getter.md

Class Getter reference

Description

Getter is used to read value.
Usually you should use Accessor instead of Getter.

Header

accessorpp/getter.h

Template parameters

template <typename Type>
class Getter;

Type: the underlying value type.

Constructors

Default constructor

Getter();

Default constructor. Note: if the Type is a reference, the default constructor won't compile.

Construct from data address

template <typename U>
explicit Getter(const U * address);

When reading from the getter, the getter gets value from address and cast the value to Type.
Example code,

int value = 3;
accessorpp::Getter<int> getter(&value);
// output 3
std::cout << (int)getter << std::endl;
value = 5;
// output 5
std::cout << (int)getter << std::endl;

Construct from class member address

template <typename U, typename C>
Getter(const U C::* address, const C * instance);

When reading from the getter, the getter gets value from member field instance->*address and cast the value to Type. Example code,

struct MyClass
{
    int value;
};
MyClass instance{ 8 };
accessorpp::Getter<int> getter(&MyClass::value, &instance);
// output 8
std::cout << (int)getter << std::endl;
instance.value = 9;
// output 9
std::cout << (int)getter << std::endl;

Construct from class member address, pass instance explicitly

template <typename U>
Getter(const U C::* address);

To read the value, call Getter::get(instance) with the object instance.
Example code,

struct MyClass
{
    int value;
};
MyClass instance{ 8 };
accessorpp::Getter<int> getter(&MyClass::value);
// output 8, can't use (int)getter here
std::cout << getter.get(&instance) << std::endl;
instance.value = 9;
// output 9
std::cout << getter.get(&instance) << std::endl;

Construct from constant data

Getter(const Type & value);

When reading from the getter, the getter always returns 'value', that's to say, the getter returns constant 'value'. Example code,

int n = 9;
// note n is passed by value
accessorpp::Getter<int> getter(n);
// output 9
std::cout << (int)getter << std::endl;
n = 8;
// still output 9
std::cout << (int)getter << std::endl;

Construct from function

template <typename F>
explicit Getter(F func);

'func' is a callable, such as function pointer, std::function, function object, or lambda. The prototype is Type ().
When reading from the getter, the getter returns func(). Example code,

int n = 9;
accessorpp::Getter<int> getter([&n]() -> int { return n++;  });
// output 9
std::cout << (int)getter << std::endl;
// output 10
std::cout << (int)getter << std::endl;

Construct from class member function

template <typename F, typename C>
explicit Getter(F func, C * instance);

func is a member function which prototype is Type (), instance is the object pointer.
When reading from the getter, the getter returns instance->*func(). Example code,

struct MyClass
{
    int getValue() const {
        return value;
    }
    int value;
};
MyClass instance{ 18 };
accessorpp::Getter<int> getter(&MyClass::getValue, &instance);
// output 18
std::cout << (int)getter << std::endl;
instance.value = 19;
// output 19
std::cout << (int)getter << std::endl;

Construct from class member function, pass instance explicitly

template <typename F>
explicit Getter(F func);

func is a member function which prototype is Type ().
To read the value, call Getter::get(instance) with the object instance.
Example code,

struct MyClass
{
    int getValue() const {
        return value;
    }
    int value;
};
MyClass instance{ 18 };
accessorpp::Getter<int> getter(&MyClass::getValue);
// output 18
std::cout << getter.get(&instance) << std::endl;
instance.value = 19;
// output 19
std::cout << getter.get(&instance) << std::endl;

Copy constructor

Getter(const Getter & other);

Copy constructor.

Move constructor

Getter(Getter && other);

Move constructor.

Member functions

get, operator Type

Type get(const void * instance = nullptr) const;
operator Type() const;

Both returns the underlying value.
If the getter is a class member and instance is not passed in constructor, the instance in the get function must be a valid object instance. Otherwise, instance can be nullptr.

Output streaming

template <typename Type>
std::ostream & operator << (std::ostream & stream, const Getter<Type> & getter);

Overloaded output stream operator.