Question

每个人我都是C ++的新手并且刚刚离开C.在研究向量时我遇到了这个：

    #include <iostream>
    #include <vector>

    using namespace std;

    int main()
    {
        vector <int> g1;
        vector <int> :: iterator i;
        vector <int> :: reverse_iterator ir;

        for (int i = 1; i <= 5; i++)
            g1.push_back(i);

        cout << "Output of begin and end\t:\t";
        for (i = g1.begin(); i != g1.end(); ++i)
            cout << *i << '\t';

        cout << endl << endl;
        cout << "Output of rbegin and rend\t:\t";
        for (ir = g1.rbegin(); ir != g1.rend(); ++ir)
            cout << '\t' << *ir;

        return 0;

    }

我的问题是为什么vector <int> :: iterator i; vector <int> :: iterator i;与int i之间存在差异？

Answer 1

为什么在这里vector <int> :: iterator i;

创建vector <int> :: iterator i;是为了遍历向量vector <int> g1;（实际上它可以遍历任何vector<int>，但在这种情况下，g1为vector <int> :: iterator i ）

int i之间有区别吗？和vector <int> :: iterator i;？

main的范围是main函数。在for (int i = 1; i <= 5; i++) g1.push_back(i);内，创建了一个新范围：

在该范围内，int是循环开始时定义的i，但它在循环结束时消失，之后vector <int> :: iterator i＆＃34;返回为＆＃34; int main() { vector <int> g1; vector <int> :: iterator i; // i is the iterator vector <int> :: reverse_iterator ir; for (int i = 1; i <= 5; i++) // here i the int g1.push_back(i); // also here // from here the int i is dead, and i is back to being an iterator cout << "Output of begin and end\t:\t"; for (i = g1.begin(); i != g1.end(); ++i) cout << *i << '\t'; cout << endl << endl; cout << "Output of rbegin and rend\t:\t"; for (ir = g1.rbegin(); ir != g1.rend(); ++ir) cout << '\t' << *ir; return 0; }

{{1}}

Answer 2

vanilla C和现代C ++之间存在差异，因为for（）中声明的变量的范围和生命长度为for（）循环体。它们被允许屏蔽具有相同名称的变量，在向上范围内声明。

这个例子几乎同样有效：

int main()
{

    int i = 5, j;

    for(int i = 5; i< 10; i++)
        j = i;

    std::cout << i << " " << j;

    return 0;
}
// Output: 5 9

你问题中的代码不够优雅，并且包含不良做法。

#include <iostream>
#include <vector>
#include <algorithm> 

int main()
{
    using namespace std; // never do this in global scope

    vector <int> g1(5);  // allocating vector of 5 elements
    // nothing is wrong with for() but you can do this too, for more complex cases.
    generate(g1.begin(), g1.end(), [](){ static int i = 1; return i++; });

    cout << "Output of begin and end\t:\t";
    // so called range-based for
    for (auto value : g1 )
        cout << value << '\t';

    cout << endl << endl;
    cout << "Output of rbegin and rend\t:\t";
    // type of ir is deduced. It's not same as C99's auto  keyword
    for (auto ir = g1.rbegin(); ir != g1.rend(); ++ir)
        cout << '\t' << *ir;

    return 0;
}

在第一个for()循环中，auto关键字被推断为容器的元素类型。使用此关键字可以避免编写长嵌套类型名称，并且在模板代码中非常有用，还有typedecl

使用C ++的向量

2 个答案: