类构造函数初始化列表

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表现形式有三种

1. 当前类进行初始化,尤其是当成员变量是一个const或者一个引用类型

class Demo
{
    private:
        int b;
    public:
        Demo(int a):b(a){};
}

2. 需要在初始化列表中调用外部类成员的初始化函数

#include <iostream>
using namespace std;
class C1
{
    private:
        int a;
        int b;
        int c;
    public:
        C1(int a,int b,int c)
        {
            cout << "Class 1 construct function" << endl;    
        };
};

class C2
{
    private:
        C1 c1Obj;
    public:
        C2():c1Obj(1,2,3)//这里用的是类1对象的
        {
            cout << "Class 2 construct function" << endl;
        };
};

int main()
{
    C2 c2Obj;
    return 1;
}

最后打印的结果是

Class 1 construct function
Class 2 construct function

3. 派生类要在初始化列表中调用父类的构造函数

#include <iostream>
using namespace std;

class Base
{
    public:
        Base(int a) : val(a) {}
    private:
        int val;
};

class Derived : public Base
{
    public:
    	//多个参数用逗号隔开
        Derived(int v) : p(v), Base(v) {}
        void print_val() { cout << "hello:" << p << endl;}
    private:
        int p;
};

int main(int argc ,char **argv)
{
    int pp = 45;
    Derived b(pp);
    b.print_val();
}

打印结果如下

hello:45