Throwing anexception transfers control to ahandler.

An exception can be thrown fromthrow expressions, the following contexts may also throw exceptions:

• allocation functions
• dynamic_cast
• typeid
• new expressions
• standard library functions

[edit]Exception object

Throwing an exception initializes an object with dynamicstorage duration, called theexception object.

If the type of the exception object would be one of the following types, the program is ill-formed:

• anincomplete type
• anabstract class type
• a pointer to an incomplete type other than (possibly cv-qualified)void

[edit]Constructing and destructing exception objects

Given the type of the exception object asT:

• Letobj be an lvalue of typeconst T, thecopy-initialization of an object of typeT fromobj must be well-formed.
• IfT is a class type:

• The selectedconstructor isodr-used.
• Thedestructor ofT ispotentially invoked.

The memory for the exception object is allocated in an unspecified way. The only guarantee is that the storage will never be allocated by globalallocation functions.

If ahandler exits byrethrowing, control is passed to another handler for the same exception object. The exception object is not destructed in this case.

[edit]throw expressions

When a new exception is thrown, its exception object is determined as follows:

1. Thearray-to-pointer andfunction-to-pointer standard conversions are performed onexpression .
2. Letex be the conversion result:

• The type of the exception object is determined by removing any top-level cv-qualifiers from the type ofex.
• The exception object iscopy-initialized fromex.

If a program attempts to rethrow an exception when no exception is presently being handled,std::terminate will be invoked. Otherwise, the exception is reactivated with the existing exception object (no new exception object is created), and the exception is no longerconsidered to be caught.

try{// throwing a new exception 123throw123;}catch(...)// catch all exceptions{// respond (partially) to exception 123throw;// pass the exception to some other handler}

[edit]Stack unwinding

Once the exception object is constructed, the control flow works backwards (up the call stack) until it reaches the start of atry block, at which point the parameters of all associated handlers are compared, in order of appearance, with the type of the exception object to find amatch. If no match is found, the control flow continues to unwind the stack until the nexttry block, and so on. If a match is found, the control flow jumps to the matching handler.

As the control flow moves up the call stack, destructors are invoked for all objects withautomatic storage duration that are constructed, but not yet destroyed, since the correspondingtry block was entered, in reverse order of completion of their constructors. If an exception is thrown from a destructor of a local variable or of a temporary used in areturn statement, the destructor for the object returned from the function is also invoked.

If an exception is thrown from a constructor or (rare) from a destructor of an object (regardless of the object's storage duration), destructors are called for all fully-constructed non-static non-variant members and base classes, in reverse order of completion of their constructors. Variant members ofunion-like classes are only destroyed in the case of unwinding from constructor, and if the active member changed between initialization and destruction, the behavior is undefined.

If the exception is thrown from a constructor that is invoked by anew-expression, the matchingdeallocation function is called, if available.

This process is calledstack unwinding.

If any function that is called directly by the stack unwinding mechanism, after initialization of the exception object and before the start of the exception handler, exits with an exception,std::terminate is called. Such functions includedestructors of objects with automatic storage duration whose scopes are exited, and the copy constructor of the exception object that is called (if notelided) to initialize catch-by-value arguments.

If an exception is thrown and not caught, including exceptions that escape the initial function ofstd::thread, the main function, and the constructor or destructor of any static or thread-local objects, thenstd::terminate is called. It is implementation-defined whether any stack unwinding takes place for uncaught exceptions.

[edit]Notes

When rethrowing exceptions, the second form must be used to avoid object slicing in the (typical) case where exception objects use inheritance:

try{std::string("abc").substr(10);// throws std::out_of_range}catch(conststd::exception& e){std::cout<< e.what()<<'\n';//  throw e; // copy-initializes a new exception object of type std::exceptionthrow;// rethrows the exception object of type std::out_of_range}

Thethrow-expression is classified asprvalue expression of typevoid. Like any other expression, it may be a sub-expression in another expression, most commonly in theconditional operator:

double f(double d){return d>1e7?throwstd::overflow_error("too big"): d;} int main(){try{std::cout<< f(1e10)<<'\n';}catch(conststd::overflow_error& e){std::cout<< e.what()<<'\n';}}

[edit]Keywords

throw

[edit]Example

#include <iostream>#include <stdexcept> struct A{int n;     A(int n=0): n(n){std::cout<<"A("<< n<<") constructed successfully\n";}    ~A(){std::cout<<"A("<< n<<") destroyed\n";}}; int foo(){throwstd::runtime_error("error");} struct B{    A a1, a2, a3;     B()try: a1(1), a2(foo()), a3(3){std::cout<<"B constructed successfully\n";}catch(...){std::cout<<"B::B() exiting with exception\n";}     ~B(){std::cout<<"B destroyed\n";}}; struct C: A, B{    C()try{std::cout<<"C::C() completed successfully\n";}catch(...){std::cout<<"C::C() exiting with exception\n";}     ~C(){std::cout<<"C destroyed\n";}}; int main()try{// creates the A base subobject// creates the a1 member of B// fails to create the a2 member of B// unwinding destroys the a1 member of B// unwinding destroys the A base subobject    C c;}catch(conststd::exception& e){std::cout<<"main() failed to create C with: "<< e.what();}

A(0) constructed successfullyA(1) constructed successfullyA(1) destroyedB::B() exiting with exceptionA(0) destroyedC::C() exiting with exceptionmain() failed to create C with: error

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]References

[edit]See also

• copy elision
• try block
• handler
• noexcept specifier
• Exception handling

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