C++基类带参数构造函数问题

Question

class test{
protected:
    int a;
public:
    test(int a = 0);
    ~test();
    void show();
};

class B :public test {
private:
    int b;
public:
    B(int b = 0);
    void show();
};

#include <iostream>
#include "test.h"
using namespace std;

test::test(int a) {
    cout << "a = " << a << endl;
    cout << "this a = " << this->a << endl;
    this->a = a;
    cout << "基类实例化：a : " << this->a << endl;
}

test::~test() {
    cout << "退出基类实例化" << endl;
}

void test::show() {
    cout << "a : " << a << endl;
}

B::B(int b) : test(a)
{
    cout << "派生类实例化" << endl;
    cout << "b = " << b << endl;
    cout << "this b = " << this->b << endl;
    this->b = a;
}

void B::show() {
    cout << "b : " << b << endl;
}

#include <iostream>
#include "test.h"
using namespace std;

int main() {
    B t1;
    t1.show();
    return 0;
}

请问为什么第一行的输出a不是0？
怎么改才能让第一行的输出a是0？

Answer 1

在基类声明时赋值，即：

class test{
protected:
    int a = 0;
public:
    test(int a);
    ~test();
    void show();
};