c++11中新型for循环,auto, 类成员的初始化及左右值引用

标签：使用   size   version   als   void   address   默认值   test   ring   

#include <typeinfo>
#include <iostream>
#include <string>
#include <vector>
#include <iterator>

static void autoValue();
static void autoPointer();
static void newVersionFor();
static void newVersionConstruct();
static void defaultInitValue();
static void leftRefAndRightRef();

//自定义类实现新型for循环
class MyVector {
    public:
        using GroupType = std::vector<int>;
    public:
        GroupType m_group;
        void push_back(int x) {
            m_group.push_back(x);
        }
};

MyVector::GroupType::iterator begin(MyVector &v) {
    return v.m_group.begin();
}
MyVector::GroupType::iterator end(MyVector &v) {
    return v.m_group.end();
}
void testMyClass() {

    MyVector mv;
    mv.push_back(1);
    mv.push_back(2);
    mv.push_back(3);
    for(auto i : mv) {
        std::cout << i << " ";  
    }
    std::cout << std::endl;
}


int main(int /*argc*/, char** /*argv*/) {
    testMyClass();
    //new version auto
    //autoValue();
    //autoPointer();
//  newVersionFor();
    //newVersionConstruct();
//  defaultInitValue();
    leftRefAndRightRef();
    return 0;
}

static void autoValue() {
    auto age = 10;
    auto name = std::string("Yt");
    auto height = 160.0f;
    auto wight = 72.0;
    std::cout << "age is type " << typeid(age).name() << std::endl;
    std::cout << "name is type " << typeid(name).name() << std::endl;
    std::cout << "height is type " << typeid(height).name() << std::endl;
    std::cout << "weight is type " << typeid(wight).name() << std::endl;
}
static void autoPointer() {
    auto age = new int(10);
    auto name = "Yt";
    auto height = new float(160.0f);
    auto wight = new double(72.0);

    std::cout << "age is type " << typeid(age).name() << std::endl;
    std::cout << "name is type " << typeid(name).name() << std::endl;
    std::cout << "height is type " << typeid(height).name() << std::endl;
    std::cout << "weight is type " << typeid(wight).name() << std::endl;
}
static void newVersionFor() {
    int ids[] = {1, 2, 3, 4, 5};
    std::cout << "new version";
    for(auto v : ids) {
        std::cout <<  v << " "; 
    }
    std::cout << std::endl;

    std::cout << "old version";

    for(int i = 0; i < sizeof(ids) / sizeof(ids[0]); ++i) {
        std::cout << ids[i] <<  " ";    
    }
    std::cout << std::endl;

    //vector::
    std::vector<int> group;
    for(int i = 0; i < 4; ++i) group.push_back(i);
    //old version:
    //老版本遍历方式 
    for(std::vector<int>::size_type i = 0, size = group.size(); i < size; ++i) {
        //cout << group[i] << " ";  
    }
    //通过迭代器来遍历
    std::cout << "iterator: " << std::endl;
    //-------对于迭代器自增的时候,要用前置操作符,不要用后置操作符. ++iter, 若使用后置操作符号
    //多了一个步骤,就是把自增前的值先保留起来, 然后在自增.无用功
    for(std::vector<int>::const_iterator iter = group.begin(); iter != group.end(); ++iter) {
        std::cout << *iter << " ";  
    }

    std::cout << std::endl;
    //auto version
    std::cout << "vector old version:" << std::endl;
    for(auto v : group) {
        std::cout << v << " ";
    }

    std::cout << std::endl;
    std::cout << "vector new version:" << std::endl;
    for(auto &v : group) {
        v = 1;
    }
    for(const auto &v : group) {
        std::cout << v << " ";
    }
}

class A {
    public:
        //why explicit ?? 具体含义是什么?
        explicit A(int value) : m_value(value) { }

    private:
    int m_value;
};

static void newVersionConstruct() {

    A a(10);
    A b{3};

    //old
    int avector[] = {1, 2, 3};
    std::vector<int> bv;
    for(auto v : avector) bv.push_back(v);
    
    //new 
    std::vector<int> cv = {1, 2, 3};
    std::vector<int> v {1, 2, 3};

    A c(true);
    A d(false);

    A e(1.0);
    // A f{1.0} //不能做构造, 只能提高精度， 不能丢失
}

class B {
    public:
        B() : m_age(18), m_height(170), m_weight(75) {}
        explicit B(int age) : m_age(age), m_height(170), m_weight(75) {}
        B(int age, int height) : m_age(age), m_height(height) {}
        int age() const { return m_age; }
        int height() const { return m_height; }
        int weight() const { return m_weight; }
    private:
        int m_age;
        int m_height;
        int m_weight;
};

class NewB {
    public:
        NewB() {}
        explicit NewB(int age) : m_age {age} {}
        NewB(int age, int height) : m_age{age}, m_height {height} {}
        int age() const { return m_age; }
        int height() const { return m_height; }
        int weight() const { return m_weight; }
        void p() const { std::cout << m_value << " " << m_fightValue;}

    private:
        int m_age = 18; //初始化, 放置出现bug
        int m_height = 170;
        int m_weight = 75;
        int m_value{}; //代表拿int的默认值来作初始化,也就是0
        double m_fightValue{}; //0.0
};

static void defaultInitValue() {
    B a(10, 20);
    NewB b(10, 20);

    std::cout << "Old a age is" << a.age() << "height " << a.height()
        << " wight " << a.weight() << std::endl;

    std::cout << "New b age is" << b.age() << "height " << b.height()
        << " wight " << b.weight() << std::endl;

    b.p();
}

static void leftRefAndRightRef() {
    //什么是引用
    //引用类似与指针,　不同之处在于, 指针可以为nullptr, 但引用不可以
    int a = 10;
    int &refA = a;
    const int &constRefA = a;
    std::cout << " a" << a << " ref of a " << refA << " const ref a "
        << constRefA << std::endl;
    //this is a error
    // int &refB = 10;
    const int &constRefB = 10; //不管左值还是右值,都能赋给const 引用
    std::cout << "different version const ref " << constRefB << std::endl;

    //auto &&rrB = 20; //与下一样
    int &&rrB = 20;
    const int &&crrB = 20;
    
    rrB = 30; //可以改变右值, 其实只是改变那个值而已
    std::cout << " right ref of b " << rrB << " const right ref b "
        << crrB << std::endl;
    int b = 20;
    int &&newRRB = std::move(b); //强行转为右值
    const int &&cNewRRB = std::move(b);
    std::cout << " b " << b << " right ref of be " << newRRB << "const right ref b "
        << cNewRRB << std::endl;
    std::cout << "address " << &b << " ref " << &newRRB << " c ref " << &cNewRRB << std::endl;

}

c++11中新型for循环,auto, 类成员的初始化及左右值引用

标签：使用   size   version   als   void   address   默认值   test   ring   

原文地址：https://www.cnblogs.com/lyxf/p/12358350.html