在 C++ 中将成员初始化为 NULL

Question

I need to instantiate a class A without initializing its member B which must be initialized later passing special parameters to its constructor. I am trying to pass a NULL but an error is returned, here is the SSCCE:

#include<stdio.h>

class B
{
    private:
        int b;
        int c;
    public:
        B(int b, int c){this->b = b; this->c = c;};
};

class A
{
    private:
        B b;
    public:
        A(): b(NULL)
        {};
};

int main()      //main function declaration
{
    A a = A();
    return 0;       //terminating function
}

the returned error is:

g++ test.c 
test.c: In constructor ‘A::A()’:
test.c:17:14: error: no matching function for call to ‘B::B(NULL)’
   17 |   A(): b(NULL)
      |              ^
test.c:9:3: note: candidate: ‘B::B(int, int)’
    9 |   B(int b, int c){this->b = b; this->c = c;};
      |   ^
test.c:9:3: note:   candidate expects 2 arguments, 1 provided
test.c:3:7: note: candidate: ‘constexpr B::B(const B&)’
    3 | class B
      |       ^
test.c:3:7: note:   no known conversion for argument 1 from ‘long int’ to ‘const B&’
test.c:3:7: note: candidate: ‘constexpr B::B(B&&)’
test.c:3:7: note:   no known conversion for argument 1 from ‘long int’ to ‘B&&’

Answer 1

You class A aggregates a B . Each instance of A has a B , always. It's not meaning to set an A 's B to null .

Want you may want is for the B member of an A to be default initialized. You can do that by providing a default constructor for B :

class B
{
private:
    int b;
    int c;
public:
    B():b(0), c(0) {};
    B(int b, int c){this->b = b; this->c = c;};
};

Or even just provide default initializer in the class itself:

class B
{
private:
    int b=0;
    int c=0;
public:
    B() {};
    B(int b, int c){this->b = b; this->c = c;};
};

in both case, you then have, simply:

    A(){};

Answer 2

If you can compile with c++17 I will recommend using std::optional , otherwise you can consider having a default constructor for B , B() = default; or dynamic allocation together with checking for nullptr

Answer 3

you are only passing a single attribute right? try with 2 nulls

its just my suggestion,

usually that error comes ,when you intialise with wrong attributes

  A(): b(NULL,NULL)