ConceptualExample.cpp

// ===========================================================================
// ConceptualExample.cpp // Null Object Pattern
// ===========================================================================

#include <iostream>
#include <string>

/**
 * The base Component class declares common operations
 * for both real and null objects.
 */
class AbstractObject
{
public:
    virtual std::string operation() const = 0;
};

/**
 * Implementation of a real object
 */
class RealObject : public AbstractObject
{
public:
    std::string operation() const override {
        return std::string{ "computed something!" };
    }
};

/**
 * Implementation of a null object
 */
class NullObject : public AbstractObject
{
public:
    std::string operation() const override {
        return {};
    }
};

/**
 * The client code works with all of the objects via the base interface.
 * The function which requires an AbstractObject instance, will not accept null
 */
static void clientCode(const AbstractObject& obj) {

    // obj may never be null here
    std::string result{ obj.operation() };
    std::cout << "RESULT: " << result << std::endl;
}

void test_conceptual_example()
{
    RealObject realObject;
    clientCode(realObject);
    NullObject nullObject;
    clientCode(nullObject);
}

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// End-of-File
// ===========================================================================