default-constructor | 易学教程

Is default no-args constructor mandatory for Gson?

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Gson user guide states that we should define default no-args constructor for any class to work with Gson properly. Even more, in the javadoc on Gson's InstanceCreator class said that exception will be thrown if we try to deserialize instance of class missing default constructor and we should use InstanceCreator in such cases. However, I've tried to test use Gson with class lacking default constructor and both serialization and deserialization work without any trouble. Here is the piece of code for deserializaiton. A class without non-args constructor: public class Mushroom { private String

Why does the default parameterless constructor go away when you create one with parameters

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问题 In C#, C++ and Java, when you create a constructor taking parameters, the default parameterless one goes away. I have always just accepted this fact, but now I\'ve started wondering why. What is the reason for this behavior? Is it just a \"safety measure/guess\" saying \"If you\'ve created a constructor of your own, you probably don\'t want this implicit one hanging around\"? Or does it have a technical reason that makes it impossible for the compiler to add one once you have created a

Creating instance of type without default constructor in C# using reflection

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问题 Take the following class as an example: class Sometype { int someValue; public Sometype(int someValue) { this.someValue = someValue; } } I then want to create an instance of this type using reflection: Type t = typeof(Sometype); object o = Activator.CreateInstance(t); Normally this will work, however because SomeType has not defined a parameterless constructor, the call to Activator.CreateInstance will throw an exception of type MissingMethodException with the message \" No parameterless

How is “=default” different from “{}” for default constructor and destructor?

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问题 I originally posted this as a question only about destructors, but now I\'m adding consideration of the default constructor. Here\'s the original question: If I want to give my class a destructor that is virtual, but is otherwise the same as what the compiler would generate, I can use =default : class Widget { public: virtual ~Widget() = default; }; But it seems that I can get the same effect with less typing using an empty definition: class Widget { public: virtual ~Widget() {} }; Is there

Is default no-args constructor mandatory for Gson?

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问题 Gson user guide states that we should define default no-args constructor for any class to work with Gson properly. Even more, in the javadoc on Gson\'s InstanceCreator class said that exception will be thrown if we try to deserialize instance of class missing default constructor and we should use InstanceCreator in such cases. However, I\'ve tried to test use Gson with class lacking default constructor and both serialization and deserialization work without any trouble. Here is the piece of

Conditions for automatic generation of default/copy/move ctor and copy/move assignment operator?

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问题 I want to refresh my memory on the conditions under which a compiler typically auto generates a default constructor, copy constructor and assignment operator. I recollect there were some rules, but I don\'t remember, and also can\'t find a reputable resource online. Can anyone help? 回答1: In the following, "auto-generated" means "implicitly declared as defaulted, but not defined as deleted". There are situations where the special member functions are declared, but defined as deleted. The

Java default constructor

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问题 What exactly is a default constructor — can you tell me which one of the following is a default constructor and what differentiates it from any other constructor? public Module() { this.name = \"\"; this.credits = 0; this.hours = 0; } public Module(String name, int credits, int hours) { this.name = name; this.credits = credits; this.hours = hours; } 回答1: Neither of them. If you define it, it's not the default. The default constructor is the no-argument constructor automatically generated

Default constructor with empty brackets

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问题 Is there any good reason that an empty set of round brackets (parentheses) isn\'t valid for calling the default constructor in C++? MyObject object; // ok - default ctor MyObject object(blah); // ok MyObject object(); // error I seem to type \"()\" automatically everytime. Is there a good reason this isn\'t allowed? 回答1: Most vexing parse This is related to what is known as "C++'s most vexing parse". Basically, anything that can be interpreted by the compiler as a function declaration will be