NAME¶

std::rethrow_if_nested - std::rethrow_if_nested

Synopsis¶

Defined in header <exception>
template< class E > (since C++11)
void rethrow_if_nested( const E& e );

If E is not a polymorphic class type, or if std::nested_exception is an inaccessible
or ambiguous base class of E, there is no effect.

Otherwise, performs

if (auto p = dynamic_cast<const std::nested_exception*>(std::addressof(e)))
p->rethrow_nested();

Parameters¶

e - the exception object to rethrow

Return value¶

(none)

Notes¶

Unlike many related functions, this function is not intended to be called with a
std::exception_ptr but rather an actual exception reference.

Possible implementation¶

namespace details
{
template<class E>
struct can_dynamic_cast
: std::integral_constant<bool,
std::is_polymorphic<E>::value &&
(!std::is_base_of<std::nested_exception, E>::value ||
std::is_convertible<E*, std::nested_exception*>::value)
> {};

template<class T>
void rethrow_if_nested_impl(const T& e, std::true_type)
{
if (auto nep = dynamic_cast<const std::nested_exception*>(std::addressof(e)))
nep->rethrow_nested();
}

template<class T>
void rethrow_if_nested_impl(const T&, std::false_type) {}
}

template<class T>
void rethrow_if_nested(const T& t)
{
details::rethrow_if_nested_impl(t, details::can_dynamic_cast<T>());
}

Example¶

Demonstrates construction and recursion through a nested exception object.

// Run this code

#include <exception>
#include <fstream>
#include <iostream>
#include <stdexcept>
#include <string>

// prints the explanatory string of an exception. If the exception is nested,
// recurses to print the explanatory of the exception it holds
void print_exception(const std::exception& e, int level = 0)
{
std::cerr << std::string(level, ' ') << "exception: " << e.what() << '\n';
try
{
std::rethrow_if_nested(e);
}
catch (const std::exception& nestedException)
{
print_exception(nestedException, level + 1);
}
catch (...) {}
}

// sample function that catches an exception and wraps it in a nested exception
void open_file(const std::string& s)
{
try
{
std::ifstream file(s);
file.exceptions(std::ios_base::failbit);
}
catch (...)
{
std::throw_with_nested(std::runtime_error("Couldn't open " + s));
}
}

// sample function that catches an exception and wraps it in a nested exception
void run()
{
try
{
open_file("nonexistent.file");
}
catch (...)
{
std::throw_with_nested(std::runtime_error("run() failed"));
}
}

// runs the sample function above and prints the caught exception
int main()
{
try
{
run();
}
catch (const std::exception& e)
{
print_exception(e);
}
}

Possible output:¶

exception: run() failed
exception: Couldn't open nonexistent.file
exception: basic_ios::clear

See also¶

nested_exception a mixin type to capture and store current exceptions
(C++11) (class)
throw_with_nested throws its argument with std::nested_exception mixed in
(C++11) (function template)