C++11列表初始化

列表初始化:

1.旧语法中定义了初始化的几种不同形式，如下：

int data = 0; //赋值初始化

int data = {0}; //花括号初始化

int data(0); //构造初始化

int data{0}; //花括号初始化

2.C++11以旧语法中花括号初始化形式为基础，设计了列表初始化语法，统一了不同的初始化形式。

数据类型 变量{初始化列表}

#include <iostream>
#include <iterator>
using namespace std;

struct Student
{
    char name[256];
    struct Date
    {
        int year;
        int month;
        int day;
    }bday;
};

class Complex
{
public:
    Complex(double r = 0, double i = 0) :m_r(r), m_i(i) {}
    friend ostream& operator<<(ostream &os, Complex const &that)
    {
        return os << that.m_r << "+" << that.m_i << "i";
    }
    Complex(Complex const &that) :m_r(that.m_r), m_i(that.m_i)
    {
        cout << "拷贝构造函数" << &that << "->" << this << endl;
    }
private:
    double m_r, m_i;
};

int main()
{
    int a{ 123 };
    cout << a << endl;
    double b{ 3.4567 };
    cout << b << endl;
    int c[]{ 100, 200, 300 };
    copy(c, c + sizeof(c) / sizeof(c[0]), ostream_iterator<decltype(*c)>(cout, " "));
    cout << endl;
    Student d{ "张飞",{ 2000, 1, 1 } };
    cout << d.name << "," << d.bday.year << "-" << d.bday.month << "-" << d.bday.day << endl;
    Complex e{ 1.2, 3.4 };
    cout << e << endl;
    Complex *f = new Complex{ 1.2, 3.4 };
    cout << *f << endl;
    delete f;
    f = new Complex[3]{ { 1.1, 2.2 },{ 2.2, 3.3 },{ 3.3, 4.4 } };
    copy(f, f + 3, ostream_iterator<decltype(*f)>(cout, " "));
    cout << endl;
    delete[] f;
    cout << Complex{ 1.2, 3.4 } << endl;
    //Complex const (&h)[3]{{ 1.1, 2.2 }, { 2.2, 3.3 }, { 3.3, 4.4 }};
    Complex const h[3]{ { 1.1, 2.2 },{ 2.2, 3.3 },{ 3.3, 4.4 } };
    copy(h, h + 3, ostream_iterator<decltype(*h)>(cout, " "));
    cout << endl;
    Complex i = e;
    //Complex i = Complex(1.2, 3.4);
    cout << i << endl;
    return 0;
}

3. 变长初始化表，initializer_list

#include <iostream>
#include <iterator>
#include <vector>
#include <map>
using namespace std;

class A
{
public:
    A(initializer_list<int> li)
    {
        for (auto it = li.begin(); it != li.end(); ++it)
        {
            m_vi.push_back(*it);
        }
    }
    friend ostream &operator<<(ostream &os, A const &that)
    {
        copy(that.m_vi.begin(), that.m_vi.end(), ostream_iterator<decltype(that.m_vi[0])>(os, " "));
        return os;
    }
private:
    vector<int> m_vi;
};

int average(initializer_list<int> scores)
{
    if (!scores.size())
        return 0;
    int sum = 0;
    for (auto it = scores.begin(); it != scores.end(); ++it)
        sum += *it;
    return sum / scores.size();
}

