Transfer and approve flow in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the approve function in the transfer and approve flow?

The approve function allows a token owner to authorize another address (called a spender) to spend a specific amount of tokens on their behalf.

beginner

Explain the role of the transferFrom function in the transfer and approve flow.

The transferFrom function allows the spender to transfer tokens from the owner's account to another account, but only up to the amount approved earlier.

intermediate

Why is the transfer and approve flow important in blockchain token standards like ERC-20?

It enables delegated token transfers, letting users allow others (like smart contracts or exchanges) to move tokens on their behalf securely and with limits.

beginner

What happens if a spender tries to transfer more tokens than approved?

The transaction will fail because the transferFrom function checks that the amount does not exceed the approved allowance.

beginner

Describe the sequence of steps in a typical transfer and approve flow.

1. Owner calls approve to set allowance for spender.
2. Spender calls transferFrom to move tokens within allowance.
3. Tokens are transferred from owner to recipient.

What does the approve function do in token contracts?

A Burns tokens from the owner's balance

B Transfers tokens directly to another address

C Mints new tokens to the owner

D Allows a spender to spend tokens on behalf of the owner

The approve function sets an allowance for a spender to use tokens from the owner's balance.

Which function is used by the spender to move tokens after approval?

A transferFrom

B approve

C transfer

D mint

transferFrom lets the approved spender transfer tokens from the owner's account.

What happens if transferFrom tries to transfer more than the approved amount?

A Transaction fails

B Transfer succeeds anyway

C Allowance is automatically increased

D Tokens are minted to cover the difference

The transferFrom function checks allowance and fails if the amount is too high.

Why is the approve and transferFrom flow useful?

A It speeds up token minting

B It allows delegated token transfers with limits

C It burns tokens automatically

D It disables token transfers

This flow lets owners delegate token spending safely to others.

In the approve and transferFrom flow, who sets the allowance?

A The spender

B The token contract

C The token owner

D The recipient

The token owner sets the allowance by calling approve.

Describe the transfer and approve flow in blockchain token contracts.

Think about how one user lets another spend tokens for them.

Explain why the approve and transferFrom functions are important for decentralized applications.

Consider how apps like exchanges move tokens on behalf of users.

Practice

1. What is the main purpose of the approve function in a token contract?

easy

A. To allow another address to spend tokens on your behalf

B. To send tokens directly to another address

C. To check the balance of an address

D. To mint new tokens to an address

Solution

1. Step 1: Understand the role of approve

   The approve function lets a token owner give permission to another address to spend tokens from their balance.

2. Step 2: Differentiate from other functions

   transfer sends tokens directly, approve only sets allowance, so the spender can use transferFrom later.

3. Final Answer:

   To allow another address to spend tokens on your behalf -> Option A

4. Quick Check:

   approve = permission grant [OK]

Hint: Approve sets spending permission, transfer sends tokens [OK]

Common Mistakes:

• Confusing approve with transfer
• Thinking approve sends tokens immediately
• Mixing approve with balance checking

2. Which of the following is the correct syntax to approve 100 tokens for spender address 0xABC... in Solidity?

easy

A. approve(100, 0xABC...);

B. approve(address spender, uint amount);

C. approve(0xABC..., 100);

D. approve(100);

Solution

1. Step 1: Recall approve function parameters

   The approve function takes two parameters: the spender's address and the amount to approve.

2. Step 2: Match parameters order

   The correct order is approve(address spender, uint amount), so calling approve(0xABC..., 100); is correct.

3. Final Answer:

   approve(0xABC..., 100); -> Option C

4. Quick Check:

   approve(spender, amount) syntax [OK]

Hint: approve(spender, amount) order matters [OK]

Common Mistakes:

• Swapping parameters order
• Using variable names instead of values
• Calling approve with only one argument

3. Given the following Solidity code snippet, what will be the balance of recipient after execution?

address owner = msg.sender;
address spender = 0x123...;
uint amount = 50;

// Owner approves spender
approve(spender, amount);

// Spender transfers tokens from owner to recipient
transferFrom(owner, recipient, 30);

medium

A. 80 tokens

B. 50 tokens

C. 0 tokens

D. 30 tokens

Solution

1. Step 1: Understand approve and transferFrom interaction

   The owner approves spender to spend 50 tokens. Then spender transfers 30 tokens to recipient.

2. Step 2: Calculate recipient balance after transferFrom

   Since spender transfers 30 tokens, recipient's balance increases by 30 tokens.

3. Final Answer:

   30 tokens -> Option D

4. Quick Check:

   transferFrom moves approved amount [OK]

Hint: transferFrom moves approved tokens only [OK]

Common Mistakes:

• Assuming full approved amount is transferred
• Confusing approve amount with transfer amount
• Ignoring transferFrom effect on recipient balance

4. Identify the error in this Solidity code snippet for transferring tokens using approve and transferFrom:

approve(spender, 100);
transferFrom(msg.sender, recipient, 50);

medium

A. approve should be called after transferFrom

B. transferFrom should be called by spender, not msg.sender

C. recipient address is missing

D. approve amount is less than transfer amount

Solution

1. Step 1: Check who calls transferFrom

   transferFrom must be called by the spender who was approved, not the token owner (msg.sender here).

2. Step 2: Confirm approve and transferFrom roles

   Owner calls approve, spender calls transferFrom to move tokens.

3. Final Answer:

   transferFrom should be called by spender, not msg.sender -> Option B

4. Quick Check:

   transferFrom caller = spender [OK]

Hint: transferFrom caller must be approved spender [OK]

Common Mistakes:

• Calling transferFrom as token owner
• Mixing approve and transferFrom order
• Ignoring caller permissions

5. You want to allow a decentralized exchange (DEX) contract to spend exactly 200 tokens from your wallet, but only once. Which sequence of calls correctly implements this using the transfer and approve flow?

hard

A. Call approve(DEX, 200) once, then DEX calls transferFrom for 200 tokens

B. Call transfer(DEX, 200) directly, no approve needed

C. Call approve(DEX, 200) multiple times before each trade

D. Call transferFrom(DEX, yourAddress, 200) yourself

Solution

1. Step 1: Understand one-time spending permission

   To let DEX spend tokens once, you approve exactly 200 tokens once.

2. Step 2: Let DEX transfer tokens

   DEX uses transferFrom to move 200 tokens from your wallet to itself or others.

3. Final Answer:

   Call approve(DEX, 200) once, then DEX calls transferFrom for 200 tokens -> Option A

4. Quick Check:

   approve once, transferFrom once [OK]

Hint: Approve once, then let spender transferFrom once [OK]

Common Mistakes:

• Transferring tokens directly without approval
• Approving multiple times unnecessarily
• Calling transferFrom as token owner