Transfer and approve flow in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Transfer and approve flow

User A calls approve(spender, amount)

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Contract records allowance

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User B calls transferFrom(User A, User C, amount)

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Check allowance and balance

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Transfer tokens

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Update balances and allowance

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Done

User A allows User B to spend tokens by approve. Then User B uses transferFrom to move tokens from A to C if allowed.

Execution Sample

Blockchain / Solidity

approve(spender, 100)
transferFrom(owner, recipient, 50)
// Check allowance and balance
// Update balances and allowance

User A approves User B to spend 100 tokens, then User B transfers 50 tokens from A to recipient.

Execution Table

Step	Action	Allowance Before	Balance Owner Before	Condition Check	Allowance After	Balance Owner After	Balance Recipient After	Result
1	User A calls approve(User B, 100)	0	1000	N/A	100	1000	0	Allowance set to 100
2	User B calls transferFrom(User A, User C, 50)	100	1000	Allowance >= 50 and Balance >= 50: True	50	950	50	Transfer successful
3	User B calls transferFrom(User A, User C, 60)	50	950	Allowance >= 60 and Balance >= 60: False	50	950	50	Transfer rejected - insufficient allowance
4	User B calls transferFrom(User A, User C, 50)	50	950	Allowance >= 50 and Balance >= 50: True	0	900	100	Transfer successful
5	User B calls transferFrom(User A, User C, 1)	0	900	Allowance >= 1 and Balance >= 1: False	0	900	100	Transfer rejected - allowance zero

💡 Execution stops when allowance or balance is insufficient for transferFrom.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Allowance (User B)	0	100	50	50	0	0
Balance Owner (User A)	1000	1000	950	950	900	900
Balance Recipient (User C)	0	0	50	50	100	100

Key Moments - 3 Insights

Why does transferFrom fail at step 3 even though User A has enough tokens?

Because the allowance for User B is only 50 tokens after step 2, and step 3 tries to transfer 60 tokens which exceeds allowance (see execution_table row 3).

What happens to allowance after a successful transferFrom?

Allowance decreases by the transferred amount, as shown in execution_table rows 2 and 4 where allowance reduces from 100 to 50, then to 0.

Can transferFrom succeed if allowance is zero?

No, transferFrom will reject the transaction if allowance is zero, as shown in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the allowance for User B after step 2?

A 0

B 50

C 100

D 150

💡 Hint

Check the 'Allowance After' column in row 2 of the execution_table.

At which step does transferFrom fail due to insufficient allowance?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Result' and 'Condition Check' columns in execution_table row 3.

If User A approves 200 tokens instead of 100 at step 1, what would be the allowance after step 4?

A 0

B 100

C 150

D 50

💡 Hint

Allowance after step 4 is original allowance minus transferred amounts (see variable_tracker and execution_table).

Concept Snapshot

Transfer and approve flow:
- User approves a spender allowance with approve(spender, amount).
- Spender uses transferFrom(owner, recipient, amount) to move tokens.
- transferFrom checks allowance and owner's balance.
- On success, balances and allowance update.
- On failure, transfer is rejected.

Full Transcript

This visual execution shows how the transfer and approve flow works in blockchain token contracts. First, User A calls approve to allow User B to spend tokens on their behalf. The contract records this allowance. Then User B calls transferFrom to move tokens from User A to another user. The contract checks if User B has enough allowance and if User A has enough balance. If both are true, the transfer happens and balances and allowance update accordingly. If not, the transfer is rejected. The execution table traces each step with allowance and balances before and after, showing when transfers succeed or fail. The variable tracker shows how allowance and balances change over time. Key moments clarify common confusions about allowance usage and transfer failures. The quiz tests understanding of allowance changes and failure conditions. This flow is essential for safe token spending delegation.