Why doesn't this piece of code run without error? Isn't bad_exception supposed to be called automatically when an unexpected exception is encountered? Or is it necessary to set a handler for it?
#include <iostream>
#include <exception>
using namespace std;
class C{};
void test() throw(bad_exception)
{
throw C();
}
int main()
{
try
{
test();
} catch(bad_exception& e)
{
cout << "Caught ";
}
}
In C++03 theory:
If you throw an exception that is not in the exception specification, unexpected() gets called. If you have not set the unexpected handler via set_unexpected() this means terminate() gets called which is the case you observed. If you have set an unexpected handler that does not call terminate but throws an exception, and if that exception is not listed in your exception specification, it gets translated into bad_exception. So in order to get the expected result, call set_unexpected() first with an appropriate handler.
In C++03 practice:
Some compilers do not support exception specifications at all (other than throw()), others just don't evaluate/check them for correctness. As Herb Sutter points out, exception specifications create a clumsy "shadow type system" that is not easy to handle right (if it is possible at all). Therefore..
... in C++11:
Exception specifications are deprecated. You should rather not use them. However, there is a nothrow operator that has a slightly different functionality than throw()
PS: so why have std::bad_exception in C++03?
You hav three different regions of code:
The function you are writing an exception specification for.
The "foreign" code you are calling from that function that might or might not throw an exception that does not match you specification.
The (maybe also unknown) unexpected handler that can be anything and should either terminate/exit/abort the program or throw anything.
So if the "foreign" code throws an exception that violates your exception specification, you have three possible outcomes of that:
The handler terminates the program. There is not much you can do about that unless you set your own handler in the function.
The handler throws an exception that matches your exception specification. And all is well.
The handler throws something else. What do you want the runtime to do now? That's where bad_exception comes in: If it is in your specification, the "something else" gets translated into a bad_exception, and the program continues. If it's not, terminate gets called.
Setting your own handler inside the function disables any handler that was previously set by anyone else who just wants to use your function. He will not expect you to disable his handler. Besides, the handler is meant as a global what-happens-if policy and thus is nothing you should care about in a single function implementation.
This should call std::unexpected which by default call std::terminate() as the thrown exception isn't part of the exception specification.
(It does here with g++ on Linux, SunOracle CC on Solaris, IBM xlC on AIX BTW)
If you install an unexpected handler, it works as you expect here:
include <iostream>
#include <exception>
using namespace std;
class C{};
void myunexpected()
{
throw std::bad_exception();
}
void test() throw(std::bad_exception)
{
throw C();
}
int main()
{
try
{
set_unexpected(myunexpected);
test();
} catch(std::bad_exception& e)
{
std::cout << "Caught ";
}
}
As per [except.unexpected], when a function throws an exception not listed in its dynamic exception specification, std::unexpected() is called. According to [unexpected.handler], the default implementation of std::unexpected() simply calls std::terminate().
However, the program can install its own handler, which can (re-)throw an exception. If the exception thus thrown is not allowed by the dynamic exception specification, it can be replaced by std::bad_exception when that is allowed.
Your catch-block would only catch exceptions of the type bad_exception, or a subtype thereof. C does not meet this criterion.
Actually, this question is very similar to yours. The accepted answer explains that if your function throws an exception that does not match the specification (if there is any), then std::unexpected is called, which by default calls std::terminate.
Related
Under exceptional circumstances, I want my program to stop processing, output an error to std::cerr, clean up, and exit.
However, calling exit() will not call all the destructors of any objects that have been constructed. I would like to have the destructors called nicely, so I wrapped all the code in a try-catch block, something like this:
int main(int argc, char** argv){
try {
bool something_is_not_right = false;
/* lots of variables declared here */
/* some code that might set something_is_not_right to true goes here */
if(something_is_not_right){
std::cerr << "something is not right!!!" << std::endl;
throw '\0'; // dummy unused variable for throwing.
}
}
catch (...) {}
return 0;
}
In this way, I get guaranteed destruction of all my variables. But I can't seem to find a way to get C++ to throw nothing. throw; has a special meaning in C++; it isn't throwing nothing.
Is there a way to throw nothing?
No
It's not possible to throw nothing. You need to throw something. While you may have seen people use the throw keyword without anything, this just means they are re-throwing the currently handled exception.
This is not a direct answer to your question, but a personal recommendation.
You might want to take a look at the predefined exceptions of stdexcept which cover almost any exceptional behaviour that occurs in a program. In your case I would throw a std::runtime_error. Also, only catch what you expect to be thrown and not catch 'em all. If you really want to catch everything, then catch std::exception (the base class of all standard exceptions).
In my opinion handling unknown exceptions doesn't really make sense and the only logical consequence is to just abort execution.
#include <iostream>
#include <stdexcept>
int main()
{
try
{
bool something_is_not_right = true;
if ( something_is_not_right )
throw std::runtime_error("something is not right!!!");
}
catch (std::runtime_error& e)
{
std::cerr << e.what() << '\n';
throw;
}
}
How do you know that the reason for the catch is your error or something else (out of memory is always a good one). If you detect an error, then you should create and throw the reason for that error in a custom exception. Then in main, you can tell the difference between your detected error and something you didn't expect. It's just good practice.
