C++ 不能使用继承成员问题

C++

谁可以帮忙解答下,为什么这样做不行(代码里面注有error的方法)?

说是不能使用继承成员

如果我想实现这样的效果, 该怎么做?

#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <map>
using namespace std;  
class A {
public:
    A() {}
    ~A(){}
    virtual void printInfo();
    void printInfo1();
protected:
    virtual void printInfo2();
    void printInfo3();
    int a;
};
class B : public A {
public:
    B() {
        A::a = 5;
        a = 10;
        b = 8;
    }
    void print_a() {
        std::cout << "parentClass: " << A::a << std::endl;
        std::cout << "subClass: " << a << std::endl;
    }
    ~B(){}
protected:
    int b;
    int a;
};
// error
void B::printInfo() {
    std::cout << "info from B - 0" << std::endl;
}
// error
void B::printInfo1() {
    std::cout << "info from B - 1" << std::endl;
}
// error
void B::printInfo2() {
    std::cout << "info from B - 2" << std::endl;
}
// error
void B::printInfo3() {
    std::cout << "info from B - 3" << std::endl;
}
int main() {
    B b;
    b.print_a();
    return 0;
}