To customize how a class initializes its members, or to invoke functions when an object of your class is created, define a constructor. A constructor has the same name as the class and no return value. You can define as many overloaded constructors as needed to customize initialization in various ways. Typically, constructors have public accessibility so that code outside the class definition or inheritance hierarchy can create objects of the class. But you can also declare a constructor as protected or private.
Constructors can optionally take a member initializer list. It's a more efficient way to initialize class members than assigning values in the constructor body. The following example shows a class Box with three overloaded constructors. The last two use member init lists:
A constructor can optionally have a member initializer list, which initializes class members before the constructor body runs. (A member initializer list isn't the same thing as an initializer list of type std::initializer_list.)
Prefer member initializer lists over assigning values in the body of the constructor. A member initializer list directly initializes the members. The following example shows the member initializer list, which consists of all the identifier(argument) expressions after the colon:
If you rely on an implicit default constructor, be sure to initialize members in the class definition, as shown in the previous example. Without those initializers, the members would be uninitialized and the Volume() call would produce a garbage value. In general, it's good practice to initialize members in this way even when not relying on an implicit default constructor.
A compiler-generated default constructor will be defined as deleted if any class members aren't default-constructible. For example, all members of class type, and their class-type members, must have a default constructor and destructors that are accessible. All data members of reference type and all const members must have a default member initializer.
This statement is an example of the "Most Vexing Parse" problem. You could interpret myclass md(); either as a function declaration or as the invocation of a default constructor. Because C++ parsers favor declarations over other things, the expression is treated as a function declaration. For more information, see Most Vexing Parse.
If a class has no default constructor, an array of objects of that class can't be constructed by using square-bracket syntax alone. For example, given the previous code block, an array of Boxes can't be declared like this:
A copy constructor initializes an object by copying the member values from an object of the same type. If your class members are all simple types such as scalar values, the compiler-generated copy constructor is sufficient and you don't need to define your own. If your class requires more complex initialization, then you need to implement a custom copy constructor. For example, if a class member is a pointer then you need to define a copy constructor to allocate new memory and copy the values from the other's pointed-to object. The compiler-generated copy constructor simply copies the pointer, so that the new pointer still points to the other's memory location.
A move constructor is a special member function that moves ownership of an existing object's data to a new variable without copying the original data. It takes an rvalue reference as its first parameter, and any later parameters must have default values. Move constructors can significantly increase your program's efficiency when passing around large objects.
The compiler chooses a move constructor when the object is initialized by another object of the same type, if the other object is about to be destroyed and no longer needs its resources. The following example shows one case when a move constructor is selected by overload resolution. In the constructor that calls get_Box(), the returned value is an xvalue (eXpiring value). It's not assigned to any variable and is therefore about to go out of scope. To provide motivation for this example, let's give Box a large vector of strings that represent its contents. Rather than copying the vector and its strings, the move constructor "steals" it from the expiring value "box" so that the vector now belongs to the new object. The call to std::move is all that's needed because both vector and string classes implement their own move constructors.
If a class doesn't define a move constructor, the compiler generates an implicit one if there's no user-declared copy constructor, copy assignment operator, move assignment operator, or destructor. If no explicit or implicit move constructor is defined, operations that would otherwise use a move constructor use the copy constructor instead. If a class declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted.
You can explicitly default copy constructors, default constructors, move constructors, copy assignment operators, move assignment operators, and destructors. You can explicitly delete all of the special member functions.
If a constructor takes a std::initializer_list as its parameter, and any other parameters have default arguments, that constructor is selected in overload resolution when the class is instantiated through direct initialization. You can use the initializer_list to initialize any member that can accept it. For example, assume the Box class (shown previously) has a std::vector member m_contents. You can provide a constructor like this:
If a class has a constructor with a single parameter, or if all parameters except one have a default value, the parameter type can be implicitly converted to the class type. For example, if the Box class has a constructor like this:
Such conversions can be useful in some cases, but more often they can lead to subtle but serious errors in your code. As a general rule, you should use the explicit keyword on a constructor (and user-defined operators) to prevent this kind of implicit type conversion:
The following example shows the order in which base class and member constructors are called in the constructor for a derived class. First, the base constructor is called. Then, the base-class members are initialized in the order in which they appear in the class declaration. Finally, the derived constructor is called.
If the constructor of a base class is non-public, but accessible to a derived class, then you can't use empty braces to initialize an object of the derived type under /std:c++17 mode and later in Visual Studio 2017 and later.
In C++17, Derived is now considered an aggregate type. It means that the initialization of Base via the private default constructor happens directly, as part of the extended aggregate initialization rule. Previously, the Base private constructor was called via the Derived constructor, and it succeeded because of the friend declaration.
A delegating constructor calls a different constructor in the same class to do some of the work of initialization. This feature is useful when you have multiple constructors that all have to perform similar work. You can write the main logic in one constructor and invoke it from others. In the following trivial example, Box(int) delegates its work to Box(int,int,int):
Visual Studio 2017 and later: The using statement in /std:c++17 mode and later brings into scope all constructors from the base class except ones that have an identical signature to constructors in the derived class. In general, it's best to use inheriting constructors when the derived class declares no new data members or constructors.
Classes that contain class-type members are known as composite classes. When a class-type member of a composite class is created, the constructor is called before the class's own constructor. When a contained class lacks a default constructor, you must use an initialization list in the constructor of the composite class. In the earlier StorageBox example, if you change the type of the m_label member variable to a new Label class, you must call both the base class constructor and initialize the m_label variable in the StorageBox constructor:
Numerical example: Analog FM requires 0.2 MHz per programme. The frequency reuse factor in most countries is approximately 15 for stereo transmissions (with lesser factors for mono FM networks), meaning (in the case of stereo FM) that only one out of 15 transmitter sites can use the same channel frequency without problems with co-channel interference, i.e. cross-talk. Assuming a total availability of 102 FM channels at a bandwidth of 0.2MHz over the Band II spectrum of 87.5 to 108.0 MHz, an average of 102/15 = 6.8 radio channels are possible on each transmitter site (plus lower-power local transmitters causing less interference). This results in a system spectral efficiency of 1 / 15 / (0.2 MHz) = 0.30 programmes/transmitter/MHz. DAB with 192 kbit/s codec requires 1.536 MHz * 192 kbit/s / 1,136 kbit/s = 0.26 MHz per audio programme. The frequency reuse factor for local programmes and multi-frequency broadcasting networks (MFN) is typically 4 or 5, resulting in 1 / 4 / (0.26 MHz) = 0.96 programmes/transmitter/MHz. This is 3.2 times as efficient as analog FM for local stations. For single frequency network (SFN) transmission, for example of national programmes, the channel re-use factor is 1, resulting in 1/1/0.25 MHz = 3.85 programmes/transmitter/MHz, which is 12.7 times as efficient as FM for national and regional networks.
Leonardo da Vinci's manuscripts, written between 1487 and 1519, add up to a total of worksheets of approximately 7000 pages: most worksheets are in notebooks plus approximately fifty loose sheets distributed by various museums and libraries of various cities in Europe and America (Pooler, 2014). 2b1af7f3a8