Transfer and approve flow in Blockchain / Solidity

Introduction

This flow helps you safely send tokens from one person to another and allow others to spend tokens on your behalf.

When you want to send tokens directly to a friend.

When you want to let a smart contract spend tokens for you.

When you want to set a spending limit for another user.

When you want to manage token transfers without sharing your private keys.

Syntax

Blockchain / Solidity

function transfer(address to, uint256 amount) returns (bool);
function approve(address spender, uint256 amount) returns (bool);
function transferFrom(address from, address to, uint256 amount) returns (bool);

transfer sends tokens directly to another address.

approve lets another address spend tokens from your balance.

Examples

Sends 100 tokens directly to address 0x1234...

Blockchain / Solidity

token.transfer(0x1234..., 100);

Allows address 0x5678... to spend up to 50 tokens from your balance.

Blockchain / Solidity

token.approve(0x5678..., 50);

Spends 30 tokens from 0x9abc... to 0xdef0... if approved.

Blockchain / Solidity

token.transferFrom(0x9abc..., 0xdef0..., 30);

Sample Program

This smart contract lets users transfer tokens, approve others to spend tokens, and transfer tokens on behalf of others if approved.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleToken {
    mapping(address => uint256) public balances;
    mapping(address => mapping(address => uint256)) public allowances;

    constructor() {
        balances[msg.sender] = 1000;
    }

    function transfer(address to, uint256 amount) public returns (bool) {
        require(balances[msg.sender] >= amount, "Not enough tokens");
        balances[msg.sender] -= amount;
        balances[to] += amount;
        return true;
    }

    function approve(address spender, uint256 amount) public returns (bool) {
        allowances[msg.sender][spender] = amount;
        return true;
    }

    function transferFrom(address from, address to, uint256 amount) public returns (bool) {
        require(balances[from] >= amount, "Not enough tokens");
        require(allowances[from][msg.sender] >= amount, "Not approved for this amount");

        balances[from] -= amount;
        balances[to] += amount;
        allowances[from][msg.sender] -= amount;
        return true;
    }
}

Important Notes

Always check balances before transferring tokens.

Approvals set spending limits, so be careful who you approve.

transferFrom requires prior approval to work.

Summary

transfer sends tokens directly to another address.

approve lets someone else spend tokens from your balance.

transferFrom moves tokens from one address to another using approval.

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