std::rethrow_if_nested

 

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

If E is not a polymorphic class type, does nothing.

Otherwise, if dynamic_cast<const std::nested_exception&>(e) would succeed (i.e., if E or the dynamic type (the most derived type) of the object referenced by e is std::nested_exception or publicly and unambiguously derived from std::nested_exception), extracts and throws the nested exception as if by calling dynamic_cast<const std::nested_exception&>(e).rethrow_nested(). Otherwise, does nothing.

Parameters

e

-

the exception object to rethrow

Return value

(none)

Possible implementation

template <class T> void rethrow_if_nested(const T& e, std::enable_if_t<std::is_polymorphic<T>::value>* = 0) { if(auto nep = dynamic_cast<const std::nested_exception*>(&e)) nep->rethrow_nested(); }   template <class T> void rethrow_if_nested(const T&, std::enable_if_t<!std::is_polymorphic<T>::value>* = 0) { }

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& e) {
        print_exception(e, 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);
    }
}

Output:

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

See also

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