C++ 代码 - 在同一 class 的构造函数中调用一个 class 的构造函数

Question

不幸的是，我正在尝试编译这个 C++ 代码，我未能编译下面的代码。 你能帮我解释为什么我收到这个错误吗？

#include <iostream>
#include <cstring>
using namespace std;

class student
{
   private:
    char name[10];
    int id;
    int fee;
       public:
           student(char name[10],int id)
           {
               strcpy(this->name,name);  //string copy
               this->id=id;
               fee=0;
           }
           student(char name[10],int id,int fee)
           {
               student::student(name,id); //calling a constructor of one class 
                                         // within a constructor of same class
               this->fee=fee;
           }
           void show() //print function
           {
               cout<<"Name:"<<name<<endl;
               cout<<"id:"<<id<<endl;
               cout<<"fee:"<<fee<<endl<<endl;
           }
};

int main()
{
 student s1("DAVID",123);
 student s2("WILLIAM",124,5000);
 s1.show();
 s2.show();

    return 0;
}

下面是 GCC 抱怨的错误： main.cpp|20|error: cannot call constructor 'student::student' directly [-fpermissive]|

Answer 1

正如@sweenish 正确建议的那样，您必须在初始化部分执行构造函数委托。 像这样的东西：

#include <iostream>
#include <cstring>
using namespace std;

class student
{
   private:
    char name[10];
    int id;
    int fee;
       public:
           student(char name[10],int id)
           {
               strcpy(this->name,name);  //string copy
               this->id=id;
               fee=0;
           }
           student(char name[10],int id,int fee): student(name, id) // constructor delegation
           {
               this->fee=fee;
           }
           void show() //print function
           {
               cout<<"Name:"<<name<<endl;
               cout<<"id:"<<id<<endl;
               cout<<"fee:"<<fee<<endl<<endl;
           }
};

int main()
{
 student s1("DAVID",123);
 student s2("WILLIAM",124,5000);
 s1.show();
 s2.show();

    return 0;
}

另外，给你一个建议。 您可以定义更专业的构造函数，并将不那么专业的构造函数的责任委托给更专业的构造函数。

#include <iostream>
#include <cstring>
using namespace std;

class student
{
   private:
    char name[10];
    int id;
    int fee;
       public:
           student(char name[10],int id, int fee): id{id}, fee{fee} // more specialized
           {
               strcpy(this->name,name);  //string copy
           }
           student(char name[10],int id): student(name, id, 0) { } // less specialized calling the more specialized constructor
           void show() //print function
           {
               cout<<"Name:"<<name<<endl;
               cout<<"id:"<<id<<endl;
               cout<<"fee:"<<fee<<endl<<endl;
           }
};

int main()
{
 student s1("DAVID",123);
 student s2("WILLIAM",124,5000);
 s1.show();
 s2.show();

    return 0;
}

Answer 2

这是您的代码，其中进行了更改以使其编译； 我用评论标记了更改。

#include <iostream>
// #include <cstring>  // CHANGED: Prefer C++ ways when writing C++
#include <string>  // CHANGED: The C++ way
// using namespace std;  // CHANGED: Bad practice

class student {
 private:
  std::string name{};  // CHANGED: Move from C-string to std::string
  int id = 0;          // CHANGED: Default member initialization
  int fee = 0;

 public:
  // CHANGED: Move from C-string to std::string
  student(std::string name, int id)
      : name(name),
        id(id)
  // CHANGED: ^^ Utilize initialization section
  {
    // CHANGED: Not needed anymore
    //  strcpy(this->name,name);  //string copy
    //  this->id=id;
    //  fee=0;
  }

  student(std::string name, int id, int fee) : student(name, id) {
    // CHANGED: Not needed anymore
    //  student::student(name,id); //calling a constructor of one class
    //                            // within a constructor of same class
    // NOTE: Inconsistency with other ctor w.r.t. this->
    this->fee = fee;
  }

  void show()  // print function
  {
    std::cout << "Name:" << name << '\n';
    std::cout << "id:" << id << '\n';
    std::cout << "fee:" << fee << "\n\n";
  }
};

int main() {
  student s1("DAVID", 123);
  student s2("WILLIAM", 124, 5000);
  s1.show();
  s2.show();

  return 0;
}

这是相同的代码，但删除了我注释掉的内容以使其更易于阅读。

#include <iostream>
#include <string>

class student {
 private:
  std::string name{};
  int id = 0;
  int fee = 0;

 public:
  student(std::string name, int id) : name(name), id(id) {}

  student(std::string name, int id, int fee) : student(name, id) {
    this->fee = fee;
  }

  void show()
  {
    std::cout << "Name:" << name << '\n';
    std::cout << "id:" << id << '\n';
    std::cout << "fee:" << fee << "\n\n";
  }
};

int main() {
  student s1("DAVID", 123);
  student s2("WILLIAM", 124, 5000);
  s1.show();
  s2.show();

  return 0;
}

初始化部分在参数列表之后并用:标记。 然后按照声明的顺序初始化每个成员。

在构造函数进行委托的情况下，您无法在初始化部分初始化fee 。 我收到的错误是a delegating constructor cannot have other mem-initializers 。

我不喜欢这样拆分我的初始化，如果你坚持委托构造函数调用这个 class，实现最具体的构造函数并用你不太具体的构造函数委托给它。 我更喜欢默认成员初始化，因为我认为它会减少混乱和整体编写代码。

然后代码编译，你会得到预期的 output：

Name:DAVID
id:123
fee:0

Name:WILLIAM
id:124
fee:5000