使用了前置类型声明，但是仍然无法使用该类声明?

#include "pch.h"
#include <iostream>
using namespace std;
class B;
class A {
public:
    friend class B;
    A(int b):a(b){}
    void do_nothing() {
        cout << "B is our friend class";
        B *p;
        B value;
    }
    ~A();
private:
    int a;
};
class B {
public:
    B()= default;
    B(int d) :c(d) {}
    void do_no() {
        cout << "sss";
    }
    ~B();
private:
    int c;
};
int main() {

}

friend strBlobptr;
->
friend class strBlobptr;