Debug Assertion Failure even when using delete[]

Question

I am a beginner at programming and recently wrote this code

#include<iostream>
using namespace std;


void removeAllCharacters(char * , char * );
void printArray(char*);

void deleteArrays(char*,char*);

int main()
{
            char * source,*remove;
            source=new char[18];
            remove=new char[10];

            source="Hello how are you";
            remove="Hi Its me";

            removeAllCharacters(source,remove);
            printArray(source);

            deleteArrays(source,remove);
            system("pause");
            return 0;
}

void removeAllCharacters(char * source, char * remove)
{
            int N1=strlen(source)+1;
            int N2=strlen(remove)+1;

            bool arr[128] = {false};

            for(int i=0;i<N2-1;i++)
            {
                arr[remove[i]]=1;
            }

            char *newSource=new char[N1];

            for(int i=0,j=0;i<N1-1;i++)
            {
                if(arr[source[i]]==0)
                {
                    newSource[j++]=source[i];
                }
            }

            delete  [] source;
            source=newSource;
            newSource=0;
}

void printArray(char* arr)
{
            int N=strlen(arr)+1;

            for(int i=0;i<N;i++)
            {
                cout << arr[i];
            }

            cout << endl << endl;
}

void deleteArrays(char* arr1,char*arr2)
{
            delete [] arr1;
            arr1=0;
            delete[]arr2;
            arr2=0;
}

I have tried debugging and found only that The error occurs at delete[] source; in the removeCharacters function.I tried looking for a solution online but i couldn't find it. Why is this error occurring? Is the error because I have allocated dynamic memory from main but i am deleting it from the function?

Answer 1

source is passed by value. Calling delete [] source will not have any effect on source in main function.

Answer 2

The return value of new[] is a pointer value. No matter what happens, when you issue a call to delete[] that same exact pointer value must be used.

Obviously this will violate the rule above:

    source=new char[18];
    remove=new char[10];

    source="Hello how are you";
    remove="Hi Its me";

You're allocating with new[], but replacing the returned value with the value of a string literal. In other words, that pointer value required to call delete[] with is now gone.

Here is an example of what you should have done:

    char str1[] = "Hello how are you";
    char str2[] = "Hi Its me";
    source=new char[sizeof(str1)];
    remove=new char[sizeof(str2)];
    strcpy(source, str1);
    strcpy(remove, str2);

Note that we created two buffers initialized with the data. Then we carefully allocate enough storage for the characters (plus the terminating null), instead of counting characters and possibly get something wrong.

Then the strcpy function copies the characters over to another buffer.

You also have another error:

source=newSource

You will see immediately that on return source does not have the pointer you just assigned. The reason is that source is a value parameter, thus local to the function -- assigning to it doesn't do anything to the actual value passed to the function, as any assignment just goes up in a puff of smoke when the function returns.

Also, it you're a beginner at C++ programming, you could have started using std::string and not char* for string data. Creating strings using new[] is a dinosaur approach in C++. Use std::string instead. You really should strive to not get into the business of creating strings like this:

 char *newSource=new char[N1];

This leads to memory leaks, and sometimes spaghetti-like logic in keeping track of the allocated memory so that delete[] functions properly. In addition, most, if not all of these issues mentioned disappear. Assignments work, no memory leaks, etc.

Answer 3

    source="Hello how are you";
    remove="Hi Its me";

This is wrong. The right way is:

    strcpy( source, "Hello how are you" );
    strcpy( remove, "Hi Its me" );

You cannot "delete" a constant string. You can delete array/memory allocated with "new".