Well you "can" but it doesn't throw nothing. It terminates.
5.17 Throwing an exception:
Evaluating a throw-expression with an operand throws an exception (15.1)
A throw-expression with no operand rethrows the currently handled exception (15.3).
If no exception is presently being handled, evaluating a throw-expression with no operand calls std::terminate()
This is valid:
int main () {
throw;
return 0;
}
Source used: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf
But it won't cleanup anything. std::terminate is used when the cleanups fail.
Otherwise you have to use operand, and then this section becomes relevant:
15.1 Throwing an exception [except.throw]
Throwing an exception copy-initializes (8.5, 12.8) a temporary object, called the exception object. The temporary is an lvalue and is used to initialize the variable declared in the matching handler (15.3).
So you have to pass something that is able to be initialized, which by definition cannot be nothing.
In order to ensure full cleanup you have to throw an exception and catch it somewhere. Implementations are not required to clean up stack objects when an exception is thrown but not caught. The requirement (in [except.handle]/9) when an exception is thrown but not caught is that the program calls std::terminate(), and it's implementation-defined whether stack objects are cleaned up.
I am not sure if this is a issue with the compiler or if I am doing something wrong. I am using Visual Studio 2013 compiler.
I have a class where I need to acquire significant amount of resources in my constructor initializer list most of which can throw an exception. I wrapped up the member initializer list in a function try block and caught the exception there. But my program still aborts even though the catch clause doesn't re-throw the exception. I am not allowed to post the actual code. So I have reproduced the issue with this equivalent demo code. Can someone please help me address this?
#include <iostream>
using namespace std;
class A{
public:
A() try : i{ 0 }{ throw 5; }
catch (...){ cout << "Exception" << endl; }
private:
int i;
};
int main(){
A obj;
}
On executing this code I get a windows alert "abort() has been called". So I guess the system is treating this as an uncaught exception and calling terminate().
On the other hand if I wrap the construction of the object in main() in a try-catch block then the exception is caught properly and the program terminates normally.
Can someone please tell me if I am doing something wrong here?
There's a relevant gotw
http://gotw.ca/gotw/066.htm
Basically even if you don't throw in your catch block, the exception will automatically be rethrown
If the handler body contained the statement "throw;" then the catch
block would obviously rethrow whatever exception A::A() or B::B() had
emitted. What's less obvious, but clearly stated in the standard, is
that if the catch block does not throw (either rethrow the original
exception, or throw something new), and control reaches the end of the
catch block of a constructor or destructor, then the original
exception is automatically rethrown.
This is normal behaviour according to the cppreference.com documentation for function-try blocks: a so-called function-try-block on a constructor or destructor must throw from its catch-clause or else there is an implicit rethrow after the catch-clause.
This makes perfect sense: the object A has not been properly constructed and hence is not in a state fit for use: it must throw an exception. You have to ensure whether the construction succeeded at the place where the object is constructed, i.e. in the case of your example in main().
Exception cannot be caught in constructor function-try-block.
n3376 15.2/15
The currently handled exception is rethrown if control reaches the end
of a handler of the function-try-block of a constructor or destructor.
You should catch it in object creation place.
I suggest you read the article: "GotW #66 Constructor Failures"on the site:http://gotw.ca/gotw/066.htm
I am not sure if this is a issue with the compiler or if I am doing something wrong. I am using Visual Studio 2013 compiler.
I have a class where I need to acquire significant amount of resources in my constructor initializer list most of which can throw an exception. I wrapped up the member initializer list in a function try block and caught the exception there. But my program still aborts even though the catch clause doesn't re-throw the exception. I am not allowed to post the actual code. So I have reproduced the issue with this equivalent demo code. Can someone please help me address this?
#include <iostream>
using namespace std;
class A{
public:
A() try : i{ 0 }{ throw 5; }
catch (...){ cout << "Exception" << endl; }
private:
int i;
};
int main(){
A obj;
}
On executing this code I get a windows alert "abort() has been called". So I guess the system is treating this as an uncaught exception and calling terminate().
On the other hand if I wrap the construction of the object in main() in a try-catch block then the exception is caught properly and the program terminates normally.
Can someone please tell me if I am doing something wrong here?
There's a relevant gotw
http://gotw.ca/gotw/066.htm
Basically even if you don't throw in your catch block, the exception will automatically be rethrown
If the handler body contained the statement "throw;" then the catch
block would obviously rethrow whatever exception A::A() or B::B() had
emitted. What's less obvious, but clearly stated in the standard, is
that if the catch block does not throw (either rethrow the original
exception, or throw something new), and control reaches the end of the
catch block of a constructor or destructor, then the original
exception is automatically rethrown.
This is normal behaviour according to the cppreference.com documentation for function-try blocks: a so-called function-try-block on a constructor or destructor must throw from its catch-clause or else there is an implicit rethrow after the catch-clause.
