Question

no default constructor exists for class"BASE"是什么错误?

Answer 1

是电脑术语
error #: no default constructor exists for class " "

A: Why I get this compilation error? line 30: error #2291: no default constructor exists for class "ABC"

B: Obviously the error is telling that class ABC does not have a default constructor.

A: What is default constructor?

B: Default constructor is a constructor with no argument or argument (with default value). Put it in other words, default constrcutor is a constructor that can be called with no argument. For example,

class ABC {
public:
ABC(); // Default constrcutor without argument
};

class ABC {
public:
ABC(int x=0, int y=1); // Default constructor, can be called without argument
};

A: So, we must write default constructor for a class?

B: If you have not write any constrcutor for a class, compiler will implicitly create a default constructor for the class. If you had create a constructor for the class, then its your responsibility to create a default constructor explicitly.

A: ABC class is actually a base class for DEF class. For some reason, I am unable to create a default construcor for ABC class. How do I solve the compilation error?

B: Call the base class constructor in derived class constructor. If you have not call the base class constructor explicitly, compiler automatically call the base class default constructor.

class DEF: public ABC {
public:
DEF(int x);
};

DEF::DEF(int x) : ABC(x,y), _x(x){
}

Answer 2

不知道您问的是哪方面的错误，但是句法上没有什么错误。好像是电脑术语！