defer

defer.zig

  1. const std = @import("std");
  2. const assert = std.debug.assert;
  3. const warn = std.debug.warn;
  5. // defer will execute an expression at the end of the current scope.
  6. fn deferExample() usize {
  7.     var a: usize = 1;
  9.     {
  10.         defer a = 2;
  11.         a = 1;
  12.     }
  13.     assert(a == 2);
  15.     a = 5;
  16.     return a;
  17. }
  19. test "defer basics" {
  20.     assert(deferExample() == 5);
  21. }
  23. // If multiple defer statements are specified, they will be executed in
  24. // the reverse order they were run.
  25. fn deferUnwindExample() void {
  26.     warn("\n", .{});
  28.     defer {
  29.         warn("1 ", .{});
  30.     }
  31.     defer {
  32.         warn("2 ", .{});
  33.     }
  34.     if (false) {
  35.         // defers are not run if they are never executed.
  36.         defer {
  37.             warn("3 ", .{});
  38.         }
  39.     }
  40. }
  42. test "defer unwinding" {
  43.     deferUnwindExample();
  44. }
  46. // The errdefer keyword is similar to defer, but will only execute if the
  47. // scope returns with an error.
  48. //
  49. // This is especially useful in allowing a function to clean up properly
  50. // on error, and replaces goto error handling tactics as seen in c.
  51. fn deferErrorExample(is_error: bool) !void {
  52.     warn("\nstart of function\n", .{});
  54.     // This will always be executed on exit
  55.     defer {
  56.         warn("end of function\n", .{});
  57.     }
  59.     errdefer {
  60.         warn("encountered an error!\n", .{});
  61.     }
  63.     if (is_error) {
  64.         return error.DeferError;
  65.     }
  66. }
  68. test "errdefer unwinding" {
  69.     deferErrorExample(false) catch {};
  70.     deferErrorExample(true) catch {};
  71. }
  1. $ zig test defer.zig
  2. 1/3 test "defer basics"...OK
  3. 2/3 test "defer unwinding"...
  4. 2 1 OK
  5. 3/3 test "errdefer unwinding"...
  6. start of function
  7. end of function
  9. start of function
  10. encountered an error!
  11. end of function
  12. OK
  13. All 3 tests passed.

See also: