C++ 读书笔记2

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Linux 下C++学习笔记1

string 表示可变长的字符序列，vector 存放的是某种给定类型对象的可变序列。

C++ 标准要求，vector 应该能在运行时搞笑快速的添加元素。因此既然vector 对象能高效的增长，那么在定义vector对象时，设定其大小也就没有什么必要了，事实上如果这么做，性能可能会更差。

范围for 语句体内不应该改变其所遍历对象序列的大小

#include <iostream>
#include <string>
#include <vector>
using std::cin;
using std::cout;
using std::endl;
using std::string;
using std::vector;
vector<int> test_int;
int main()
{
    for (int i=0; i < 100; ++i) {
        test_int.push_back(i);
    }
    for (int j : test_int) {
        cout << j << endl;
    }
    return 0;
}

不能将数组的内容拷贝给其他数组作为初始值，也不能用数组为其他数组赋值。

指针加上一个整数所得的结果还是一个指针

使用数组的时候其实真正用的是指向数组首元素的指针。

对于大多数应用来说，使用标准库string要比使用C风格字符串更加安全、更加高效

现代的C++ 程序应当尽量使用vector 和迭代器，避免使用内置数组和指针；应该尽量使用string，避免使用C风格的基于数组的字符串。

严格来说，C++中没有多维数组，通常所说的多维数组其实是数组的数组。谨记这一点，对今后理解和使用多维数组大有益处。

//遍历多维数组
int main()
{
    //大小为1的数组，每个数组中含有2个数组元素。
    //每个数组元素中含有5个整型值
    int test_int[1][2][5] = {
            {
                    {
                            {1},{2},{3},{4},{5}
                    },
                    {
                            {6},{7},{8},{9},{10}
                    },
            }
    };
    //遍历多维数组
    for (auto &a : test_int) {
        for (auto &b : a) {
            for (auto c : b) {
                cout << c << endl;
            }
        }
    }
   return 0;
}

要使用范围for语句处理多维数组，除了最内层的循环外，其他所有的循环控制变量都应该是引用类型。

int *ip[4];   // 是个数组，整形指针的数组，含有4个int型指针
int (*ip)[4]; // 是个指针，指向含有4个整数的的数组

可以在if switch 、while 、和for 语句的控制结构内定义变量。定义在控制结构当中的变量只在相应的语句内部可见，一旦语句结束，变量也就超出其作用范围了。

PHP 代码

roverliang$ cat demo.php
<?php
for($i=0; $i< 100; $i++ ) {
//do some thing;
}
echo $i.PHP_EOL;
roverliang$ php demo.php
100
roverliang$ 

C++ 代码

int main()
{
   for (int i=0; i < 100; ++i) {
       //
   }
    cout << i << endl;
   return 0;
}
//error: use of undeclared identifier 'i'
//cout << i << endl;
            ^