列表初始化:
1.舊語法中定義了初始化的幾種不同形式，如下：
int data = 0; //賦值初始化
int data = {0}; //花括號初始化
int data(0); //構造初始化
int data{0}; //花括號初始化

2.C++11以舊語法中花括號初始化形式為基礎，設計了列表初始化語法，統一了不同的初始化形式。
數據類型 變量{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 << "拷貝構造函數" << &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{ "張飛",{ 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. 變長初始化表，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[]{ "張飛", "趙云", "關羽", "黃忠", "馬超" };copy(a, a + sizeof(a) / sizeof(a[0]), ostream_iterator<decltype(a[0])>(cout, " "));cout << endl;vector<const char *> b{ "張飛", "趙云", "關羽", "黃忠", "馬超" };copy(b.begin(), b.end(), ostream_iterator<decltype(b[0])>(cout, " "));cout << endl;map<const char *, int> c{ { "張飛", 100 },{ "趙云", 50 },{ "關羽", 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; }

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4.聚合類型
(4.1)任意類型的數組
(4.2)滿足特定條件的類：
a 無自定義的構造函數
b 無私有或者保護的非靜態成員變量
c 無基類
d 無虛函數
e 無通過“=”或者“{}”在類聲明部分被初始化的非靜態成員變量
(4.3)聚合類型的元素或者成員可以是聚合類型也可以是非聚合類型
(4.4)對聚合類型使用列表初始化，相當于對其中的元素逐一初始化，
而對非聚合類型使用列表初始化，相當于用列表初始化的值作為參數，調用相應的構造函數。

5.initializer_list作為輕量級的列表容器，不但可以用在構造函數中，
也可以作為普通函數的參數，傳遞不定數量的實參，相對于傳統標準容器，
效率更高(輕量級列表容器，僅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;//輕量級列表容器內部存放初始化列表元素的引用而非其拷貝 initializer_list<int> light(void) {int a = 1000, b = 2000, c = 3000;//返回局部變量的引用將在函數返回以后失效return{ a,b,c }; }//重量級容器內部存放初始化列表元素的拷貝而非其引用 list<int> heavy(void) {int a = 1000, b = 2000, c = 3000;//所返回局部變量拷貝在函數返回后繼續有效return{ a, b, c }; }int main() {//可以接受任意長度的初始化列表，但列表中元素的類型必須相同initializer_list<int> initlist{ 10, 20, 30, 40, 50, 60 };//initilizer_list只有三個公有成員：begin，end，sizecopy(initlist.begin(), initlist.end(), ostream_iterator<decltype(*initlist.begin())>(cout, " "));cout << "[" << initlist.size() << "]" << endl;//迭代器為只讀類型，其目標元素不可修改/*for (auto it = initlist.begin(); it != initlist.end();++it){*it *= 100;}*///可以對容器整體賦值initlist = { 100, 200, 300 };copy(initlist.begin(), initlist.end(), ostream_iterator<decltype(*initlist.begin())>(cout, " "));cout << "[" << initlist.size() << "]" << endl;//提供缺省構造函數，用于實例化空容器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; }

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6.列表初始化可以防止類型收窄，即對可能造成信息損失的類型轉換，提示警告或者直接報錯
long double ld = 3.1415926;
int a{ld},b{ld}; //error,轉換未執行，因為存在丟失信息的危險
int a(ld),b(ld); //true,轉換執行，且確實丟失了部分值

轉載于:https://www.cnblogs.com/LuckCoder/p/8467656.html

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