std::rethrow_if_nested
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C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Diagnostics library
(until C++20*)
(C++17)(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
rethrow_if_nested
(C++11)
Exception handling failures
(C++11)
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)
(TM TS)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++23)
(C++23)
(C++26)
(C++26)
(C++26)
Defined in header
<exception>
template< class E >
void rethrow_if_nested( const E& e );
(since C++11) void rethrow_if_nested( const E& e );
(constexpr since C++26)
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();
[edit] Parameters
e
-
the exception object to rethrow
[edit] Notes
Unlike many related functions, this function is not intended to be called with a std::exception_ptr but rather an actual exception reference.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_constexpr_exceptions |
202411L |
(C++26) | constexpr for exception types |
[edit] 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>()); }
[edit] 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 string 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