How to test if a struct state variable is set

Question

I have the following struct defined as a state variable and conditionally set at runtime.

Transaction private tx;
struct Transaction {
    address to;
    uint value;
    bytes data;
}

I would like to check if the variable was set before executing a specific portion of my code.

if(tx == ??????){
    //do something
}

What's the best way to check if the struct is set?

Edit: It is important to note that here the struct is not stored in a mapping. Similar questions on this stackexchange are asking for structs in mapping, here this is not the case.

Answer 1

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