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# std::rethrow_if_nested

From cppreference.com
< cpp‎ | error

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

If `E` is a type that is 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.

### [edit] Parameters

e | - | the exception object to rethrow

### [edit] Return value

(none)

### [edit] Example

[edit]

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

### [edit] See also

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

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