 Hint

Check the 'Owner' column at step 4 in the execution_table.

At which step does nextTokenId become 2?

A Step 5

B Step 6

C Step 3

D Step 2

💡 Hint

Look at the 'nextTokenId' column in execution_table and variable_tracker.

If the transfer in step 6 did not happen, who would own token 0 at the end?

A AnotherAddress

B No owner

C UserAddress

D Contract owner

💡 Hint

Refer to the ownership changes in variable_tracker and execution_table steps 6 and 8.

Concept Snapshot

ERC-721 is a standard for unique NFTs.
Each token has a unique ID.
Tokens are minted to owners with _safeMint.
Ownership can be transferred.
Metadata and ownership can be queried.
nextTokenId tracks new token IDs.

Full Transcript

This visual execution traces the ERC-721 NFT standard contract. It starts with deploying the contract, then minting tokens with unique IDs assigned to the caller's address. After each mint, the nextTokenId increments to ensure uniqueness. Ownership of tokens is tracked and can be transferred to others. Queries return the current owner of each token. The execution table shows each step's action, token ID, owner, and nextTokenId value. Variable tracking highlights how ownership and nextTokenId change over time. Key moments clarify why IDs increment and how ownership updates. The quiz tests understanding of ownership and token ID changes during execution.