Suppose constructors were inherited… then because every class eventually derives from Object, every class would end up with a parameterless constructor. That’s a bad idea. What exactly would you expect: FileInputStream stream = new FileInputStream(); to do? Now potentially there should be a way of easily creating the “pass-through” constructors which are fairly common, but … Read more
The idea behind this message is for the sake of readability. We expect to find all the attributes an instance may have by reading its __init__ method. You may still want to split initialization into other methods though. In such case, you can simply assign attributes to None (with a bit of documentation) in the … Read more
Construct signatures in interfaces are not implementable in classes; they’re only for defining existing JS APIs that define a ‘new’-able function. Here’s an example involving interfaces new signatures that does work: interface ComesFromString { name: string; } interface StringConstructable { new(n: string): ComesFromString; } class MadeFromString implements ComesFromString { constructor (public name: string) { console.log(‘ctor … Read more
__construct was introduced in PHP5 and it is the right way to define your, well, constructors (in PHP4 you used the name of the class for a constructor). You are not required to define a constructor in your class, but if you wish to pass any parameters on object construction then you need one. An … Read more
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 interpreted as a function declaration. Another instance of the same problem: std::ifstream ifs(“file.txt”); std::vector<T> v(std::istream_iterator<T>(ifs), std::istream_iterator<T>()); v is interpreted as a declaration of function with … Read more
No, You can’t do that, the only place you can call the constructor from another constructor in C# is immediately after “:” after the constructor. for example class foo { public foo(){} public foo(string s ) { } public foo (string s1, string s2) : this(s1) {….} }
From the Java Language Specification, section 15.9.5.1: An anonymous class cannot have an explicitly declared constructor. Sorry 🙁 EDIT: As an alternative, you can create some final local variables, and/or include an instance initializer in the anonymous class. For example: public class Test { public static void main(String[] args) throws Exception { final int fakeConstructorArg … Read more
Yes, constructors can throw exceptions. Usually this means that the new object is immediately eligible for garbage collection (although it may not be collected for some time, of course). It’s possible for the “half-constructed” object to stick around though, if it’s made itself visible earlier in the constructor (e.g. by assigning a static field, or … Read more
Because there might be a standard way you want to instantiate data in the abstract class. That way you can have classes that inherit from that class call the base constructor. public abstract class A{ private string data; protected A(string myString){ data = myString; } } public class B : A { B(string myString) : … Read more
Yes, a constructor can be private. There are different uses of this. One such use is for the singleton design anti-pattern, which I would advise against you using. Another, more legitimate use, is in delegating constructors; you can have one constructor that takes lots of different options that is really an implementation detail, so you … Read more