std:: uncaught_exception, std:: uncaught_exceptions

Sometimes it's safe to throw an exception even while std :: uncaught_exception ( ) == true (until C++17) std :: uncaught_exceptions ( ) > 0 (since C++17) . For example, if stack unwinding causes an object to be destructed, the destructor for that object could run code that throws an exception as long as the exception is caught by some catch block before escaping the destructor.

Parameters

(none)

Return value

Notes

An example where int-returning uncaught_exceptions is used is the boost.log library: the expression BOOST_LOG ( logger ) << foo ( ) ; first creates a guard object and records the number of uncaught exceptions in its constructor. The output is performed by the guard object's destructor unless foo ( ) throws (in which case the number of uncaught exceptions in the destructor is greater than what the constructor observed).

std::experimental::scope_fail and std::experimental::scope_success in LFTS v3 rely on the functionality of uncaught_exceptions , because their destructors need to do different things that depend on whether is called during stack unwinding.

Example

#include <exception>
#include <iostream>
#include <stdexcept>
 
struct Foo
{
    char id{'?'};
    int count = std::uncaught_exceptions();
 
    ~Foo()
    {
        count == std::uncaught_exceptions()
            ? std::cout << id << ".~Foo() called normally\n"
            : std::cout << id << ".~Foo() called during stack unwinding\n";
    }
};
 
int main()
{
    Foo f{'f'};
 
    try
    {
        Foo g{'g'};
        std::cout << "Exception thrown\n";
        throw std::runtime_error("test exception");
    }
    catch (const std::exception& e)
    {
        std::cout << "Exception caught: " << e.what() << '\n';
    }
}

Exception thrown
g.~Foo() called during stack unwinding
Exception caught: test exception
f.~Foo() called normally

Defect reports

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

See also

External links