[Ethereum] the right way to create a struct instance inside a function

arraysgo-ethereummappingsoliditystruct

I read somewhere that if you create a storage variable inside a function, it could overwrite another existing one in the global scope of the smart contract, is that true?

So, if I wanted to create a new struct variable inside a function, and then push it to an array, should I create it in the storage space or memory space?

Like this:

MyStruct storage newStruct;
... // modifiying newStruct variables
myStructsArray.push(newStruct);

Or like this:

MyStruct memory newStruct;
... // modifiying newStruct variables
myStructsArray.push(newStruct);

Same question for when wanting to add a newly instantiated struct to a mapping:

MyStruct storage/memory newStruct;
...
MyStructMapping[index] = newStruct;

Best Answer

If you try to assign a memory struct into a local storage variable the compiler will give you an error:

// This doesn't work, because it would need to create a new temporary 
// unnamed struct in storage, but storage is "statically" allocated.
// MyStruct storage newStruct = MyStruct();

Instead you need to use a memory variable:

// This works.
MyStruct memory newStruct = MyStruct();

If you have a state variable (or an array/mapping containing this struct), you can assign it to a local storage variable):

// Reference type local variables are storage by default, 
// you can omit the storage keyword.
MyStruct newStruct = stateStruct;
// Any assignments to fields in newStruct will be written to storage.

I read somewhere that if you create a storage variable inside a function, it could overwrite another existing one in the global scope of the smart contract, is that true?

I think the compiler prevents such cases as it doesn't allow creating temporary unnamed storage variables.

Related Solutions

Solidity Libraries – Can Library Functions Work for Non-Storage Structs?

Libraries can operate on structs in storage (that's one of their main purposes) but are currently unable to operate on memory structs (lifting this limitation is planned, though).

For your general problem concerning fractional numbers, I would propose different solutions, though:

either combine the two uints into a single uint (256 bits is a lot) or use uint[2] instead of the struct
wait for Solidity to receive fixed point integers (they should come "soon")

Solidity – Initializing Structs to Storage Variables

The former doesn't work because a SomeStruct storage is a pointer to the contract's storage space, and the way Solidity handles storage requires deriving that address from the organization of the structs you're writing it to - that is, its address in storage is directly dependent on the variable you're going to store it in. Without knowing what variable it's going in, it can't write to storage.

Your second snippet works because it first copies the data into a known location in storage, then copies it a second time to a new location, but as you'd expect, that's very inefficient. A better approach is to make your temporary struct in memory:

contract StructExample {

    struct SomeStruct {
        int someNumber;
        string someString;
    }

    SomeStruct[] someStructs;

    function addSomeStruct() {
        SomeStruct memory someStruct = SomeStruct(123, "test");
        someStructs.push(someStruct);
    }
}

Solidity will automatically generate code that copies the struct data from memory into storage.

Best Answer

Related Solutions

Solidity Libraries – Can Library Functions Work for Non-Storage Structs?

Solidity – Initializing Structs to Storage Variables

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