Descendants of class Exception are used to communicate between Kernel#raise and rescue statements in begin ... end blocks. Exception objects carry information about the exception – its type (the exception’s class name), an optional descriptive string, and optional traceback information. Exception subclasses may add additional information like NameError#name.
Programs may make subclasses of Exception, typically of StandardError or RuntimeError, to provide custom classes and add additional information. See the subclass list below for defaults for raise and rescue.
When an exception has been raised but not yet handled (in rescue, ensure, at_exit and END blocks) the global variable $! will contain the current exception and $@ contains the current exception’s backtrace.
It is recommended that a library should have one subclass of StandardError or RuntimeError and have specific exception types inherit from it. This allows the user to rescue a generic exception type to catch all exceptions the library may raise even if future versions of the library add new exception subclasses.
For example:
class MyLibrary class Error < RuntimeError end class WidgetError < Error end class FrobError < Error end end
To handle both WidgetError and FrobError the library user can rescue MyLibrary::Error.
The built-in subclasses of Exception are:
-
StandardError– default forrescue -
fatal – impossible to rescue
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.
# File tmp/rubies/ruby-2.3.8/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
static VALUE
exc_initialize(int argc, VALUE *argv, VALUE exc)
{
VALUE arg;
rb_scan_args(argc, argv, "01", &arg);
rb_ivar_set(exc, id_mesg, arg);
rb_ivar_set(exc, id_bt, Qnil);
return exc;
}
Construct a new Exception object, optionally passing in a message.
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 status = 0;
obj = rb_protect(try_convert_to_exception, obj, &status);
if (status || 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.
# File tmp/rubies/ruby-2.3.8/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.
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
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.
Now, this method is not affected by Exception#set_backtrace().
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.
static VALUE
exc_exception(int argc, VALUE *argv, VALUE self)
{
VALUE exc;
if (argc == 0) return self;
if (argc == 1 && self == argv[0]) return self;
exc = rb_obj_clone(self);
exc_initialize(argc, argv, exc);
return exc;
}
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.
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_str_dup(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
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. By supplying a to_str method, exceptions are agreeing to be used where Strings are expected.
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.
# File tmp/rubies/ruby-2.3.8/ext/json/lib/json/add/exception.rb, line 28
def to_json(*args)
as_json.to_json(*args)
end
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).