just a quick question. Is there any difference between
void f(Foo x) try
{
...
}
catch(exception& e)
{
...
}
and
void f(Foo x)
{
try { ... }
catch (exception& e)
{
...
}
}
?
If no, why are function try blocks for (the case of initialization lists for constructors being put aside) ? What happens if the copy constructor of Foo throws an exception when x is passed to f ?
Function try blocks are only ever needed in constructors. In all other cases exactly the same effect can be achieved by enclosing the entire body of the function in a normal try/catch block.
If the copy constructor used to initialize a parameter throws an exception this happens before the function call. It cannot be caught by a function try block or exceptional handler in the function as the function doesn't get called.
Some things are allowed because it would be harder to disallow them. Allowing function try blocks on some, but not all function bodies would make the grammar and compilers more complicated.
Function try blocks were added expressly for the purpose of catching exceptions in constructor initialization lists.
In your example there are no constructor initializations, so there is no difference between the two forms.
Just spotted an interesting point in this Dr. Dobb's article (although quite old):
...remember that you can't return a value inside a function-try-block handler. So it makes no sense to use a function try block for a non-void function
and this is their code example:
int f()
try
{
...
}
catch(Error &e)
{
// oops, can't return int from here!
}
Which actually means that function try blocks are weaker than "regular" try blocks and their use should be discouraged other than in constructors.
(the article is from 2000, so it'd be nice if someone would comment on whether this is still so in the current standard)
A function-try-block is equivalent to a try block inside the function spanning the whole function unless you have a constructor or destructor.
On a constructor, the function-try-block also catches exceptions thrown by constructors of your base classes and non-static data members. Catching those is not possible with a regular try block spanning the statement-block of the constructor. This is the primary use-case for function-try-block.
On a destructor, the function-try-block also catches exceptions thrown by destructors of base classes and your non-static data members. This cannot be achieved using a try block inside the destructor. Note that destructors that throw are bad design, nonetheless they are legal in C++ and this is the way to deal with them.
In both these cases, additional rules apply: You cannot use return in the function-try-block’s catch clauses of a constructor because a data member may not be properly constructed; however, you may use throw and at the end of every catch, there is an implicit throw; statement. In a destructor’s function-try-block, at the end of every catch, there is also an implicit throw;, but an explicit return; is allowed.
Related
So the way to nest exceptions in C++ using std::nested_exception is:
void foo() {
try {
// code that might throw
std::ifstream file("nonexistent.file");
file.exceptions(std::ios_base::failbit);
}
catch(...) {
std::throw_with_nested(std::runtime_error("foo failed"));
}
}
But this technique uses explicit try/catch blocks at every level where one wishes to nest exceptions, which is ugly to say the least.
RAII, which Jon Kalb expands as "responsibility acquisition is initialization", is a much cleaner way to deal with exceptions instead of using explicit try/catch blocks. With RAII, explicit try/catch blocks are largely only used to ultimately handle an exception, e.g. in order to display an error message to the user.
Looking at the above code, it seems to me that entering foo() can be viewed as entailing a responsibility to report any exceptions as std::runtime_error("foo failed") and nest the details inside a nested_exception. If we can use RAII to acquire this responsibility the code looks much cleaner:
void foo() {
Throw_with_nested on_error("foo failed");
// code that might throw
std::ifstream file("nonexistent.file");
file.exceptions(std::ios_base::failbit);
}
Is there any way to use RAII syntax here to replace explicit try/catch blocks?
To do this we need a type that, when its destructor is called, checks to see if the destructor call is due to an exception, nests that exception if so, and throws the new, nested exception so that unwinding continues normally. That might look like:
struct Throw_with_nested {
const char *msg;
Throw_with_nested(const char *error_message) : msg(error_message) {}
~Throw_with_nested() {
if (std::uncaught_exception()) {
std::throw_with_nested(std::runtime_error(msg));
}
}
};
However std::throw_with_nested() requires a 'currently handled exception' to be active, which means it doesn't work except inside the context of a catch block. So we'd need something like:
~Throw_with_nested() {
if (std::uncaught_exception()) {
try {
rethrow_uncaught_exception();
}
catch(...) {
std::throw_with_nested(std::runtime_error(msg));
}
}
}
Unfortunately as far as I'm aware, there's nothing like rethrow_uncaught_excpetion() defined in C++.
