Exception handling provides a way of transferring control and information from a point in the execution of a thread to an exception handler associated with a point previously passed by the execution. A handler will be invoked only by a throw-expression invoked in code executed in the handler's try block or in functions called from the handler's try block .
try-block: try compound-statement handler-seq
function-try-block: try ctor-initializeropt compound-statement handler-seq
handler-seq: handler handler-seqopt
handler: catch ( exception-declaration ) compound-statement
exception-declaration: attribute-specifier-seqopt type-specifier-seq declarator attribute-specifier-seqopt type-specifier-seq abstract-declaratoropt ...
throw-expression: throw assignment-expressionopt
The optional attribute-specifier-seq in an exception-declaration appertains to the formal parameter of the catch clause ([except.handle]).
A try-block is a statement (Clause [stmt.stmt]). A throw-expression is of type void. Code that executes a throw-expression is said to “throw an exception;” code that subsequently gets control is called a “handler.” [ Note: Within this Clause “try block” is taken to mean both try-block and function-try-block. — end note ]
A goto or switch statement shall not be used to transfer control into a try block or into a handler. [ Example:
void f() {
goto l1; // Ill-formed
goto l2; // Ill-formed
try {
goto l1; // OK
goto l2; // Ill-formed
l1: ;
} catch (...) {
l2: ;
goto l1; // Ill-formed
goto l2; // OK
}
}
— end example ] A goto, break, return, or continue statement can be used to transfer control out of a try block or handler. When this happens, each variable declared in the try block will be destroyed in the context that directly contains its declaration. [ Example:
lab: try {
T1 t1;
try {
T2 t2;
if (condition)
goto lab;
} catch(...) { /* handler 2 */ }
} catch(...) { /* handler 1 */ }
Here, executing goto lab; will destroy first t2, then t1, assuming the condition does not declare a variable. Any exception raised while destroying t2 will result in executing handler 2; any exception raised while destroying t1 will result in executing handler 1. — end example ]
A function-try-block associates a handler-seq with the ctor-initializer, if present, and the compound-statement. An exception thrown during the execution of the compound-statement or, for constructors and destructors, during the initialization or destruction, respectively, of the class's subobjects, transfers control to a handler in a function-try-block in the same way as an exception thrown during the execution of a try-block transfers control to other handlers. [ Example:
int f(int);
class C {
int i;
double d;
public:
C(int, double);
};
C::C(int ii, double id)
try : i(f(ii)), d(id) {
// constructor statements
}
catch (...) {
// handles exceptions thrown from the ctor-initializer
// and from the constructor statements
}
— end example ]
Throwing an exception transfers control to a handler. An object is passed and the type of that object determines which handlers can catch it. [ Example:
throw "Help!";
can be caught by a handler of const char* type:
try {
// ...
}
catch(const char* p) {
// handle character string exceptions here
}
and
class Overflow {
public:
Overflow(char,double,double);
};
void f(double x) {
throw Overflow('+',x,3.45e107);
}
can be caught by a handler for exceptions of type Overflow
try {
f(1.2);
} catch(Overflow& oo) {
// handle exceptions of type Overflow here
}
— end example ]
When an exception is thrown, control is transferred to the nearest handler with a matching type ([except.handle]); “nearest” means the handler for which the compound-statement or ctor-initializer following the try keyword was most recently entered by the thread of control and not yet exited.
A throw-expression initializes a temporary object, called the exception object, the type of which is determined by removing any top-level cv-qualifiers from the static type of the operand of throw and adjusting the type from “array of T” or “function returning T” to “pointer to T” or “pointer to function returning T”, respectively. The temporary is an lvalue and is used to initialize the variable named in the matching handler ([except.handle]). If the type of the exception object would be an incomplete type or a pointer to an incomplete type other than (possibly cv-qualified) void the program is ill-formed. Except for these restrictions and the restrictions on type matching mentioned in [except.handle], the operand of throw is treated exactly as a function argument in a call ([expr.call]) or the operand of a return statement.
