如何通过引用或指针传递的指针正确释放main中的内存？

Question

I'm writing a function to be called by main which allocate memory to a few pointers declared in main. I learnt I can do this by using pointer to pointer or reference to pointer, but I'm having problem deallocating these memories in main. please help me with what is wrong in the following code:

1. using pointer to pointer:

void fun1(int **a, double **b, char **c)
{

    *a=new int[20];
    *b=new double[20];
    *c=new char[10];

    for (int i=0;i<20;i++){
        (*a)[i]=i;
        (*b)[i]=sqrt((double)((*a)[i]));
    }
    *c="0123456789";

}


int _tmain(int argc, _TCHAR* argv[])
{
    int *a;
    double *b;
    char *c;

    fun1(&a,&b,&c);
    cout<<"a & b are:"<<endl;
    for(int i=0;i<20;i++)
        cout<<a[i]<<"\t"<<b[i]<<endl;
    cout<<"c is: "<<c<<endl;

    delete[] a;
    delete[] b;
    delete[] c;
    return 0;
}

2. using reference to pointer:

void fun1(int*& a, double*& b, char*& c)
{

    a=new int[20];
    b=new double[20];
    c=new char[10];

    for (int i=0;i<20;i++){
        a[i]=i;
        b[i]=sqrt((double)a[i]);
    }
    c="0123456789";

}


int _tmain(int argc, _TCHAR* argv[])
{
    int *a;
    double *b;
    char *c;

    fun1(a,b,c);
    cout<<"a & b are:"<<endl;
    for(int i=0;i<20;i++)
        cout<<a[i]<<"\t"<<b[i]<<endl;
    cout<<"c is: "<<c<<endl;

    delete[] a;
    delete[] b;
    delete[] c;
    return 0;
}

thanks a lot!

Answer 1

This in fun1()

*c="0123456789";

followed by this in main()

delete[] c;

is undefined behavior since you try to delete[] a string literal.

Answer 2

The problem has nothing to do with how you are passing the pointers, and everything to do with how you are initializing the contents of c. c ends up pointing to memory that was not created by new[] and so cannot be freed by delete[].

Answer 3

int *a;
double *b;
char *c;

fun1(a,b,c);

You meant:

int* a;
double* b;
char* c;

fun1(&a,&b,&c);

And then you're ok.

However, as sharptooth says, your delete[] c is broken because you just reassigned the pointer to a string literal instead of copying the chars.

---

Please, use std::vector and std::string instead of all this nonsense:

#include <iostream>
#include <string>
#include <vector>

void fun1(std::vector<int>& a, std::vector<double>& b, std::string& c)
{
    a.resize(20);
    b.resize(20);

    for (int i = 0; i < 20; i++) {
        a[i] = i;
        b[i] = sqrt((double)a[i]);
    }

    c = "0123456789";
}


int main(int argc, char* argv[])
{
    std::vector<int> a;
    std::vector<double> b;
    std::string c;

    fun1(a,b,c);

    std::cout << "a & b are: \n";
    for (int i = 0; i < 20; i++)
        std::cout << a[i] << "\t" << b[i] << " \n";

    std::cout << "c is: " << c << std::endl;
}

Answer 4

There is no "proper" way to do that, your program design is flawed. The very same code module that is allocating the dynamic memory shall be the same one that frees it. So if fun1() is located in fun.c, there should also be another function in fun.c called cleanup() or whatever, which cleans up the mess fun1() made. Otherwise your programs are soon turning into happy-memory-leak-land.

A better design in this case is to let main() allocate the memory, then pass the allocated buffers as parameter to the function.

This concept is known as "private encapsulation" in object-oriented programming.

Answer 5

What you are doing is a ticket for disaster. I strongly recommend using stl containers. It is usually recommended to use containers that handle deallocation for you, because deallocation is a frequent cause of annoying bugs. Try to avoid raw pointers, they are very rarely an advantage and make your life more complicated.

#include <vector>
#include <string>
#include <iostream>
using namespace std;

void fun1(vector<int>& a, vector<double>& b, string& c)
{
    size_t size = 20;
    a.resize(size);
    b.resize(size);
    for (int i=0;i<size;i++){
        a.at(i) = i;
        b.at(i) = sqrt(i); // auto conversion to double
    }
    c="0123456789";
}

int _tmain(int argc, _TCHAR* argv[])
{
    vector<int> a;
    vector<double> b;
    string c;

    fun1(a,b,c);
    cout<<"a & b are:"<<endl;
    for(int i=0;i<a.size();i++)
        cout<<a.at(i)<<"\t"<<b.at(i)<<endl;
    cout<<"c is: "<<c<<endl;

    return 0;
}