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std::rethrow_if_nested

Defined in header <exception>
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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

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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.

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#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:

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

See also

(C++11)
 a mixin type to capture and store current exceptions
(class)
(C++11)
 throws its argument with std::nested_exception mixed in
(function template)
doc_CPP
doc_CPP
2025-01-10 15:47:30
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