Effective C++ Item 18 Make interfaces easy to use correctly and hard to use incorrectly

1. A good API will provide easy to use interfaces but also provide hard to miss-use interfaces. Usually the later one is more fundamental than that of the previous one. Consider you want to write a data class, there are thousands ways to write it. Here is one example:

class Date {
public:
    Date(int month, int day, int year);
    ...
};

Date(3, 4, 2014);
Date(4, 3, 2014);

Both are OK, but only one is logical right.
Under such situation, a better way to implement that is to constrict clients what they can do and force them to right direction:

class Month {
public:
    static Month Jan() {
        return Month(1);
    }
    ...
private:
    explicit Month(int m);
};

Data d(Month::Mar(), Day(10), Year(2011));

 
2. Let's see another example, suppose your interface returns a dynamic allocated resource and will be deleted after all. There are chances that a programmer will forget to delete it or delete it multi times. So, return a smart pointer would be a great idea.

std::tr1::shared_ptr<Investment> createInvestment() {
    std::tr1::shared_ptr<Investment> retVal(static_cast<Investment>(0),
        getRidOfInvestment());

    retVal = ...; //let retVal point to right object
    return retVal;
}

Besides, smart pointer have another advantage that it will use default or assigned deleter, and you don't need to worry about "cross-DLL problems".