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As the evaluate_rpn function accepts iterators, it is very easy to feed it with different inputs than the standard input stream. This makes it very easy to test, or to adapt to different sources of user input.

Feeding it with iterators from a string stream or from a string vector, for example, looks like the following code, for which evaluate_rpn does not have to be changed at all:

int main()
{
    stringstream s {"3 2 1 + * 2 /"};
    cout << evaluate_rpn(istream_iterator<string>{s}, {}) << '\n';

    vector<string> v {"3", "2", "1", "+", "*", "2", "/"};
    cout << evaluate_rpn(begin(v), end(v)) << '\n';
}