Statements: Using

A using statement is used to enforce object disposal. It has two forms: block and non-block. Here is an example of the block form:

  using ($f1 = new TextFile("file1.txt", "rw")) {
    // ... work with the file
  } // __dispose is called here

The type of the expression inside the parentheses must implement either IDisposable (or IAsyncDisposable). The scope of $f1 is the using block, and at the end of that scope, __dispose (or __disposeAsync) is called. If the assignment (as in, $f1 =) is omitted, we cannot access the object directly inside the block.

Within the block, there are limits to what we can do with $f1. Specifically, we cannot assign to it again or make copies of it. And to pass it to a function, we must mark the function's corresponding parameter with the attribute __AcceptDisposable. We can also call methods on the object that $f1 designates. Consider the following:

  using ($f1 = new TextFile("file1.txt", "rw")) {
//  echo "\$f1 is >" . $f1 . "<\n";  // usage not permitted
    echo "\$f1 is >" . $f1->__toString() . "<\n";
    // ...

Note the commented-out trace statement at the start of the block. Under the hood, we're trying to pass a copy of a TextFile to echo, but echo doesn't know anything about TextFile's object cleanup, so that is rejected. We can, however, directly call a method on that object, which is why __toString is called explicitly in the statement following.

Here is an example of the non-block form:

function foo(): void {
  using $f4 = TextFile::open_TextFile("file4.txt", "rw");
  using $f5 = new TextFile("file5.txt", "rw");
  // ... work with both files
} // __dispose is called here for both $f4 and $f5

The difference here is that no parentheses are required around the controlling expression, we use a trailing semicolon instead of a block, and the scope of the assigned-to variables ends at the end of the parent block, which avoids the need to use nested using statements.

See object disposal for a detailed example of the use of both forms.

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