Throwing exceptions

Throwing an exception transfers control to a handler.

# Notes

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

The throw-expression is classified as prvalue expression of type void. Like any other expression, it may be a sub-expression in another expression, most commonly in the conditional operator:

# 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()
{
    throw std::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 (const std::exception& e)
{
    std::cout << "main() failed to create C with: " << e.what();
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
CWG 499	C++98	an array with unknown bound could not be thrown becauseits type is incomplete, but an exception object can becreated from the decayed pointer without any problem	apply the type completionrequirement to theexception object instead
CWG 668	C++98	std::terminate was not called if an exception is thrownfrom the destructor of a local non-automatic object	call std::terminatein this case
CWG 1863	C++11	copy constructor was not required for move-onlyexception objects when thrown, but copying allowed later	copy constructor required
CWG 1866	C++98	variant members were leaked on stack unwinding from constructor	variant members destroyed
CWG 2176	C++98	throw from a local variable destructorcould skip return value destructor	function return valueadded to unwinding
CWG 2699	C++98	throw “EX” would actually throw char* rather than const char*	corrected
CWG 2711	C++98	the source of the copy-initialization ofthe exception object was not specified	copy-initializedfrom expression
CWG 2775	C++98	the exception object copy-initialization requirement was unclear	made clear
CWG 2854	C++98	the storage duration of exception objects was unclear	made clear
P1825R0	C++11	implicit move from parameters was forbidden in throw	allowed

Throwing exceptions

# Notes

# Example

# Defect reports

# See also