c++11 - 关于在 C++11 中初始化向量

Question

在 Stroustrup 的著作《Programming: Principles and Practices of Programming Using C++(第二版)》中，作者创建了一个 struct如下:

const int not_a_reading = –7777;

struct Day {
vector<double> hour {vector<double>(24,not_a_reading)};
};
// As the author says: "That is, a Day has 24 hours, 
// each initialized to not_a_reading."

我知道 vector<double> hour{24, not_a_reading}不会这样做，因为它初始化了一个包含两个元素的向量，24 和 -7777，这不是所需的对象。

但是有什么理由说明作者的初始化技巧比仅仅做的要好:

vector<double> hour(24, not_a_reading)

(?)

Answer 1

在上面的代码中，下面是一个类(结构体)非静态数据成员hour :

vector<double> hour {vector<double>(24,not_a_reading)};

它有一个 default member initializer : {vector<double>(24,not_a_reading)}

But is there any reason why the author's initialization technique is superior to just doing:

vector<double> hour(24, not_a_reading)

是的，您将无法以这种方式编写类成员的初始化程序。您需要类(结构)定义中的花括号使其成为初始值设定项，或者您可以使用以下语法: vector<double> hour = vector<double>(24,not_a_reading);这意味着同样的事情。

#include <vector>

using namespace std;

int main()
{
    const int not_a_reading = -7777;

    struct Day {
        vector<double> hour{vector<double>(24,not_a_reading)}; // create a vector of doubles object with the constructor and then initialize hour with 24 doubles
        vector<double> hour2 = vector<double>(24,not_a_reading); // same as above
    };

    //struct Day2 {
    //  vector<double> hour(24,not_a_reading); // syntax error
    //};

    struct Day3 {
      vector<double> hour(int,int); // function declaration!
    };

    vector<double> other_hour(24,not_a_reading); // ok here
    vector<double> other_hour2(); // function declaration, most vexing parse!
    vector<double> another_hour{vector<double>(24,not_a_reading)}; // also ok here

    return 0;
}

一个可能的原因是 vector<double> hour(24,not_a_reading);不允许创建 hour object 是因为在某些情况下它可能与函数声明混淆。所谓的 most vexing parse .