The memory for the exception object is allocated in an unspecified way, except as noted in [basic.stc.dynamic.allocation]. If a handler exits by rethrowing, control is passed to another handler for the same exception. The exception object is destroyed after either the last remaining active handler for the exception exits by any means other than rethrowing, or the last object of type std::exception_ptr ([propagation]) that refers to the exception object is destroyed, whichever is later. In the former case, the destruction occurs when the handler exits, immediately after the destruction of the object declared in the exception-declaration in the handler, if any. In the latter case, the destruction occurs before the destructor of std::exception_ptr returns. The implementation may then deallocate the memory for the exception object; any such deallocation is done in an unspecified way. [ Note: an exception thrown by a throw-expression does not propagate to other threads unless caught, stored, and rethrown using appropriate library functions; see [propagation] and [futures]. — end note ]
When the thrown object is a class object, the copy/move constructor and the destructor shall be accessible, even if the copy/move operation is elided ([class.copy]).
An exception is considered caught when a handler for that exception becomes active ([except.handle]). [ Note: An exception can have active handlers and still be considered uncaught if it is rethrown. — end note ]
If the exception handling mechanism, after completing evaluation of the expression to be thrown but before the exception is caught, calls a function that exits via an exception, std::terminate is called ([except.terminate]). [ Example:
struct C {
C() { }
C(const C&) { throw 0; }
};
int main() {
try {
throw C(); // calls std::terminate()
} catch(C) { }
}
— end example ]
A throw-expression with no operand rethrows the currently handled exception ([except.handle]). The exception is reactivated with the existing temporary; no new temporary exception object is created. The exception is no longer considered to be caught; therefore, the value of std::uncaught_exception() will again be true. [ Example: code that must be executed because of an exception yet cannot completely handle the exception can be written like this:
try {
// ...
} catch (...) { // catch all exceptions
// respond (partially) to exception
throw; // pass the exception to some
// other handler
}
— end example ]
If no exception is presently being handled, executing a throw-expression with no operand calls std::terminate() ([except.terminate]).
As control passes from a throw-expression to a handler, destructors are invoked for all automatic objects constructed since the try block was entered. The automatic objects are destroyed in the reverse order of the completion of their construction.
An object of any storage duration whose initialization or destruction is terminated by an exception will have destructors executed for all of its fully constructed subobjects (excluding the variant members of a union-like class), that is, for subobjects for which the principal constructor ([class.base.init]) has completed execution and the destructor has not yet begun execution. Similarly, if the non-delegating constructor for an object has completed execution and a delegating constructor for that object exits with an exception, the object's destructor will be invoked. If the object was allocated in a new-expression, the matching deallocation function ([basic.stc.dynamic.deallocation], [expr.new], [class.free]), if any, is called to free the storage occupied by the object.
The process of calling destructors for automatic objects constructed on the path from a try block to a throw-expression is called “stack unwinding.” If a destructor called during stack unwinding exits with an exception, std::terminate is called ([except.terminate]). [ Note: So destructors should generally catch exceptions and not let them propagate out of the destructor. — end note ]
The exception-declaration in a handler describes the type(s) of exceptions that can cause that handler to be entered. The exception-declaration shall not denote an incomplete type, an abstract class type, or an rvalue reference type. The exception-declaration shall not denote a pointer or reference to an incomplete type, other than void*, const void*, volatile void*, or const volatile void*.
A handler of type “array of T” or “function returning T” is adjusted to be of type “pointer to T” or “pointer to function returning T”, respectively.
A handler is a match for an exception object of type E if
The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or
the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or
the handler is of type cv1 T* cv2 and E is a pointer type that can be converted to the type of the handler by either or both of
a standard pointer conversion ([conv.ptr]) not involving conversions to pointers to private or protected or ambiguous classes
a qualification conversion
the handler is a pointer or pointer to member type and E is std::nullptr_t.
[ Note: A throw-expression whose operand is an integral constant expression of integer type that evaluates to zero does not match a handler of pointer or pointer to member type. — end note ]
[ Example:
class Matherr { /* ... */ virtual void vf(); };
class Overflow: public Matherr { /* ... */ };
class Underflow: public Matherr { /* ... */ };
class Zerodivide: public Matherr { /* ... */ };
void f() {
try {
g();
} catch (Overflow oo) {
// ...
} catch (Matherr mm) {
// ...
}
}
Here, the Overflow handler will catch exceptions of type Overflow and the Matherr handler will catch exceptions of type Matherr and of all types publicly derived from Matherr including exceptions of type Underflow and Zerodivide. — end example ]
The handlers for a try block are tried in order of appearance. That makes it possible to write handlers that can never be executed, for example by placing a handler for a derived class after a handler for a corresponding base class.
A ... in a handler's exception-declaration functions similarly to ... in a function parameter declaration; it specifies a match for any exception. If present, a ... handler shall be the last handler for its try block.
If no match is found among the handlers for a try block, the search for a matching handler continues in a dynamically surrounding try block of the same thread.
A handler is considered active when initialization is complete for the formal parameter (if any) of the catch clause. [ Note: The stack will have been unwound at that point. — end note ] Also, an implicit handler is considered active when std::terminate() or std::unexpected() is entered due to a throw. A handler is no longer considered active when the catch clause exits or when std::unexpected() exits after being entered due to a throw.
The exception with the most recently activated handler that is still active is called the currently handled exception.
If no matching handler is found, the function std::terminate() is called; whether or not the stack is unwound before this call to std::terminate() is implementation-defined ([except.terminate]).
Referring to any non-static member or base class of an object in the handler for a function-try-block of a constructor or destructor for that object results in undefined behavior.
The fully constructed base classes and members of an object shall be destroyed before entering the handler of a function-try-block of a constructor for that object. Similarly, if a delegating constructor for an object exits with an exception after the non-delegating constructor for that object has completed execution, the object's destructor shall be executed before entering the handler of a function-try-block of a constructor for that object. The base classes and non-variant members of an object shall be destroyed before entering the handler of a function-try-block of a destructor for that object ([class.dtor]).
The scope and lifetime of the parameters of a function or constructor extend into the handlers of a function-try-block.
Exceptions thrown in destructors of objects with static storage duration or in constructors of namespace-scope objects with static storage duration are not caught by a function-try-block on main(). Exceptions thrown in destructors of objects with thread storage duration or in constructors of namespace-scope objects with thread storage duration are not caught by a function-try-block on the initial function of the thread.
If a return statement appears in a handler of the function-try-block of a constructor, the program is ill-formed.
The currently handled exception is rethrown if control reaches the end of a handler of the function-try-block of a constructor or destructor. Otherwise, a function returns when control reaches the end of a handler for the function-try-block ([stmt.return]). Flowing off the end of a function-try-block is equivalent to a return with no value; this results in undefined behavior in a value-returning function ([stmt.return]).
If the exception-declaration specifies a name, it declares a variable which is copy-initialized ([dcl.init]) from the exception object. If the exception-declaration denotes an object type but does not specify a name, a temporary ([class.temporary]) is copy-initialized ([dcl.init]) from the exception object. The lifetime of the variable or temporary ends when the handler exits, after the destruction of any automatic objects initialized within the handler.
When the handler declares a non-constant object, any changes to that object will not affect the temporary object that was initialized by execution of the throw-expression. When the handler declares a reference to a non-constant object, any changes to the referenced object are changes to the temporary object initialized when the throw-expression was executed and will have effect should that object be rethrown.
A function declaration lists exceptions that its function might directly or indirectly throw by using an exception-specification as a suffix of its declarator.
exception-specification: dynamic-exception-specification noexcept-specification
dynamic-exception-specification: throw ( type-id-listopt )
type-id-list: type-id ...opt type-id-list , type-id ...opt
noexcept-specification: noexcept ( constant-expression ) noexcept
In a noexcept-specification, the constant-expression, if supplied, shall be a constant expression ([expr.const]) that is contextually converted to bool (Clause [conv]). A noexcept-specification noexcept is equivalent to noexcept(true).
An exception-specification shall appear only on a function declarator for a function type, pointer to function type, reference to function type, or pointer to member function type that is the top-level type of a declaration or definition, or on such a type appearing as a parameter or return type in a function declarator. An exception-specification shall not appear in a typedef declaration or alias-declaration. [ Example:
void f() throw(int); // OK void (*fp)() throw (int); // OK void g(void pfa() throw(int)); // OK typedef int (*pf)() throw(int); // ill-formed
— end example ] A type denoted in an exception-specification shall not denote an incomplete type. A type denoted in an exception-specification shall not denote a pointer or reference to an incomplete type, other than void*, const void*, volatile void*, or const volatile void*. A type cv T, “array of T”, or “function returning T” denoted in an exception-specification is adjusted to type T, “pointer to T”, or “pointer to function returning T”, respectively.
Two exception-specifications are compatible if:
both are non-throwing (see below), regardless of their form,
both have the form noexcept(constant-expression) and the constant-expressions are equivalent, or
both are dynamic-exception-specifications that have the same set of adjusted types.
