Exception

Class

Class Exception and its subclasses are used to communicate between Kernel#raise and rescue statements in begin ... end blocks.

An Exception object carries information about an exception:

Its type (the exception’s class).
An optional descriptive message.
Optional backtrace information.

Some built-in subclasses of Exception have additional methods: e.g., NameError#name.

Defaults

Two Ruby statements have default exception classes:

raise: defaults to RuntimeError.
rescue: defaults to StandardError.

Global Variables

When an exception has been raised but not yet handled (in rescue, ensure, at_exit and END blocks), two global variables are set:

$! contains the current exception.
$@ contains its backtrace.

Custom Exceptions

To provide additional or alternate information, a program may create custom exception classes that derive from the built-in exception classes.

A good practice is for a library to create a single “generic” exception class (typically a subclass of StandardError or RuntimeError) and have its other exception classes derive from that class. This allows the user to rescue the generic exception, thus catching all exceptions the library may raise even if future versions of the library add new exception subclasses.

For example:

class MyLibrary
  class Error < ::StandardError
  end

  class WidgetError < Error
  end

  class FrobError < Error
  end

end

To handle both MyLibrary::WidgetError and MyLibrary::FrobError the library user can rescue MyLibrary::Error.

Built-In `Exception` Classes

The built-in subclasses of Exception are:

Class Methods

exc.exception(string) → an_exception or exc

error.c View on GitHub

With no argument, or if the argument is the same as the receiver, return the receiver. Otherwise, create a new exception object of the same class as the receiver, but with a message equal to string.to_str.

json_create(object)

ext/json/lib/json/add/exception.rb View on GitHub

          
            # File tmp/rubies/ruby-2.7.6/ext/json/lib/json/add/exception.rb, line 10
def self.json_create(object)
  result = new(object['m'])
  result.set_backtrace object['b']
  result
end

Deserializes JSON string by constructing new Exception object with message m and backtrace b serialized with to_json

Exception.new(msg = nil) → exception

error.c View on GitHub

          
            static VALUE
exc_initialize(int argc, VALUE *argv, VALUE exc)
{
    VALUE arg;

    arg = (!rb_check_arity(argc, 0, 1) ? Qnil : argv[0]);
    return exc_init(exc, arg);
}

Construct a new Exception object, optionally passing in a message.

Exception.to_tty? → true or false

error.c View on GitHub

          
            static VALUE
exc_s_to_tty_p(VALUE self)
{
    return rb_stderr_tty_p() ? Qtrue : Qfalse;
}

Returns true if exception messages will be sent to a tty.

Instance Methods

exc == obj → true or false

error.c View on GitHub

          
            static VALUE
exc_equal(VALUE exc, VALUE obj)
{
    VALUE mesg, backtrace;

    if (exc == obj) return Qtrue;

    if (rb_obj_class(exc) != rb_obj_class(obj)) {
        int state;

        obj = rb_protect(try_convert_to_exception, obj, &state);
        if (state || obj == Qundef) {
            rb_set_errinfo(Qnil);
            return Qfalse;
        }
        if (rb_obj_class(exc) != rb_obj_class(obj)) return Qfalse;
        mesg = rb_check_funcall(obj, id_message, 0, 0);
        if (mesg == Qundef) return Qfalse;
        backtrace = rb_check_funcall(obj, id_backtrace, 0, 0);
        if (backtrace == Qundef) return Qfalse;
    }
    else {
        mesg = rb_attr_get(obj, id_mesg);
        backtrace = exc_backtrace(obj);
    }

    if (!rb_equal(rb_attr_get(exc, id_mesg), mesg))
        return Qfalse;
    if (!rb_equal(exc_backtrace(exc), backtrace))
        return Qfalse;
    return Qtrue;
}

Equality—If obj is not an Exception, returns false. Otherwise, returns true if exc and obj share same class, messages, and backtrace.

as_json(*)

ext/json/lib/json/add/exception.rb View on GitHub

          
            # File tmp/rubies/ruby-2.7.6/ext/json/lib/json/add/exception.rb, line 18
def as_json(*)
  {
    JSON.create_id => self.class.name,
    'm'            => message,
    'b'            => backtrace,
  }
end

Returns a hash, that will be turned into a JSON object and represent this object.

exception.backtrace → array or nil

error.c View on GitHub

          
            static VALUE
exc_backtrace(VALUE exc)
{
    VALUE obj;

    obj = rb_attr_get(exc, id_bt);

    if (rb_backtrace_p(obj)) {
        obj = rb_backtrace_to_str_ary(obj);
        /* rb_ivar_set(exc, id_bt, obj); */
    }

    return obj;
}

Returns any backtrace associated with the exception. The backtrace is an array of strings, each containing either “filename:lineNo: in ‘method”’ or “filename:lineNo.”

def a
  raise "boom"
end

def b
  a()
end

begin
  b()
rescue => detail
  print detail.backtrace.join("\n")
end

produces:

prog.rb:2:in `a'
prog.rb:6:in `b'
prog.rb:10

In the case no backtrace has been set, nil is returned

ex = StandardError.new
ex.backtrace
#=> nil

exception.backtrace_locations → array or nil

error.c View on GitHub

          
            static VALUE
exc_backtrace_locations(VALUE exc)
{
    VALUE obj;

    obj = rb_attr_get(exc, id_bt_locations);
    if (!NIL_P(obj)) {
        obj = rb_backtrace_to_location_ary(obj);
    }
    return obj;
}

Returns any backtrace associated with the exception. This method is similar to Exception#backtrace, but the backtrace is an array of Thread::Backtrace::Location.

