Solidity Struct Initialization – How to Initialize a Struct

contract-developmentsoliditystruct

I have a struct like so :

struct fooStruct {
  uint foo;
  uint figther;
}

I would like to initialize that struct but it won't be stored in a mapping but inside an array.

Is there a way to initialize the struct like

fooStruct myStruct = fooStruct.new(par1,2,3)

Best Answer

Yes, just use

fooStruct myStruct = fooStruct(1,2);

fooStruct myStruct = fooStruct({foo:1, fighter:2});

fooStruct memory myStruct; // for temporary data
myStruct.figther = 2; // will only write to memory

fooStruct storage myStruct = ...; // for persistent data, has to be initialized from a state variable. `storage` is the default and a warning will be thrown by Solidity compiler versions starting with 4.17
myStruct.fighter = 2; // will write directly to storage

See the docs for more examples

Related Solutions

[Ethereum] Solidity: Initialize struct containing string array

The other answer which didnt work brought me on track to find it out myself:

contract testStruct {
    struct stru{
        string[] s;
    }

    stru myStru;

    function add(string s) {
        myStru.s.push(s);
    }

    function getAt(uint256 i) constant returns (string s) {
        s = myStru.s[i];
    }
}

Solidity – Test if a Struct State Variable is Set

Question is possibly a duplicate of this: What is the zero, empty or null value of a struct?

There are a few ways.

You can check a value if you're sure it implicitly indicates a set/not set condition.

if(tx.to > 0) {} // the address is set

You can be explicit if you prefer:

struct Transaction {
    address to;
    uint value;
    bytes data;
    bool isValid; // set to true whenever the struct holds data
}

Then you can:

if(tx.isValid) {
  // do something
}

You'll want to use the explicit method in the case that 0 or empty has meaning your application; meaning you can't use 0 as an indication of valid/inValid.

Not sure it applies in this case, but elimination of doubt about this sort thing is a bonus feature if you buy into the idea of maintaining an index of valid keys to a mapped structure.

Hope it helps.

Update: Some general examples here: Blog: Simple Storage Patterns in Solidity

Best Answer

Related Solutions

[Ethereum] Solidity: Initialize struct containing string array

Solidity – Test if a Struct State Variable is Set

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