If any declaration of a function has an exception-specification that is not a noexcept-specification allowing all exceptions, all declarations, including the definition and any explicit specialization, of that function shall have a compatible exception-specification. If any declaration of a pointer to function, reference to function, or pointer to member function has an exception-specification, all occurrences of that declaration shall have a compatible exception-specification In an explicit instantiation an exception-specification may be specified, but is not required. If an exception-specification is specified in an explicit instantiation directive, it shall be compatible with the exception-specifications of other declarations of that function. A diagnostic is required only if the exception-specifications are not compatible within a single translation unit.
If a virtual function has an exception-specification, all declarations, including the definition, of any function that overrides that virtual function in any derived class shall only allow exceptions that are allowed by the exception-specification of the base class virtual function. [ Example:
struct B {
virtual void f() throw (int, double);
virtual void g();
};
struct D: B {
void f(); // ill-formed
void g() throw (int); // OK
};
The declaration of D::f is ill-formed because it allows all exceptions, whereas B::f allows only int and double. — end example ] A similar restriction applies to assignment to and initialization of pointers to functions, pointers to member functions, and references to functions: the target entity shall allow at least the exceptions allowed by the source value in the assignment or initialization. [ Example:
class A { /* ... */ };
void (*pf1)(); // no exception specification
void (*pf2)() throw(A);
void f() {
pf1 = pf2; // OK: pf1 is less restrictive
pf2 = pf1; // error: pf2 is more restrictive
}
— end example ]
In such an assignment or initialization, exception-specifications on return types and parameter types shall be compatible. In other assignments or initializations, exception-specifications shall be compatible.
An exception-specification can include the same type more than once and can include classes that are related by inheritance, even though doing so is redundant. [ Note: An exception-specification can also include the class std::bad_exception ([bad.exception]). — end note ]
A function is said to allow an exception of type E if the constant-expression in its noexcept-specification evaluates to false or its dynamic-exception-specification contains a type T for which a handler of type T would be a match ([except.handle]) for an exception of type E.
Whenever an exception is thrown and the search for a handler ([except.handle]) encounters the outermost block of a function with an exception-specification that does not allow the exception, then,
if the exception-specification is a dynamic-exception-specification, the function std::unexpected() is called ([except.unexpected]),
otherwise, the function std::terminate() is called ([except.terminate]).
[ Example:
class X { };
class Y { };
class Z: public X { };
class W { };
void f() throw (X, Y) {
int n = 0;
if (n) throw X(); // OK
if (n) throw Z(); // also OK
throw W(); // will call std::unexpected()
}
— end example ]
[ Note: A function can have multiple declarations with different non-throwing exception-specifications; for this purpose, the one on the function definition is used. — end note ]
The function unexpected() may throw an exception that will satisfy the exception-specification for which it was invoked, and in this case the search for another handler will continue at the call of the function with this exception-specification (see [except.unexpected]), or it may call std::terminate().
An implementation shall not reject an expression merely because when executed it throws or might throw an exception that the containing function does not allow. [ Example:
extern void f() throw(X, Y);
void g() throw(X) {
f(); // OK
}
the call to f is well-formed even though when called, f might throw exception Y that g does not allow. — end example ]
A function with no exception-specification or with an exception-specification of the form noexcept(constant-expression) where the constant-expression yields false allows all exceptions. An exception-specification is non-throwing if it is of the form throw(), noexcept, or noexcept(constant-expression) where the constant-expression yields true. A function with a non-throwing exception-specification does not allow any exceptions.
An exception-specification is not considered part of a function's type.
An implicitly declared special member function (Clause [special]) shall have an exception-specification. If f is an implicitly declared default constructor, copy constructor, move constructor, destructor, copy assignment operator, or move assignment operator, its implicit exception-specification specifies the type-id T if and only if T is allowed by the exception-specification of a function directly invoked by f's implicit definition; f shall allow all exceptions if any function it directly invokes allows all exceptions, and f shall allow no exceptions if every function it directly invokes allows no exceptions. [ Example:
struct A {
A();
A(const A&) throw();
A(A&&) throw();
~A() throw(X);
};
struct B {
B() throw();
B(const B&) throw();
B(B&&) throw(Y);
~B() throw(Y);
};
struct D : public A, public B {
// Implicit declaration of D::D();
// Implicit declaration of D::D(const D&) throw();
// Implicit declaration of D::D(D&&) throw(Y);
// Implicit declaration of D::~D() throw(X, Y);
};
Furthermore, if A::~A() or B::~B() were virtual, D::~D() would not be as restrictive as that of A::~A, and the program would be ill-formed since a function that overrides a virtual function from a base class shall have an exception-specification at least as restrictive as that in the base class. — end example ]
A deallocation function ([basic.stc.dynamic.deallocation]) with no explicit exception-specification is treated as if it were specified with noexcept(true).
