cpp2.0/cpp11/auto.cpp - master - githubmirror/CPlusPlusThings

cpp2.0/cpp11/auto.cpp
103 строки3 KB
zhangxing
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30 мар 2024, 17:00
30 мар 2024, 17:003c8a3f2
//
// Created by light on 19-11-2.
//

#include <iostream>
#include <list>
#include <algorithm>
#include <vector>
#include <functional>
#include <iterator>

using namespace std;

// iterator  stl_iterator.h里面的源码 使用auto
template<typename _Iterator>
#if __cplusplus < 201103L
inline typename reverse_iterator<_Iterator>::difference_type
    operator-(const reverse_iterator<_Iterator>& __x,
          const reverse_iterator<_Iterator>& __y)
#else
inline auto
operator-(const reverse_iterator<_Iterator> &__x,
          const reverse_iterator<_Iterator> &__y)
-> decltype(__x.base() - __y.base())
#endif
{ return __y.base() - __x.base(); }


// auto 关键字(c++11)  语法糖
// for 循环  ranged-base for



class C {
public:
    explicit C(const string &s) : mystr(s) {

    }

    friend ostream &operator<<(ostream out, const C &c) {
        out << c.mystr << endl;
    }

private:
    string mystr;
};

int main() {

    auto i = 42;  // 编译器具备实参推导

    auto ll1 = [](int x) -> int {
        return x + 10;
    };
    // lambda 匿名函数
    function<int(int x)> ll = [](int x) -> int {
        return x + 10;
    };

    cout << ll(10) << endl;
    list<int> l{1, 2, 3};
    list<int>::iterator iterator;
    iterator = find(l.begin(), l.end(), 10);

    auto ite = find(l.begin(), l.end(), 11);    // auto 关键字

    vector<int> vec;
    //=====================range-based for statement=========================
    // (1)
    for (auto elem:vec) // pass  by value
        cout << elem << endl;
    // c++2.0 之前 (2)   编译器会把(1)转换为(2)或(3)
    for (auto _pos = l.begin(), _end = l.end(); _pos != _end; ++_pos)
        cout << *_pos << " ";
    cout << endl;
    // (3)
    // c++2.0之后 全局函数begin(container)与end(container) 可以接受容器
    for (auto _pos = begin(l), _end = end(l); _pos != _end; ++_pos)
        cout << *_pos << " ";
    cout << endl;

    // (4)
    for (auto &elem:vec) // pass by reference
        elem *= 3;

    // (5) 编译器会把(4)转换为(5)
    for (auto _pos = l.begin(), _end = l.end(); _pos != _end; ++_pos) {
        auto &elem = *_pos;
        elem *= 3;
    }
    cout << endl;

    vector<string> vs = {"hello", "world"};
    // 加了explicit就不能转了 error
//    for (const C &elem:vs) {
//    }
    // auto 不利：降低代码可读性、可能得不到你预想的类型、配合decltype有意想不到的结果
    // auto可能得不到你预想的类型，如vector<bool>[]的返回类型。
    vector<bool> v(true);
    auto var = v.at(0);
    cout<< typeid(var).name()<<endl;
    return 0;
}