int main()
{
    char const *a[]{ "张飞", "赵云", "关羽", "黄忠", "马超" };
    copy(a, a + sizeof(a) / sizeof(a[0]), ostream_iterator<decltype(a[0])>(cout, " "));
    cout << endl;
    vector<const char *> b{ "张飞", "赵云", "关羽", "黄忠", "马超" };
    copy(b.begin(), b.end(), ostream_iterator<decltype(b[0])>(cout, " "));
    cout << endl;
    map<const char *, int> c{ { "张飞", 100 },{ "赵云", 50 },{ "关羽", 25 } };
    for (auto it = c.begin(); it != c.end(); ++it)
        cout << it->first << ":" << it->second << endl;
    /*for (map<const char *, int>::iterator it = c.begin(); it != c.end(); ++it)
    cout << it->first << ":" << it->second << endl;*/
    A a1{ 1, 3, 5, 7, 9 };
    cout << a1 << endl;
    A a2{ 2,4,6,8,10 };
    cout << a2 << endl;
    int d = 60, e = 70, f = 80;
    cout << average({ d,e,f }) << endl;
    cout << average({ 50,d, e, f,90 }) << endl;
    getchar();
    return 0;
}

4.聚合类型

(4.1)任意类型的数组

(4.2)满足特定条件的类：

a 无自定义的构造函数

b 无私有或者保护的非静态成员变量

c 无基类

d 无虚函数

e 无通过“=”或者“{}”在类声明部分被初始化的非静态成员变量

(4.3)聚合类型的元素或者成员可以是聚合类型也可以是非聚合类型

(4.4)对聚合类型使用列表初始化，相当于对其中的元素逐一初始化，

而对非聚合类型使用列表初始化，相当于用列表初始化的值作为参数，调用相应的构造函数。

5.initializer_list作为轻量级的列表容器，不但可以用在构造函数中，

也可以作为普通函数的参数，传递不定数量的实参，相对于传统标准容器，

效率更高(轻量级列表容器，仅保存初始化列表元素的引用，而非其副本)

#include <iostream>
#include <iterator>
#include <list>
using namespace std;

//轻量级列表容器内部存放初始化列表元素的引用而非其拷贝
initializer_list<int> light(void)
{
    int a = 1000, b = 2000, c = 3000;
    //返回局部变量的引用将在函数返回以后失效
    return{ a,b,c };
}

//重量级容器内部存放初始化列表元素的拷贝而非其引用
list<int> heavy(void)
{
    int a = 1000, b = 2000, c = 3000;
    //所返回局部变量拷贝在函数返回后继续有效
    return{ a, b, c };
}

int main()
{
    //可以接受任意长度的初始化列表，但列表中元素的类型必须相同
    initializer_list<int> initlist{ 10, 20, 30, 40, 50, 60 };
    //initilizer_list只有三个公有成员：begin，end，size
    copy(initlist.begin(), initlist.end(), ostream_iterator<decltype(*initlist.begin())>(cout, " "));
    cout << "[" << initlist.size() << "]" << endl;

    //迭代器为只读类型，其目标元素不可修改
    /*for (auto it = initlist.begin(); it != initlist.end();++it)
    {
    *it *= 100;
    }*/

    //可以对容器整体赋值
    initlist = { 100, 200, 300 };
    copy(initlist.begin(), initlist.end(), ostream_iterator<decltype(*initlist.begin())>(cout, " "));
    cout << "[" << initlist.size() << "]" << endl;
    //提供缺省构造函数，用于实例化空容器
    initlist = {};
    copy(initlist.begin(), initlist.end(), ostream_iterator<decltype(*initlist.begin())>(cout, " "));
    cout << "[" << initlist.size() << "]" << endl;
    initlist = light();
    copy(initlist.begin(), initlist.end(), ostream_iterator<decltype(*initlist.begin())>(cout, " "));
    cout << "[" << initlist.size() << "]" << endl;
    list<int> li = heavy();
    copy(li.begin(), li.end(), ostream_iterator<decltype(*li.begin())>(cout, " "));
    cout << "[" << li.size() << "]" << endl;
    return 0;
}

6.列表初始化可以防止类型收窄，即对可能造成信息损失的类型转换，提示警告或者直接报错

long double ld = 3.1415926;

int a{ld},b{ld}; //error,转换未执行，因为存在丢失信息的危险

int a(ld),b(ld); //true,转换执行，且确实丢失了部分值