Question

UPDATE: Allocated memory for str1’s new data. Still memory error.

I’m trying to rewrite the += method for a string class I created.

Class mystring{

public:
    friend void operator+=(mystring& str1, const mystring& str2){
        mystring temp;

        delete[] temp.data;
        temp.length = str1.length + str2.length;
        temp.data = new char[temp.length + 1];

        strcpy(temp.data, str1.data);
        strcat(temp.data, str2.data);

        delete[] str1.data;
        str1.length = temp.length;

        strcpy(str1.data, temp.data);

    }

private:
    char *data;
    int length;

}

Then in the main class:

mystring str1("hi");
mystring str2("matt");

str1 += str2;
cout << str1 << endl;

This function is working as it should be, but I am getting memory errors all over when I run valgrind. I am unable to figure out why that is. If anyone could give me any tips that would be awesome.

Thanks

Answer 1

Firstly, you meant not:

 strcat(str1.data, str1.data);

but:

 strcat(str1.data, str2.data);

Secondly, where do you expect str2.data to go? It is a memory scribble and hence the valgrind errors. Surprised it does not just crash.

You need to reallocate enough storage for the combined length, copy over both original strings and free str1.data before re-assigning it to the new storage.

Based on the updated post:

friend void operator+=(mystring& str1, const mystring& str2)
    {
        // Not using a temp mystring here, as the temp never really maintains its state as a mystring
        // I am assuming length is the length of the string, not the storage. Not the best design if you consider resizing the the string to less than the storage
        int newStringLength = str1.length + str2.length;
        char* newStorage = new char[newStringLength +  1];

        strcpy(newStorage, str1.data);
        // strcat has to scan from the start of the string; we do not need to.
        strcpy(newStorage + str1.length, str2.data);

        delete[] str1.data;

        str1.length = newStringLength ;
        str1.data = newStorage;

         // Haven't though about the case where str2 is an alias for str1.
    }