没有默认构造函数的对象数组初始化 [英] Object array initialization without default constructor

问题描述

#include <iostream>
class Car
{
private:
  Car(){};
  int _no;
public:
  Car(int no)
  {
    _no=no;
  }
  void printNo()
  {
    std::cout<<_no<<std::endl;
  }
};
void printCarNumbers(Car *cars, int length)
{
    for(int i = 0; i<length;i++)
         std::cout<<cars[i].printNo();
}

int main()
{
  int userInput = 10;
  Car *mycars = new Car[userInput];
  for(int i =0;i < userInput;i++)
         mycars[i]=new Car[i+1];
  printCarNumbers(mycars,userInput);
  return 0;
}    

我想创建一个汽车数组，但出现以下错误:

I want to create a car array but I get the following error:

cartest.cpp: In function ‘int main()’:
cartest.cpp:5: error: ‘Car::Car()’ is private
cartest.cpp:21: error: within this context

有没有办法在不公开 Car() 构造函数的情况下进行初始化?

is there a way to make this initialization without making Car() constructor public?

推荐答案

没有.

但是你看！如果您使用 std::vector<Car>，就像您应该使用的那样(永远不要使用 new[])，那么您可以准确指定应该如何构造元素*.

But lo! If you use std::vector<Car>, like you should be (never ever use new[]), then you can specify exactly how elements should be constructed*.

*好吧.您可以指定要复制的值.

*Well sort of. You can specify the value of which to make copies of.

像这样:

#include <iostream>
#include <vector>

class Car
{
private:
    Car(); // if you don't use it, you can just declare it to make it private
    int _no;
public:
    Car(int no) :
    _no(no)
    {
        // use an initialization list to initialize members,
        // not the constructor body to assign them
    }

    void printNo()
    {
        // use whitespace, itmakesthingseasiertoread
        std::cout << _no << std::endl;
    }
};

int main()
{
    int userInput = 10;

    // first method: userInput copies of Car(5)
    std::vector<Car> mycars(userInput, Car(5)); 

    // second method:
    std::vector<Car> mycars; // empty
    mycars.reserve(userInput); // optional: reserve the memory upfront

    for (int i = 0; i < userInput; ++i)
        mycars.push_back(Car(i)); // ith element is a copy of this

    // return 0 is implicit on main's with no return statement,
    // useful for snippets and short code samples
} 

具有附加功能:

void printCarNumbers(Car *cars, int length)
{
    for(int i = 0; i < length; i++) // whitespace! :)
         std::cout << cars[i].printNo();
}

int main()
{
    // ...

    printCarNumbers(&mycars[0], mycars.size());
} 

注意 printCarNumbers 确实应该设计不同的，以接受表示一个范围的两个迭代器.

Note printCarNumbers really should be designed differently, to accept two iterators denoting a range.

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