Solidity Variables Explained: State, Local, and Global

In Solidity, variables act as containers for storing data on the blockchain. But not all variables are created equal! Understanding their types and lifetimes is crucial for writing efficient and secure smart contracts.

Quick Comparison

Type	Lifetime	Gas Cost	Example
State	Contract lifetime	High (storage)	uint256 public count;
Local	Function execution	Low (memory)	uint256 temp = 5;
Global	Always available	None (pre-defined)	msg.sender

1. State Variables: The Blockchain's Database

contract MyContract {
    // State variable (stored on-chain)
    uint256 public persistentData;
    
    function updateData(uint256 _new) public {
        persistentData = _new; // Modifies blockchain state
    }
}

• 🌐 Stored in contract storage
• 💸 Modifying them costs gas
• 🔒 Accessible across all functions

2. Local Variables: Temporary Workers

function calculateTotal() public pure returns(uint256) {
    uint256 a = 5; // Local variable
    uint256 b = 10;
    return a + b; // Dies after execution
}

• ⚡ Exist only during function execution
• 🆓 No gas cost (memory-based)
• 🚫 Cannot be accessed externally

3. Global Variables: Blockchain Oracles

function getInfo() public view returns(
    address, uint256, uint256
) {
    return (
        msg.sender, // Caller's address
        block.timestamp, // Current time
        address(this).balance // Contract balance
    );
}

Common Global Variables:

• msg.value: ETH sent with call
• tx.origin: Original sender
• block.number: Current block

⚠️ Common Mistakes

• Confusing memory and storage for reference types
• Using global variables like tx.origin for authorization
• Overusing state variables ($$$ gas costs!)

Best Practices

1. Minimize state variables to reduce gas costs
2. Use memory for temporary arrays/structs
3. Validate global inputs (e.g., check msg.value > 0)