指向不是类成员的函数的指针(是类成员) [英] Pointer(is a class member) to a function that is not a class member

问题描述

 double getSalary(int v,double s){return v*s;}
 double getSalary(double s){return s/2;}
 double getSalary(){return 1000;}

class person{        //the class is not inherited, so I can''t use virtual 
public:
	char name[20];
        int age;
	double salary;

 person(char* num="Anonim",int a=0,double s=0):age(a),salary(s)
{	
	strcpy(name,num);
}
//...methods...
       double (*person::getSalary)() //I have also tried without personand many others...
};
void main(){
   person p("John",25,3000);
   cout<<"Salary is "<<p.getSalary  /*not sure that this is the right way to call  it, but i don''t think this is the main mistake, also please correct me here*/
}

请告诉我如何在类中声明一个指向getSalary()的指针，在此示例中，我需要函数的第3个版本.我认为编译器会基于函数自动知道要调用的3个之一'' s签名.我没有编译错误，但在运行时会刹车.我尝试调试它，它表明指针为空.

Please tell me how to declare inside the class a pointer to getSalary(),in this example I want the 3rd version of the function.I thought that the compiler automatically knows wich one of the 3 I want to call, baseing on function''s signature.I have no compileing error but it brakes on run-time.I have tried to debug it, it seams that the pointer is empty.

推荐答案

未初始化函数指针person::getSalary.取消引用此指针会导致崩溃.

在调用p.getSalary();之前使用p.getSalary= getSalary;，希望编译器知道选择哪种重载.

You do not initialize the function pointer person::getSalary. Dereferencing this pointer leads to a crash.

Use p.getSalary= getSalary; before invoking p.getSalary(); hoping that the compiler will know what overload to pick.

您的示例将始终仅应调用

Hi,

Your example will always should call

getSalary()

.

因为您的函数指针是

only.

Because your function pointer is of type

(*person::getSalary)()

类型，其中没有参数.

我没有用真实的代码对其进行测试，但这在逻辑上是隐含的.

where there is no parameters.

I have not tested it with the real code , but this is what logically implies.

该示例有几处错误，但我会坚持提出的问题.

声明"double (*person::getSalary)();"不应已编译.

指向成员函数的指针的声明将是
double (person::*getSalary)();.

由于您的代码示例暗示您要指向全局函数，因此正确的声明应为
double (*getSalary)();.

以下内容将为您提供总体思路.

There are several things wrong with the example, but I will stick with the question asked.

The declaration "double (*person::getSalary)();" should not have compiled.

The declaration of a pointer to a member function would be
double (person::*getSalary)();.

Since your code example implies you want to point to a global function the correct declaration would be
double (*getSalary)();.

The following should give you the general idea.

#include <cstring>
double getSalary(int v, double s) { return v * s; }
double getSalary(double s) { return s/2; }
double getSalary() { return 1000; }

class person
{
public:
    char name[20];
    int age;
    double salary;

    person(char* num="Anonim",int a=0, double s=0) : age(a), salary(s)
    {
        strcpy(name,num); // dangerous: length of num is unknown
        getSalary = ::getSalary;
        getIt = ::getSalary;
    }
    // pointer to global function with signature of
    // double (*)()
    double (*getSalary)();
    // same thing with a different varaible name for the pointer
    double (*getIt)();
};

void test()
{
    person p("John",25,3000);
    std::cout << "Salary is " << p.getSalary() << std::endl;
    std::cout << "Salary is " << p.getIt() << std::endl;
}

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