This makes perfect sense: the object A has not been properly constructed and hence is not in a state fit for use: it must throw an exception. You have to ensure whether the construction succeeded at the place where the object is constructed, i.e. in the case of your example in main().
Exception cannot be caught in constructor function-try-block.
n3376 15.2/15
The currently handled exception is rethrown if control reaches the end
of a handler of the function-try-block of a constructor or destructor.
You should catch it in object creation place.
I suggest you read the article: "GotW #66 Constructor Failures"on the site:http://gotw.ca/gotw/066.htm
If I have a method marked throw(), e.g.
void method() throw()
{
// do some stuff, call other functions
}
and yet exception does happen inside, gcc will terminate the application (with message "terminate called after throwing an instance of 'xyz'").
Is there a way to avoid this behaviour?
For example, a command-line switch to ignore throw() stuff or force eh_frame generation. Etc.
Did you try the GCC manual?
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining `NDEBUG'. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.
It doesn't force generate of EH frames, but it should stop the call to std::unexpected() and so might be useful for your case.
As the docs say, "the compiler still optimizes based on the specifications" so e.g. it doesn't help when the call to method() can be inlined into the catch site, because the compiler assumes the catch is not needed, because the empty exception spec says no exception will be throw, and so if an exception is thrown it doesn't get caught. If the call to method() is not inlinable into the catch site it seems to work, the exception leaves method() without calling std::unexpected() and can be caught higher up the stack.
Edit: This will still call std::terminate() even with -fno-enforce-eh-specs:
void func() throw() { throw ""; }
void func2() { func(); }
int main() { try { func2(); } catch (...) { } }
The compiler can see that the call to func2 only calls a no-throw function, so the catch will never be needed and so is optimised away. When the exception is thrown, it isn't caught.
This does work with -fno-enforce-eh-specs and doesn't terminate:
/* func2.cc */
void func() throw();
void func2() { func(); }
/* main.cc */
void func2();
int main() { try { func2(); } catch (...) { } }
Here, when compiling main.cc, the compiler can't tell whether func2 is going to throw or not because it has no exception specification and its definition is not visible in main.cc, so the catch cannot be omitted. When the exception is thrown it will be caught.
You can provide your own terminate handler by calling std::set_terminate ... however I don't think it's legal to return from there, so the only thing it can really do is terminate somehow.
You can provide your own unexpected exception handler with std::set_unexpected: the default one calls std::terminate but you can do something different (like logging and swallowing the exception). Whether or not you can recover usefully depends on your program though. Also, I've seen it marked deprecated, so relying on this probably isn't the greatest idea.
I know this is a valid c++ program.
What is the point of the throw in the function declarement? AFAIK it does nothing and isnt used for anything.
#include <exception>
void func() throw(std::exception) { }
int main() { return 0; }
It specifies that any std::exception can be thrown from func(), and nothing else. If something else is thrown, it will call an unexpected() function which by default calls terminate().
What this means is that throwing something else will almost certainly terminate the program, in the same manner as an uncaught exception, but the implementation will have to enforce this. This is normally much the same as putting a try{...}catch(){...} block around func(), which can inhibit performance.
Usually, exception specifications aren't worth it, according to the Guru of the Week column about it. The Boost guidelines say that there might be a slight benefit with a blank throws() for a non-inline function, and there are disadvantages.
That is an exception specification, and it is almost certainly a bad idea.
It states that func may throw a std::exception, and any other exception that func emits will result in a call to unexpected().
This is a C++ exception specification. It declares that the particular function will potentially throw a std::exception type.
In general though exception specifications in C++ are considered a feature to avoid. It's an odd feature in that it's behavior is declared at compile time but only checked at runtime (very different from say Java's version).
Here is a good article which breaks down the feature
http://www.gotw.ca/publications/mill22.htm
This is an exception specification. It says that the only exception that func() can throw is std::exception (or a derivative thereof). Attempting to throw any other exception will give std::unexpected instead.
Exception specification. The type(s) following the throw keyword specifies exactly what all, if any, exceptions the function can throw. See 15.4 of the draft.
Note: A function with no exception-specification allows all exceptions. A function with an empty exception-specification, throw(), does not allow any exceptions.
Basically this:
void func() throw(std::exception,B) { /* Do Stuff */}
Is just shorthand fro this:
void func()
{
try
{
/* Do Stuff */
}
catch(std::exception const& e)
{
throw;
}
catch(B const& e)
{
throw;
}
catch(...)
{
unexpected(); // This calls terminate
// i.e. It never returns.
}
}
Calling terminate() is rarely what you want, as the stack is not unwound and thus all your efforts in RAII is wasted. The only exception to the rule is declaring an empty throw list and this is mainly for documentation purposes to show that you are supporting the no-throw exception gurantee (you should still manually catch all exceptions in this situation).
Some important (imho) places that should be no-throw are destructors and swap() methods. Destructors are rarely explicitly marked no-throw but swap() are quite often marked no-throw.
void myNoThrowFunc() throws() // No-Throw (Mainlly for doc purposes).
{
try
{
/* Do Stuff */
}
catch(...) // Make sure it does not throw.
{
/* Log and/or do cleanup */
}
}