构造函数是否必须在C ++中初始化成员变量？

Question

I have learned that constructors are used to initialize the member variables of a class in C++. In all the examples I have seen the constructors initialize all the member variables of the class when called.

What happens if I write a constructor that only initializes some or none of the member variables?

Answer 1

It really depends on what member variables you have. If you provide a constructor and don't explicitly initialize a variable in the member initialization list, then it will be default initialized . And this is for every variable.

Now, default initialization does something else depending on what variable you have. If you have a builtin type, like int or bool , then it will not be initialized to 0 or any other value, just like if you had:

int value; // it has an indeterminate value

This also applies to arrays. If it is another class, then the default constructor of that class will be called, just like if you had:

struct Foo { /*something*/ };
Foo value; // calls default constructor, i.e. initializes object

Answer 2

it's fine.. you can also initialize your member variables in your member functions, then, just call the function in the constructor.. the important thing is to don't forget to initialize variables before using them..

in short.. it's fine to not initialize your member variables in the constructor as long as you initialize them somewhere in the class before using them..

Answer 3

You are allowed to do that. It isn't great practise though as you'll have a series of uninitialised member variables that could produce unexpected results

Answer 4

Depends on the class and your program. Does your class and program needs all of those variables in all of the cases. if your class does require them then no errors or exception will occur otherwise your program might crash or something.