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

boost::optional. It was specifically designed for this kind of situation.

Note, it will be included in upcoming C++14 standard as std::optional. Update: After reviewing national body comments to N3690, std::optional was voted out from C++14 working paper into a separate Technical Specification. It is not a part of the draft C++14 as of n3797.

Compared to std::unique_ptr, it avoids dynamic memory allocation, and expresses more clearly its purpose. std::unique_ptr is better for polymorphism (e.g. factory methods) and storing values in containers, however.

Usage example:

#include 
#include 
#include 

class A
{
private:
    std::string value;
public:
    A(std::string s) : value(s) {}

    boost::optional find_response(const std::string& id) const
    {
        if(id == value)
            return std::string("Found it!");
        else
            return boost::none;
        //or
        //return boost::make_optional(id == value, std::string("Found it!"));
    }

    //You can use boost::optional with references,
    //but I'm unfamiliar with possible applications of this.
    boost::optional get_id() const
    {
        return value;
    }
};

#include 

int main()
{
    A a("42");
    boost::optional response = a.find_response("42"); //auto is handy
    if(response)
    {
        std::cout << *response;
    }
}