In a dynamic-exception-specification, a type-id followed by an ellipsis is a pack expansion ([temp.variadic]).
[ Note: The use of dynamic-exception-specifications is deprecated (see Annex [depr]). — end note ]
The functions std::terminate() ([except.terminate]) and std::unexpected() ([except.unexpected]) are used by the exception handling mechanism for coping with errors related to the exception handling mechanism itself. The function std::current_exception() ([propagation]) and the class std::nested_exception ([except.nested]) can be used by a program to capture the currently handled exception.
In some situations exception handling must be abandoned for less subtle error handling techniques. [ Note: These situations are:
when the exception handling mechanism, after completing the initialization of the exception object but before activation of a handler for the exception ([except.throw]), calls a function that exits via an exception, or
when the exception handling mechanism cannot find a handler for a thrown exception ([except.handle]), or
when the search for a handler ([except.handle]) encounters the outermost block of a function with a noexcept-specification that does not allow the exception ([except.spec]), or
when the destruction of an object during stack unwinding ([except.ctor]) terminates by throwing an exception, or
when initialization of a non-local variable with static or thread storage duration ([basic.start.init]) exits via an exception, or
when destruction of an object with static or thread storage duration exits via an exception ([basic.start.term]), or
when execution of a function registered with std::atexit or std::at_quick_exit exits via an exception ([support.start.term]), or
when a throw-expression with no operand attempts to rethrow an exception and no exception is being handled ([except.throw]), or
when std::unexpected throws an exception which is not allowed by the previously violated dynamic-exception-specification, and std::bad_exception is not included in that dynamic-exception-specifica-tion ([except.unexpected]), or
when the implementation's default unexpected exception handler is called ([unexpected.handler]), or
when the function std::nested_exception::rethrow_nested is called for an object that has captured no exception ([except.nested]), or
when execution of the initial function of a thread exits via an exception ([thread.thread.constr]), or
when the destructor or the copy assignment operator is invoked on an object of type std::thread that refers to a joinable thread ([thread.thread.destr], [thread.thread.assign]).
— end note ]
In such cases, std::terminate() is called ([exception.terminate]). In the situation where no matching handler is found, it is implementation-defined whether or not the stack is unwound before std::terminate() is called. In the situation where the search for a handler ([except.handle]) encounters the outermost block of a function with a noexcept-specification that does not allow the exception ([except.spec]), it is implementation-defined whether the stack is unwound, unwound partially, or not unwound at all before std::terminate() is called. In all other situations, the stack shall not be unwound before std::terminate() is called. An implementation is not permitted to finish stack unwinding prematurely based on a determination that the unwind process will eventually cause a call to std::terminate().
If a function with a dynamic-exception-specification throws an exception that is not listed in the dynamic-exception-specification, the function std::unexpected() is called ([exception.unexpected]) immediately after completing the stack unwinding for the former function.
[ Note: By default, std::unexpected() calls std::terminate(), but a program can install its own handler function ([set.unexpected]). In either case, the constraints in the following paragraph apply. — end note ]
The std::unexpected() function shall not return, but it can throw (or re-throw) an exception. If it throws a new exception which is allowed by the exception specification which previously was violated, then the search for another handler will continue at the call of the function whose exception specification was violated. If it throws or rethrows an exception that the dynamic-exception-specification does not allow then the following happens: If the dynamic-exception-specification does not include the class std::bad_exception ([bad.exception]) then the function std::terminate() is called, otherwise the thrown exception is replaced by an implementation-defined object of the type std::bad_exception and the search for another handler will continue at the call of the function whose dynamic-exception-specification was violated.
Thus, a dynamic-exception-specification guarantees that only the listed exceptions will be thrown. If the dynamic-exception-specification includes the type std::bad_exception then any exception not on the list may be replaced by std::bad_exception within the function std::unexpected().
The function std::uncaught_exception() returns true after completing the initialization of the exception object ([except.throw]) until completing the activation of a handler for the exception ([except.handle], [uncaught]). This includes stack unwinding. If the exception is rethrown ([except.throw]), std::uncaught_exception() returns true from the point of rethrow until the rethrown exception is caught again.