This method is not affected by Exception#set_backtrace().

exception.cause → an_exception or nil

error.c View on GitHub

          
            static VALUE
exc_cause(VALUE exc)
{
    return rb_attr_get(exc, id_cause);
}

Returns the previous exception ($!) at the time this exception was raised. This is useful for wrapping exceptions and retaining the original exception information.

exc.exception(string) → an_exception or exc

error.c View on GitHub

          
            static VALUE
exc_exception(int argc, VALUE *argv, VALUE self)
{
    VALUE exc;

    argc = rb_check_arity(argc, 0, 1);
    if (argc == 0) return self;
    if (argc == 1 && self == argv[0]) return self;
    exc = rb_obj_clone(self);
    rb_ivar_set(exc, id_mesg, argv[0]);
    return exc;
}

exception.full_message(highlight: bool, order: [:top or :bottom]) → string

error.c View on GitHub

          
            static VALUE
exc_full_message(int argc, VALUE *argv, VALUE exc)
{
    VALUE opt, str, emesg, errat;
    enum {kw_highlight, kw_order, kw_max_};
    static ID kw[kw_max_];
    VALUE args[kw_max_] = {Qnil, Qnil};

    rb_scan_args(argc, argv, "0:", &opt);
    if (!NIL_P(opt)) {
        if (!kw[0]) {
#define INIT_KW(n) kw[kw_##n] = rb_intern_const(#n)
            INIT_KW(highlight);
            INIT_KW(order);
#undef INIT_KW
        }
        rb_get_kwargs(opt, kw, 0, kw_max_, args);
        switch (args[kw_highlight]) {
          default:
            rb_raise(rb_eArgError, "expected true or false as "
                     "highlight: %+"PRIsVALUE, args[kw_highlight]);
          case Qundef: args[kw_highlight] = Qnil; break;
          case Qtrue: case Qfalse: case Qnil: break;
        }
        if (args[kw_order] == Qundef) {
            args[kw_order] = Qnil;
        }
        else {
            ID id = rb_check_id(&args[kw_order]);
            if (id == id_bottom) args[kw_order] = Qtrue;
            else if (id == id_top) args[kw_order] = Qfalse;
            else {
                rb_raise(rb_eArgError, "expected :top or :bottom as "
                         "order: %+"PRIsVALUE, args[kw_order]);
            }
        }
    }
    str = rb_str_new2("");
    errat = rb_get_backtrace(exc);
    emesg = rb_get_message(exc);

    rb_error_write(exc, emesg, errat, str, args[kw_highlight], args[kw_order]);
    return str;
}

Returns formatted string of exception. The returned string is formatted using the same format that Ruby uses when printing an uncaught exceptions to stderr.

If highlight is true the default error handler will send the messages to a tty.

order must be either of :top or :bottom, and places the error message and the innermost backtrace come at the top or the bottom.

The default values of these options depend on $stderr and its tty? at the timing of a call.

exception.inspect → string

error.c View on GitHub

          
            static VALUE
exc_inspect(VALUE exc)
{
    VALUE str, klass;

    klass = CLASS_OF(exc);
    exc = rb_obj_as_string(exc);
    if (RSTRING_LEN(exc) == 0) {
        return rb_class_name(klass);
    }

    str = rb_str_buf_new2("#<");
    klass = rb_class_name(klass);
    rb_str_buf_append(str, klass);
    rb_str_buf_cat(str, ": ", 2);
    rb_str_buf_append(str, exc);
    rb_str_buf_cat(str, ">", 1);

    return str;
}

Return this exception’s class name and message.

exception.message → string

error.c View on GitHub

          
            static VALUE
exc_message(VALUE exc)
{
    return rb_funcallv(exc, idTo_s, 0, 0);
}

Returns the result of invoking exception.to_s. Normally this returns the exception’s message or name.

exc.set_backtrace(backtrace) → array

error.c View on GitHub

          
            static VALUE
exc_set_backtrace(VALUE exc, VALUE bt)
{
    return rb_ivar_set(exc, id_bt, rb_check_backtrace(bt));
}

Sets the backtrace information associated with exc. The backtrace must be an array of String objects or a single String in the format described in Exception#backtrace.

to_json(*args)

ext/json/lib/json/add/exception.rb View on GitHub

          
            # File tmp/rubies/ruby-2.7.6/ext/json/lib/json/add/exception.rb, line 28
def to_json(*args)
  as_json.to_json(*args)
end

Stores class name (Exception) with message m and backtrace array b as JSON string

exception.to_s → string

error.c View on GitHub

          
            static VALUE
exc_to_s(VALUE exc)
{
    VALUE mesg = rb_attr_get(exc, idMesg);

    if (NIL_P(mesg)) return rb_class_name(CLASS_OF(exc));
    return rb_String(mesg);
}

Returns exception’s message (or the name of the exception if no message is set).

Exception

Defaults

Global Variables

Custom Exceptions

Built-In Exception Classes

exc.exception(string) → an_exception or exc

json_create(object)

Exception.new(msg = nil) → exception

Exception.to_tty? → true or false

exc == obj → true or false

as_json(*)

exception.backtrace → array or nil

exception.backtrace_locations → array or nil

exception.cause → an_exception or nil

exc.exception(string) → an_exception or exc

exception.full_message(highlight: bool, order: [:top or :bottom]) → string

exception.inspect → string

exception.message → string

exc.set_backtrace(backtrace) → array

to_json(*args)

exception.to_s → string

Built-In `Exception` Classes