what does default constructor do when it's empty?

I wonder if anyone could explai开发者_运维问答n what the default ctor does after memory allocated, how it initializes the allocated memory?

I don't know which languange you asked the question for, but I will try to answer anyway for C++ and Java

In C++, it :

• leaves the built-in types (int, float, pointers, etc.) to an uninitialized value
• calls the default constructor on class members

In Java, I think all class members are initialized to their default value (0 or NULL).

Default constructors invoke the default constructors of all non-static data members, except those of built-in types, which remain uninitialised.

[2003: 12.1/5]:

A default constructor for a class X is a constructor of class X that can be called without an argument. If there is no user-declared constructor for class X, a default constructor is implicitly declared.

[2003: 12.1/8]:

Default constructors are called implicitly to create class objects of static or automatic storage duration (3.7.1, 3.7.2) defined without an initializer (8.5).

Have a look at this (in C and C++ implementation).

Yes, implementation varies from language to language.