c++ - 我的 C++ 代码给出了在代码中看不到的错误。错误是什么？

Question

当我编译这段代码时，它给出了如下所示的运行时错误。但它并没有告诉我的代码中哪一行有问题。

Debug Assertion Failed!

Program: C:\Windows\system32\MSVCP110D.dll
File: c:\program files (x86)\microsoft visual studio 11.0\vc\include\xstring
Line: 1143

Expression: invalid null pointer

For information on how your program can cause an assertion failure, see the Visual C++ documentation on asserts.

(Press Retry to debug the application.)

以下是我的 C++ 代码。包含一个基类:Vehicle 和另一个派生类:Car，它是从基类公开继承的。

class Vehicle {

private:
    string VehicleNo, color;

public:
    Vehicle():VehicleNo(NULL),color(NULL){};
    string getVehicleNo(){return VehicleNo;}
    string getColor(){return color;}

    void setVehicleNo(){
        //getline(cin,VehicleNo);
        cin>>VehicleNo;
    }

    void setVehicleColor(){cin>>color;}
};



class Car: public Vehicle {

private:
    int distance;

public:
    void setDistance(int x){distance=x;}
    void setCarNo(){setVehicleNo();}
    void setCarColor(){setVehicleColor();}

    int calculateFare(int x){return 5*x;};


    void displayInformation()
    {
        cout<<"Your Car Number is: "<<getVehicleNo()<<endl;
        cout<<"The color of your Car is: "<<getColor()<<endl;
        cout<<"Total fare which you have to pay: "<<calculateFare(distance);

    }


};

int main()
{
    Car c1;
    int distance;
    char choice;

    cout<<"Enter car number: ";
    c1.setCarNo();

    cout<<"\nEnter Car Color: ";
    c1.setCarColor();

    cout<<"\nHow long would you like to go? Enter distance in kilometers: ";
    cin>>distance;
    c1.setDistance(distance);

    cout<<"\n----------------------------------\n";
    c1.displayInformation();
    cout<<"\n----------------------------------\n";

    cout<<"\nDo you want to calculate Fare of different distance (y/Y for yes and another character for No?  ";
    cin>>choice;

    do{

        cout<<"\nHow long would you like to go? Enter distance in Kilometers: ";
        cin>>distance;
        cout<<"\n----------------------------------\n";
        c1.setDistance(distance);

        c1.displayInformation();

        cout<<"\nDo you want to calculate Fare of different distance (y/Y for yes and another character for No?  ";
        cin>>choice;
    } 
    while(choice=='y' || choice=='Y');
}

Answer 1

C++ 提供了 9 个 string 构造函数:http://en.cppreference.com/w/cpp/string/basic_string/basic_string

其中 2 个接受指针:

1. basic_string(const CharT* s, size_type count, const Allocator& alloc = Allocator())
2. basic_string(const CharT* s, const Allocator& alloc = Allocator())

当您调用 VehicleNo(NULL) 和 color(NULL) 时，您只是将空指针而不是 count 传递给 string 构造函数，以便编译器将您的 null 参数传递给选项 2。 s 应该是:

A pointer to a character string to use as source to initialize the string with

当 string 构造函数试图取消引用 s 以将其内容复制到正在构造的 string 中时，它会出现段错误。

您在这里尝试构造的是一个空的字符串。当您使用默认构造函数时，C++ 已经这样做了:string()。

如果在构造函数初始化列表中没有指定构造函数，将调用成员对象的默认构造函数。因此，您无需将 VehicleNo 或 color 放入构造函数初始化列表中即可将它们构造为空 string。这意味着您可以使用编译器生成的默认构造函数，并完全摆脱您的构造函数。