C++11 introduces the possibility of Delegating Constructors (aka Constructor Delegation) that can be used by C++ in 2023. In the Constructor Delegation feature, class constructors can be invoked within other constructors of the same class. This is a very useful feature that helps programmers to write less and more expressive code. Constructor Delegation reduce the repetitive code by adding a function that does all of the validation, in this method one constructor could delegate some of the work to another construction method.
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What Is a constructor in Modern C++?
Object-Oriented Programming (OOP) is a way to represent data and functionality with programming constructs we call objects. Objects can contain data in the form (attributes or properties of objects), and code blocks in the form of procedures (methods, functions of objects). These attributes and methods are variables and functions that belong to the class (the collective name for the object’s code), they are generally referred to as a class member. Constructors are one of the members of classes.
Classes are defined in C++ using the keyword class followed by the name of the class. Classes are the blueprint for the objects, and they are user-defined data types that we can use in our program.
Objects are an instantiation of a class. In C++ programming, most of the commands are associated with classes and objects, along with their attributes and methods.
The Constructor in C++ is a function, a method in the class, but it is a ‘special method’ that is automatically called when an object of a class is created. We don’t need to call this function; it gets called automatically. Whenever a new object of a class is created, the Constructor allows the class to initialize member variables or allocate storage. This is why the name Constructor is given to this special method.
If you are new to OOP and want to know more about Constructs please start with this two posts.
We have many different constructor examples in C++, please check this dynamic search for more examples: https://learncplusplus.org/page/2/?s=constructor
What are delegating constructors in Modern C++?
The Delegating Constructors (aka Constructor Delegation) come with the C++11 standard and later. In Constructor Delegation, class constructors can be invoked within other constructors of the same class. This is a very useful feature that helps programmers to write less and more expressive code.
In C++ you may have different constructor methods in a class. Constructor Delegation reduce the repetitive code by adding another constructor that does all of the validation, in this method one constructor could delegate some of the work to another construction method.
For more information about this feature, see Delegating Constructors (revision 3) Delegating constructors Proposal document [PDF link].
Is there a simple delegating constructor example in modern C++?
The following example shows how delegating constructors can be used in modern C++.
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class B { B(int newA) : a(newA) { /* other initializations */ } public: B() : B(15) { /* other initializations */ } public: int a; }; |
Is there full delegating constructor examples in modern C++?
This example is good to understand how you can delegate some of the work to another construction method.
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class Tdata { public: Tdata() { std::cout << "reading ..." << std::endl; } Tdata(int x) : Tdata() // Delegating constructor above { std::cout << "analyzing with method " << x << std::endl; }; }; int main() { Tdata mydat; // only read in construction Tdata mydata(7); // read and analyze in construction } |
Here Tdata() constructor is reading data in construction, and the Tdata(int x) constructor delegate Tdata() constructor, reads and analyses in its construction.
Delegation can be as below too.
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#include <iostream> #include <string> class Tdimension { public: Tdimension( std::string s, float x, float y, float z) { std::cout << s << "," << x << "," << y << "," << z << "," << std::endl; } Tdimension(float x) : Tdimension( "1D", x, 0, 0){}; Tdimension(float x, float y) : Tdimension( "2D", x, y, 0){}; Tdimension(float x, float y, float z) : Tdimension( "3D", x, y, z){}; }; int main() { Tdimension d1(5); Tdimension d2(9, 3); Tdimension d3(3, 15, 12); Tdimension d4("3 Dimensional", 7, 21, 13); system("pause"); return 0; } |
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