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Exception: Exception

Overview

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:

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 Attribute Summary

Class Method Summary

Instance Method Summary

Constructor Details

.new(msg = nil) ⇒ Exception

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

[ GitHub ] 

  


# File 'error.c', line 997



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);
}


  







Class Attribute Details



  

    .to_tty?  ⇒ Boolean  (readonly)  



  

    

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


  



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# File 'error.c', line 1067



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


  







Class Method Details



  

    .exception(string)  ⇒ Exception   



  

    

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.


  



  [ GitHub ]





Instance Method Details



  

    #==(obj)  ⇒ Boolean   



  

    

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


  



  [ GitHub ]


  

  


# File 'error.c', line 1347



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;
}


  








  

    #backtrace  ⇒ Array?   



  

    

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


  



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# File 'error.c', line 1213



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;
}


  








  

    #backtrace_locations  ⇒ Array?   



  

    

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



This method is not affected by #set_backtrace().


  



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# File 'error.c', line 1261



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;
}


  








  

    #cause  ⇒ Exception?   



  

    

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


  



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# File 'error.c', line 1326



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


  








  

    #exception(string)  ⇒ Exception   



  

    

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.


  



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# File 'error.c', line 1019



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;
}


  








  

    #full_message(highlight: bool, order: [:top or :bottom])  ⇒ String   



  

    

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.


  



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# File 'error.c', line 1091



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;
}


  








  

    #inspect  ⇒ String   



  

    

Return this exception’s class name and message.


  



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# File 'error.c', line 1157



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;
}


  








  

    #message  ⇒ String   



  

    

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


  



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# File 'error.c', line 1144



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


  








  

    #set_backtrace(backtrace)  ⇒ Array   



  

    

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 #backtrace.


  



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# File 'error.c', line 1305



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


  








  

    #to_s  ⇒ String   



  

    

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


  



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# File 'error.c', line 1040



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);
}