# Exceptions and Cancellation

oneAPI Threading Building Blocks (oneTBB) supports exceptions and cancellation. When code inside an oneTBB algorithm throws an exception, the following steps generally occur:

1. The exception is captured. Any further exceptions inside the algorithm are ignored.

2. The algorithm is cancelled. Pending iterations are not executed. If there is oneTBB parallelism nested inside, the nested parallelism may also be cancelled as explained in Cancellation and Nested Parallelism.

3. Once all parts of the algorithm stop, an exception is thrown on the thread that invoked the algorithm.

As compilers evolve to support this functionality, future versions of oneTBB might throw the original exception. So be sure your code can catch either type of exception. The following example demonstrates exception handling:

    #include "oneapi/tbb.h"
    #include <vector>
    #include <iostream>

    using namespace oneapi::tbb;
    using namespace std;

    vector<int> Data;

    struct Update {
        void operator()( const blocked_range<int>& r ) const {
            for( int i=r.begin(); i!=r.end(); ++i )
                Data.at(i) += 1;
        }
    };

    int main() {
        Data.resize(1000);
        try {
            parallel_for( blocked_range<int>(0, 2000), Update());
        } catch( out_of_range& ex ) {
           cout << "out_of_range: " << ex.what() << endl;
        }
        return 0;
    }

The `parallel_for` attempts to iterate over 2000 elements of a vector with only 1000 elements. Hence the expression `Data.at(i)` sometimes throws an exception `std::out_of_range` during execution of the algorithm. When the exception happens, the algorithm is cancelled and an exception thrown at the call site to `parallel_for`.

- Cancellation Without An Exception
- task_group Cancellation Example
- Cancellation and Nested Parallelism