C++11特性（5）：explicit、列表初始化

#include <iostream>
#include <vector>
#include <map>

//1、显示转化操作符explicit
struct Rational1
{
//没有explicit修饰的构造函数可以隐式调用，可以将某种类型隐式转化为类类型
Rational1(int n = 0, int d = 1):num(n), den(d)
{
std::cout << "Rational(" << n << "/" << d << ")" << std::endl;
}
int num;      //被除数
int den;      //除数
};

struct Rational2
{
//而使用explicit修饰的构造函数则禁止隐式调用，禁止将某种类型隐式转化为类类型
explicit Rational2(int n = 0, int d = 1) :num(n), den(d)
{
std::cout << "Rationa2(" << n << "/" << d << ")" << std::endl;
}
int num;      //被除数
int den;      //除数
};

void Display1(Rational1 r)
{
std::cout << "Numerator: " << r.num << ",Denominator: " << r.den << std::endl;
}

void Display2(Rational2 r)
{
std::cout << "Numerator: " << r.num << ",Denominator: " << r.den << std::endl;
}

//C++11标准中，将explicit使用范围扩展到自定义的类型转换操作符上，以支持所谓的“显式类型转换”
class ConvertTo{};
class Convertable
{
public:
//定义了一个显式转换到ConvertTo类型的类型转换符
explicit operator ConvertTo () const{ return ConvertTo(); }
};

void Func(ConvertTo ct){}
void ExplicitTest()
{
Convertable c;
ConvertTo ct(c);                                                //直接编译通过
ConvertTo ct2 = c;                                          //拷贝构造初始化，编译失败
ConvertTo ct3 = static_cast<ConvertTo>(c);    //强制转化，ok
Func(c);                                                            //拷贝构造初始化，编译失败
}

//2、列表初始化
//通过initializer_list来实现列表初始化
#include <initializer_list>
#include <vector>

enum Gender{ male, female };
class Person
{
public:
Person(std::initializer_list<std::pair<Gender, std::string>> ls)
{
auto lse = ls.begin();
/*for (; lse != ls.end(); ++lse)
pl.push_back(*lse);*/
std::for_each(ls.begin(), ls.end(), [&](std::pair<Gender, std::string> pe){pl.push_back(pe); });
}
std::vector<std::pair<Gender, std::string>> pl;
};

void Fun(std::initializer_list<int> ls){}

#include <algorithm>     //for  for_each()
class Mydata
{
public:
Mydata& operator[](std::initializer_list<int> ls)
{
std::for_each(ls.begin(), ls.end(), [&](const int e){idx.push_back(e); });
/*for (auto i = ls.begin(); i != ls.end(); ++i)
idx.push_back(*i);*/
return *this;
}
Mydata& operator=(int v)
{
if (idx.empty() != true)
{
for (auto i = idx.begin(); i != idx.end(); ++i)
{
id.resize((*i > id.size() ? *i : id.size()));
id[*i - 1] = v;
}
idx.clear();
}
return *this;
}
void Print()
{
std::for_each(id.begin(), id.end(), [](const int i) {std::cout << i << " "; });
std::cout << std::endl;
}
public:
std::vector<int> idx;
std::vector<int> id;
};

int main()
{
Rational1 r1_1 = 11;           //Rational1(11/1)
Rational1 r1_2(12);             //Rational1(12/1)
Rational2 r2_1 = 12;           //无法通过编译
Rational2 r2_2(5);               //Rational2(5/1)
Display1(1);                        //Rational1(1/1)
Display2(2);                        //无法通过编译
Display2(Rational2(3));      //Rational2(3/1)

//列表初始化,在｛｝之间的=号，加与不加效果相同
int a[]{1, 2, 4};     // int a[] = {1,2,4};
std::vector<int> v{ 1, 2, 3 };
std::map<int, float> m = { { 1, 1.0f }, { 2, 2.0f } };
int c = { 3 + 4 };     //int c{3 + 4};     等于 7

//通过initializer_list实现列表初始化
//例子1
Person p = { { male, "Tom" }, { female, "Kitty" } };

//例子2
Fun({ 1, 2 });      //列表元素｛1，2｝
Fun({});           //空列表

//例子3
Mydata d;
d[{2, 3, 5}] = 7;
d[{1, 4, 6, 9}] = 4;
d.Print();            //4 7 7 4 7 4 0 0 4

return 0;
}

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