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

// 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