CODEWITHSUNDEEP
rustrust-0.8
Brian Oh
Brian Oh

Reputation: 10720

Why using Rust does passing a mutable struct to a function result in immutable fields?

I'm learning Rust using 0.8 on Win8-64. I have a test program I'm working on where a function that handles input of parameters returned a struct containing those parameters. That worked OK. I then altered the program to pass the &struct to the function, and I now get a compiler error that I'm attempting to assign to an immutable field.

How should I be passing a pointer/reference to the struct to prevent this error?

The code that results in the error (I've tried a few variations) :

let mut ocParams : cParams = cParams::new();     //!!!!!! This is the struct passed

fInputParams(&ocParams);               // !!!!!!! this is passing the struct

if !ocParams.tContinue {
    return;
}

.......

struct cParams {
  iInsertMax : i64,
  iUpdateMax : i64,
  iDeleteMax : i64,
  iInstanceMax : i64,
  tFirstInstance : bool,
  tCreateTables : bool,
  tContinue : bool
}

impl cParams {
  fn new() -> cParams {
     cParams {iInsertMax : -1, iUpdateMax : -1, iDeleteMax : -1, iInstanceMax : -1,
              tFirstInstance : false, tCreateTables : false, tContinue : false}
  }   
}

.....

fn fInputParams(mut ocParams : &cParams) {

    ocParams.tContinue = (sInput == ~"y");    // !!!!!! this is one of the error lines

All of the assignments to the struct fields in the function result in an error at compile-time. An example of the errors that result from compile :

testli007.rs:240:2: 240:20 error: cannot assign to immutable field
testli007.rs:240   ocParams.tContinue = (sInput == ~"y");

Upvotes: 8

Views: 7050

Answers (1)

Cyrille Ka
Cyrille Ka

Reputation: 15538

In the declaration of your function:

fn fInputParams(mut ocParams : &cParams) {

ocParams is a mutable variable containing a borrowed pointer to an immutable struct. What you want is for that struct to be mutable, not the variable. Therefore, the signature of the function should be:

fn fInputParams(ocParams : &mut cParams) {

Then you have to change the call itself to fInputParams:

fInputParams(&mut ocParams);  // pass a pointer to mutable struct.

Upvotes: 17

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