What should the return type be when the function might not have a value to return?

Question

In the old days, you might have a function like this:

const char* find_response(const char* const id) const;

If the item could not be found

Answer 1

There are several good solutions here already. But for the sake of completeness I'd like to add this one. If you don't want to rely on boost::optional you may easily implement your own class like

class SearchResult
{
    SearchResult(std::string stringFound, bool isValid = true)
        : m_stringFound(stringFound),
        m_isResultValid(isValid)
    { }

    const std::string &getString() const { return m_stringFound; }
    bool isValid() const { return m_isResultValid; }

private:
    std::string m_stringFound;
    bool m_isResultValid;
};

Obviously your method signature looks like this then

const SearchResult& find_response(const std::string& id) const;

But basically that's the same as the boost solution.