提问人：KAMLESH SINGH 提问时间：9/3/2023 最后编辑：Carlo WoodKAMLESH SINGH 更新时间：9/12/2023 访问量：64
是否仍调用私有基类构造函数？
Are private base class constructors still called?
问：
#include <iostream>
// first base class
class Vehicle
{
public:
Vehicle() { std::cout << "This is a Vehicle\n"; }
};
// second base class
class FourWheeler
{
public:
FourWheeler()
{
std::cout << "This is a 4 wheeler Vehicle\n";
}
};
// sub class derived from two base classes
class Car : private Vehicle, private FourWheeler
{
};
// main function
int main()
{
// Creating object of sub class will
// invoke the constructor of base classes?
Car obj;
return 0;
}
我本来以为没有输出，但它给出了输出（即它运行超类的构造函数）。为什么会这样？请启发我。
答：
1赞
Siddharth Sagar
9/3/2023
#1
Private 用于描述类的访问控制。继承的类将运行其各自的构造函数，而不考虑私有继承或公共继承。
Private 应该用于定义您不希望暴露给类外部的方法和函数的方法和函数。它并不意味着隐藏或锁定功能。
0赞
Mostafa Mohamed Farag Beshier
9/3/2023
#2
继承中使用的 public、private 和 protected 关键字称为访问修饰符。这些更改继承的基类成员的最大控制级别。 构造函数是无法继承的公共方法，它们在创建对象时仅调用一次。 默认情况下，基类的构造函数由子类的构造函数调用。 如果同一子类中存在多个继承，则其构造函数由相同的继承序列调用。 在您的示例中：
class Car : private Vehicle, private FourWheeler
{
// The access modifier is private for both base classes.So, all the public and protected inherited members are treated here as private members.
// All the private members of the base classes will not be inherited to the Car class.
// You cannot access the inherited members outside the class or when inheriting this class.
// The constructor of Vehicle class will be called first then FourWheeler class.
// There is no constructor in the Car class so the compiler will generate it by default constructor.
// It also will generate the copy constructor and other functions like assignment operator and move operator by default.
};
在这里，编译器将此类代码扩展到此。
class Car : private Vehicle, private FourWheeler
{
public:
Car() = default; // default constructor
Car(Car& source) = default; // copy constructor
};
构造函数将按如下方式扩展：
class Car : private Vehicle, private FourWheeler
{
public:
Car(){
//default things.
}
};
但是基类的构造函数调用在哪里。 即使你这样编写子类的构造函数，它也会被扩展为调用所有基类的构造函数，并将其默认参数传递给调用。 您的班级代码将扩展为：
class Car : private Vehicle, private FourWheeler
{
public:
Car(): Vehicle(), FourWheeler(){
//default things.
}
};
“：” 后面的部分称为直接初始化。直接初始化的优先级高于大括号中的部分。 因此，如果子类构造函数中有这样的语句：
class Car : private Vehicle, private FourWheeler
{
public:
Car() : Vehicle(), FourWheeler() {
cout << "I have created a car!\n";
}
};
这将是输出：
This is a Vehicle
This is a 4 wheeler Vehicle
I have created a car!
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