CODEWITHSUNDEEP
c++pointers
Ian Ling
Ian Ling

Reputation: 334

Pointer not getting set to nullptr after delete

I delete the pointer aStudent in the destroyStudent() function, then I set aStudent to nullptr. However, after running the function, aStudent is not set to nullptr anymore, so I have to set it to nullptr again.

#include <cstring>

using namespace std;

struct Student {
   char *     name;
   float      gpa;
};

Student * createStudent(const char name[], float gpa) {
    struct Student * student = new Student;
    student->name = (char*)malloc(strlen(name + 1)); //allocate only enough memory to fit the given name
    strcpy(student->name, name);
    student->gpa = gpa;

    return student;
}

bool destroyStudent(Student * aStudent) {
    if(aStudent) { //check whether this pointer is already null.
        free(aStudent->name);
        delete aStudent; // ******This is where the issue is******
        aStudent = nullptr;
        return true;
    }
    return false; //aStudent is already null
}


int main() {
    Student * student1 = createStudent("Charles", 2.5);
    cout << student1->name << " and " << student1->gpa << endl;
    destroyStudent(student1);
    if(student1) {
        cout << "Pointer is NOT null!!!" << endl;
        student1 = nullptr;
    }

    if(!student1) {
        cout << "The pointer is null now." << endl;
    }

    return 0;
}

Upvotes: 1

Views: 1281

Answers (2)

Andrew Henle
Andrew Henle

Reputation: 1

C++ uses pass-by-value.

You're setting the variable local to your destroyStudent() method to nullptr, not the variable in your main().

Upvotes: 0

Galik
Galik

Reputation: 48635

The problem is that aStudent is a local copy of the pointer.

You need to pass the pointer in by reference like this:

bool destroyStudent(Student*& aStudent) {
    if(aStudent) { //check whether this pointer is already null.
        free(aStudent->name);
        delete aStudent; // ******This is where the issue is******
        aStudent = nullptr;
        return true;
    }
    return false; //aStudent is already null
}

That way it is the outside pointer you change, not a local copy.

Upvotes: 6

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