Solidity – Why Was Memory Keyword Used in This Example

solidity

I am doing the cryptozombies.io course on Solidity.

In Chapter 7 of Making the Zombie Factory, it introduces function declarations. The instructions on this chapter are:

Create a public function named createZombie. It should take two parameters: _name (a string), and _dna (a uint). Don't forget to pass the first argument by value by using the memory keyword

My question is why is the _name parameter required to be declared with keyword memory (and similarly, why is the _dna parameter NOT required to be declared with keyword memory)?

In general, when does one declare a parameter using keyword memory and without it? I did Google about the memory keyword but I did not understand when it should be used and when it shouldn't.

Best Answer

Unsigned Integer or integers do not require memory specification when declared as arguments. Strings, arrays and structs require memory specification.

Related Solutions

Solidity Memory Keyword – What Does ‘Memory’ Do Exactly?

The Solidity FAQ on "memory" is highly recommended reading in entirety, and a snippet is provided below.

The Ethereum Virtual Machine has three areas where it can store items.

The first is “storage”, where all the contract state variables reside. Every contract has its own storage and it is persistent between function calls and quite expensive to use.

The second is “memory”, this is used to hold temporary values. It is erased between (external) function calls and is cheaper to use.

The third one is the stack, which is used to hold small local variables. It is almost free to use, but can only hold a limited amount of values.

For almost all types, you cannot specify where they should be stored, because they are copied everytime they are used.

The types where the so-called storage location is important are structs and arrays. If you e.g. pass such variables in function calls, their data is not copied if it can stay in memory or stay in storage. This means that you can modify their content in the called function and these modifications will still be visible in the caller.

There are defaults for the storage location depending on which type of variable it concerns (source):
state variables are always in storage
function arguments are always in memory
local variables of struct, array or mapping type reference storage by default
local variables of value type (i.e. neither array, nor struct nor mapping) are stored in the stack

The FAQ has code samples to help explain further.

A common mistake is to declare a local variable (of struct, array or mapping) and assume that it will be created in memory, although it will be created in storage.

The subtleties explanation of memory explains gas costs for using memory, which is a byte-array that "starts off zero-size, but can be expanded in 32-byte chunks by simply accessing or storing memory at indices greater than its current size".

[Ethereum] memory keyword in function argument

If you don't specify storage type of a function argument then function arguments are always in memory and you are passing an argument to a function by value.

When you are explicitly defining storage type of an argument asstorage you are passing an argument to a function by reference.

Best Answer

Related Solutions

Solidity Memory Keyword – What Does ‘Memory’ Do Exactly?

[Ethereum] memory keyword in function argument

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