Inout Parameters in

About Inout Parameters

Within the body of a function, parameters are treated as constants, not variables; trying to modify the value of a parameter will raise a compile-time error. If a function wishes to modify the value of a parameter, it must use an in-out parameter to make this mutability explicit.

To use in-out parameters, a programmer must be sure of three things:

1. The function definition must explicitly mark in-out parameters by writing the keyword inout between the : and the type name in the parameter's type annotation.
2. The value passed in to the in-out parameter must be declared as a variable, not a constant. Similarly it cannot be a literal.
3. The variable name must be preceded by an & in the call to the function, explicitly marking it as modifiable.

When this is done, the function will make an internal copy of the in-out parameter, modify it as directed in the body of the function and then copy the modified value back into the variable that was passed in when the function returns.

func updateVersion(_ record: inout (version: Int, title: String)) {
  record.version += 1
}

var dbRecord = (version: 2, title: "Exercism")
updateVersion(&dbRecord)

// dbRecord is now (3, "Exercism")

There are a couple of extra rules one should be aware of regarding in-out parameters.

1. Inside a function with in-out parameters, you are not allowed to reference the variable that was passed in as the in-out parameter.
2. The same variable cannot be passed as multiple in-out parameters in the same function.

func inoutFunc(_ ioVar1: inout Int, _ ioVar2: inout Int) {
  ioVar1 += 1
  ioVar2 += 2
}

var mutVar = 0
inoutFunc(&mutVar, &mutVar)
// raises a compiler error: "Inout arguments are not allowed to alias each other"