In the absence of a method to catch (and consume) the uncaught exception in the destructor, there is no way to rethrow an exception, nested or not, in the context of the destructor without std::terminate being called (when the exception is thrown in the context of exception handling).
std::current_exception (combined with std::rethrow_exception) will only return a pointer to a currently handled exception. This precludes its use from this scenario as the exception in this case is explicitly unhandled.
Given the above, the only answer to give is from an aesthetic perspective. Function level try blocks make this look slightly less ugly. (adjust for your style preference):
void foo() try {
// code that might throw
std::ifstream file("nonexistent.file");
file.exceptions(std::ios_base::failbit);
}
catch(...) {
std::throw_with_nested(std::runtime_error("foo failed"));
}
It's impossible with RAII
Considering the simple rule
Destructors must never throw.
it is impossible with RAII to implement the thing you want. The rule has one simple reason: If a destructor throws an exception during stack unwinding due to an exception in flight, then terminate() is called and your application will be dead.
An alternative
In C++11 you can work with lambdas which can make life a little easier. You can write
void foo()
{
giveErrorContextOnFailure( "foo failed", [&]
{
// code that might throw
std::ifstream file("nonexistent.file");
file.exceptions(std::ios_base::failbit);
} );
}
if you implement the function giveErrorContextOnFailure in the following way:
template <typename F>
auto giveErrorContextOnFailure( const char * msg, F && f ) -> decltype(f())
{
try { return f(); }
catch { std::throw_with_nested(std::runtime_error(msg)); }
}
This has several advantages:
You encapsulate how the error is nested.
Changing the way errors are nested can be changed for the whole program, if this technique is followed strictly program wide.
The error message can be written before the code just as in RAII. This technique can be used for nested scopes as well.
There's less code repetition: You don't have to write try, catch, std::throw_with_nested and std::runtime_error. This makes your code more easily maintainable. If you want to change the behavior of your program you need to change your code in one place only.
The return type will be deduced automatically. So if your function foo() should return something, then you just add return before giveErrorContextOnFailure in your function foo().
In release mode there will typically be no performance panelty compared to the try-catch-way of doing things, since templates are inlined by default.
One more interesting rule to follow:
Do not use std::uncaught_exception().
There's a nice article about this topic by Herb Sutter which explains this rule perfectly. In short: If you have a function f() which is called from within a destructor during stack unwinding looking like this
void f()
{
RAII r;
bla();
}
where the destructor of RAII looks like
RAII::~RAII()
{
if ( std::uncaught_exception() )
{
// ...
}
else
{
// ...
}
}
then the first branch in the destructor will always be taken, since in the outer destructor during stack unwinding std::uncaught_exception() will always return true, even inside functions called from that destructor including the destructor of RAII.
I've encountered an example (for a trivial function-try-block), from which seems to utilize one of the more obscure of aspects of C++ syntax to which (like most obscure aspects of the language) I can not seem to find documentation for.
I've searched the Google to no avail, with all searches returning (rather uselessly) only for constructor initialization (which I'm familiar with), but what I would like to know is the importance of the body: and handler: statements in this constructor block:
class foo{
foo() try
//initalizer list that could throw
{
body:
//do stuff that could possibly throw
} catch(std::exception &e) {
handler:
throw;
}
};
The original author abruptly adds this syntax in a single example and I would like to know the implications of such linguistic constructs
The XXX: is a label. It doesn't serve any functional purpose in the code you posted. It may have been put in by the author to help them organize the code.
You can use labels with goto statements, as in goto XXX; to jump to that point in your code, although I'll leave it up to you to decide if that is a good think or now.
foo::foo()
try
{
body:
//... do stuff that could possibly throw
}
catch(std::exception &e){
handler:
throw;
}
That piece of code has two rather unusual constructs. The first is what you already pointed out and has already been explained in mjk's answer: body and handler are labels. Labels are used for gotos, and that in turn can be used for flow control (determining where the code should continue executing). Now, gotos should rarely be used, as there are very few cases where they cannot be substituted by other flow controls (if,for,while,do...) that would make it more readable. Since gotos are rarely used, labels are also rarely used.
The second interesting construct is the function-level try block:
foo::foo()
try {
} catch (std::exception const &) {
}
Note that there try catch is not inside the constructor body, but actually outside the {}. This is also a rare construct. It was designed to support catching an exception during the evaluation of the initializer list in the constructor (which in the code above is implicit). If the type has a base, or members whose constructors could throw, the constructor's body would never be evaluated and a regular try-catch would not be useful to treat that exception. The function level try block encloses also the initializer list and will catch any exception thrown during the evaluation of the different submember's constructors. It is a rarely used construct since you cannot really do much in the catch block. The exact state of what has been or has not been constructed is unknown and cannot be verified so the only possible use is to rethrow either the same exception or a different one. The syntax in a more complete example would be:
T::T()
try
: base1(args), base2(), member1(), member2() //...
