# std::nested_exception

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Defined in header `<exception>` |  |
class nested_exception; |  | (since C++11)

`std::nested_exception` is a polymorphic mixin class which can capture and store the current exception, making it possible to nest exceptions of arbitrary types within each other.

### Member functions

(constructor) | constructs a nested_exception (public member function)
(destructor) [virtual] | destructs a nested exception (virtual public member function)
operator= | replaces the contents of a nested_exception (public member function)
rethrow_nested | throws the stored exception (public member function)
nested_ptr | obtains a pointer to the stored exception (public member function)

### Non-member functions

throw_with_nested (C++11) | throws its argument with std::nested_exception mixed in (function template)
rethrow_if_nested (C++11) | throws the exception from a std::nested_exception (function template)

### 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

exception_ptr (C++11) | shared pointer type for handling exception objects (typedef)
throw_with_nested (C++11) | throws its argument with std::nested_exception mixed in (function template)
rethrow_if_nested (C++11) | throws the exception from a std::nested_exception (function template)

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