C++笔记:构造函数

• 必须使用构造函数初始化列表的情况
  • 没有默认构造函数的类类型的成员 (不能创建对象赋值)
    
  • const类型的成员 (不能赋值)
  • 引用类型的成员 (必须进行初始化)
• 成员的初始化次序
  • 造函数初始化列表仅指定用于初始化成员的值,并不指定这些初始化执行的次序。
    
  • 成员被初始化的次序就是定义成员的次序
• 与构造函数内赋值操作区别
  • 构造函数中的对成员进行设置值是赋值操作,不是初始化操作,其初始化操作已经在初始化表中进行。
  • 构造函数初始化列表调用的是类相匹配的构造函数,构造函数内赋值操作首先调用类默认构造函数初始化对象,然后在调用赋值构造函数进行赋值操作.(见下面Code1.1).
  • 构造函数的执行先是初始化阶段

#include <iostream>
#include <stdio.h>
using namespace std;

class Book {
public:
Book(const string &str = "", double num = 0.0):
isbn(str),price(num) { cout << "Book::init" << endl; }
Book(const Book &book):
isbn(book.isbn),price(book.price) { cout << "Book::copy" << endl; }
Book& operator=(const Book &book)
{
cout << "Book::=" << endl;
isbn = book.isbn;
price = book.price;
return *this;
}

private:
string isbn;
double price;
};

class MyTest {
public:
MyTest(const Book &book):
books(book),num(0.0) { cout << "Mytest::init:" << endl; }

MyTest():
books("0-990-09",2.2),num(0.0) { cout << "Mytest::()" << endl; }

/*
MyTest(const Book &book)
{
cout << "Mytest::init" << endl;
books = book;
num = 0.0;
}
*/

private:
Book books;
double num;
};

int main()
{
string isbn("9-999-9999");
cout << "create book===" << endl;
Book book(isbn,1.1);
cout << "create test===" << endl;
MyTest test(book);
cout << "create test1===" << endl;
MyTest test1;

return 0;
}

create book===
Book::init
create test===
Book::copy
Mytest::init:
create test1===
Book::init
Mytest::()

构造函数内赋值操作首先调用类默认构造函数初始化对象,然后在调用赋值构造函数进行赋值操作:

修改构造函数为

MyTest(const Book &book)
{
cout << "Mytest::init" << endl;
books = book;
num = 0.0;
}

create book===
Book::init
create test===
Book::init
Mytest::init
Book::=
create test1===
Book::init
Mytest::()