[Ethereum] What does “_” mean before a variable name

eventssoliditystate-variable

I'm a beginner, just recently started reading the docs for Solidity.
I can't seem to find this anywhere on google. What does the underscore mean when placed with the indexed keyword?
Examples:
_from
_id
_amount
_account
How are these different from other variables, and how are they used in events?

Best Answer

There's no meaning as far as Solidity is concerned.

Some programmers choose to use a leading underscore for all function parameters, just as a convention to indicate that they're function parameters.

They're also often used so as to avoid collisions, e.g.:

uint256 totalSupply;

constructor(uint256 _totalSupply) public {
    totalSupply = _totalSupply;
}

There, calling the parameter totalSupply would shadow the existing state variable totalSupply, so the leading underscore is used to avoid the naming collision.

Related Solutions

[Ethereum] Local storage variable

Value assignment to a variable can be done by either giving a reference or creating an independent copy. If it's a reference when the original one which is assigned to the variable get changed value of the variable also gets changed. Consider the following,

a = 10 // value of a is 10
b = a // variable b is assigned a value
a = 20

if the assignment above assigned a reference now the value of b will also be 20. If the assignment was done by creating an independent copy value of b will be still 10.

In the solidity doc what it says is if the assignment is between the storage and memory,or the assignment is to a state variable it will always create an independent copy. If it's a local storage variable it will only make a reference.

Referring to the example provided in the solidity docs, local storage variable is a variable defined inside a function. (The scope of the variable is limited to the function)

[Ethereum] This contract does not implement all functions and thus cannot be created

The problem is that totalSupply() implementation is missing. Seems like autogenerated getters do not count. So you need to explicitly create the function.

contract StandardToken is Token {
    uint256 _totalSupply;

    function totalSupply() constant returns (uint256 totalSupply) {
        totalSupply = _totalSupply;
    }
}

Best Answer

Related Solutions

[Ethereum] Local storage variable

[Ethereum] This contract does not implement all functions and thus cannot be created

Related Topic