{
// body
} catch (exception1 const& ex1) {
// ...
throw; // rethrow same exception
} catch (exception2 const& ex2) {
// ...
throw different_exception();
}
I know that having global variables is not appreciated. But in the book, The C++ programming language, by Bjarne Stroustrup, Author says that " The only way to gain control in case of throw from an initializer of a non local static object is set_unexpected() ". How is it done?
I posted the question in some other forums and got some answers.
First one was to declare a pointer rather than having an object and to initialize it in main()
Second one was to derive the class (whose constructor throws the exception ) from another class which performs set_terminate so as to set a meaningful handler. The second one seems to work fine in codeblocks ide.
The code I used to test check it is:
void f() //Unexpected exception handler
{
cout<<"Terminate called "<<endl;
exit (0);
}
class A //Base class that performs set_unexpected
{
terminate_handler h;
public:
A()
{
h=set_terminate(f);
}
~A()
{
set_terminate(h);
}
};
class B:public A //Derived class that throws unexpected_exception
{
public:
B()
{
throw 1;
}
};
B b;
int main()
{
}
The program displays the message: "Terminate called" and exits.
try/catch block are within or along with function scope. Thus global objects cannot reside within try/catch scope, so you can never catch those exceptions.
You can explore the special try/catch syntax:
class Test {
public:
Test ()
try { throw 0; }
catch(...) {}
};
This provides some relief in handling such exceptions. Here is one thread discussing the same.
However, you have to provide such syntax for every class for which you declare global objects
You might be wondering that what's the difference if you put try/catch inside the constructor itself? Well if a global object fails to initialize then the internal try/catch will silently absorb it, which is bad.
But the above syntax will give you a chance for error diagnosis and then again rethrow the exception which will terminate the program.
See this thread for more details.
On the other hand, std::unexpected() is called when there is no appropriate catch() for a thrown exception. This function is standard, but you can configure your own by using std::set_unexpected().
It's very easy to miss the point with error handling in C++, with exceptions, terminate, abort, and so on. The premise that regular code really fears to brake is that destructors will clean up everything constructed so far. So stack unwinding should be the focus, not the means you use for error handling, be it exceptions or something else.
To end the program in a catch in main(), usually you just return an error, or exit(EXIT_FAILURE), so after the stack unwinding you already performed, static duration variables are also destroyed normally.
With global variables that are initialized before main(), that's often all you need. You can preserve destructors functionality simply by using exit instead of throw in their constructors. Other global variables constructed so far are destroyed before the program ends, unlike with throw in a default setup. exit does not unwind the stack, but at this moment there are no destructors to be called by unwinding (except as edited below).
This might be a simpler solution for later readers with the problem in the question title.
EDIT: Besides automatic duration variables, stack unwinding also destroys completed bases and members of a class that throws on construction, which the exit solution does not cover. If you have a global with subobjects that need destruction, you still have to wrap its constructor in a try/catch, or manage its constructor failure manually. The catch in this case needs to call exit for other already constructed globals to be destroyed.
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 */
}
}
Note that this function does not have a "{" and "}" body. Just a try/catch block:
void func( void )
try
{
...
}
catch(...)
{
...
}
Is this intentionally part of C++, or is this a g++ extension?
Is there any purpose to this other than bypass 1 level of {}?
I'd never heard of this until I ran into http://stupefydeveloper.blogspot.com/2008/10/c-function-try-catch-block.html
Yes, that is valid C++. One purpose i've found for it is to translate exceptions into return values, and have the code translating the exceptions in return values separate from the other code in the function. Yes, you can return x; from a catch block like the one you showed (i've only recently discovered that, actually). But i would probably just use another level of braces and put the try/catch inside the function in that case. It will be more familiar to most C++ programmers.
Another purpose is to catch exceptions thrown by an constructor initializer list, which uses a similar syntax:
struct f {
g member;
f() try {
// empty
} catch(...) {
std::cerr << "thrown from constructor of g";
}
};
Yes, it is standard. Function try blocks, as they're called, aren't that much use for regular functions, but for constructors, they allow you to catch exceptions thrown in the initialiser list.
Note that, in the constructor case, the exception will always be rethrown at the end of any catch blocks.