c ++ set const char* 类的成员变量

Question

When I am trying to assign a member variable, which is const char*, it just ends the program. However, if I get some cin inputs before I assign a member variable, it works. this is my NameCard.h

class NameCard {

private:
    const char* name;
    const char* companyName;
    const char* phoneNumber;
    enum grade{CLERK, SENIOR, ASSIST, MANAGER};
    grade my_grade; 

    NameCard* nextCard; 
    NameCard* head;

public:
    NameCard(const char*, const char*, const char*, int);
    void setName();
    void setCompanyName();
    void setPhoneNumber();
    void setMyGrade();
    void setHead(NameCard*);
    void setNextCard(NameCard*);

    void findInfo(char*);
    void printAll();

};

This is main.cpp

#include <iostream>
#include <cstring>
#include "NameCard.h"

using namespace std;
NameCard::NameCard(const char* a, const char* b, const char* c, int d){
    name = a;
    companyName = b;
    phoneNumber = c;
    switch(d){
        case CLERK: my_grade = CLERK; break;
        case SENIOR: my_grade = SENIOR; break;
        case ASSIST: my_grade = ASSIST; break;
        case MANAGER: my_grade = MANAGER; break;
    }
    cout << name << " " << companyName << " " << phoneNumber << " " << my_grade << endl;
}
void NameCard::setName(){
    char* name;
    cout << "type name." << endl;
    cin >> name;
    this->name = name;
    cout << this->name << endl;
}

void NameCard::setCompanyName(){
    char* company_name;
    cout << "type company name." << endl;
    cin >> company_name;
    this->companyName = company_name;
    cout << this->companyName << endl;
}
void NameCard::setPhoneNumber(){
    char* phone_number;
    cout << "type phone number." << endl;
    cin >> phone_number;
    phoneNumber = phone_number;
    cout << phoneNumber;
};
void NameCard::setMyGrade(){
    int input_grade;
    cout << "type position. 1)CLERK 2) SENOIR 3) ASSIST 4)MANAGER" << endl;
    cin >> input_grade;
    switch(input_grade){
        case CLERK: my_grade = CLERK; break;
        case SENIOR: my_grade = SENIOR; break;
        case ASSIST: my_grade = ASSIST; break;
        case MANAGER: my_grade = MANAGER; break;
    }
    cout << input_grade;
};

void NameCard::setHead(NameCard* head){
    this->head = head;
};

void NameCard::setNextCard(NameCard* next){
    this->nextCard = next;
}

void NameCard::findInfo(char* target_name){
    NameCard* temp;
    temp = head;
    while(temp != NULL){
        if(temp->name == target_name){
            cout << "name : " << temp->name << endl;
            cout << "companyName : " << temp->companyName << endl;
            cout << "phoneNumber : " << temp->phoneNumber << endl;
            cout << "grade : " << temp->my_grade << endl;
            break;
        }
        temp = temp->nextCard;
    }
};

void NameCard::printAll(){
    NameCard* temp;
    temp = head;
    while(temp!= NULL){
        cout<< "----------------------------" << endl;
        cout << "name : " << temp->name << endl;
        cout << "companyName : " << temp->companyName << endl;
        cout << "phoneNumber : " << temp->phoneNumber << endl;
        cout << "grade : " << temp->my_grade << endl;
        temp = temp->nextCard;
    }
}


int main(){
    int index = 0;
    NameCard* head = new NameCard("test", "test", "test", 3);
    NameCard* bef = NULL; 
    // char input[50];
    // cout << "nmae ";
    // cin >>input; 
    head->setName();
    head->setPhoneNumber();
    head->setCompanyName();
    head->setMyGrade();
    
    return 0;
}

And the problem is here. This is not working. The program ends right after it gets name from setName(). It just ends at the setName() cin >> name. it did not print name after then, and stopped.

int main(){
    int index = 0;
    NameCard* head = new NameCard("test", "test", "test", 3);
    NameCard* bef = NULL; 
    // char input[50];
    // cout << "nmae ";
    // cin >>input; 
    head->setName();
    head->setPhoneNumber();
    head->setCompanyName();
    head->setMyGrade();
    
    return 0;
}

However, this is working fine. it gets the test input and executes all the code below.

int main(){
    int index = 0;
    NameCard* head = new NameCard("test", "test", "test", 3);
    NameCard* bef = NULL; 
    char input[50];
    cout << "test input ";
    cin >>input; 
    head->setName();
    head->setPhoneNumber();
    head->setCompanyName();
    head->setMyGrade();
    
    return 0;
}

Answer 1

You are writing to memory you didn't allocate and most likely don't own. Check this for example

void NameCard::setName(){
    char* name;
    cout << "type name." << endl;
    cin >> name;
    this->name = name;
    cout << this->name << endl;
}

You declare char* name and try to read from cin into it. But name is just an uninitialized pointer.

One fix may be to declare name as an array, which also decays to a pointer (so it can be used as a pointer), but does have associated memory. Like this

char name[50];

Or if you want it to be dynamically allocated

char* name = new char[50];
// Use it and when you are done delete it
// Never forget to release memory allocated with new or you get a memory leak
delete[] name; 

The key point is that the pointer needs to point to some memory block you know you can use. Note that in the example I allocated 50 chars, you have to make sure you never use (read/write) past the allocated memory, or you get the same error you have been getting and potentially program termination.

A safer alternative is to use std::string , which automatically handles memory for you, so you don't have to mind about allocation and releasing. You could do something like

std::string name;
std::cin >> name;