perplexing things in c++

I am trying to copy an array of chars, into an other array of chars in reversed order.

This is my method:

void reversString(char* str){
    char* ptr = str;

    int i = 0;
    // getting length of str/ptr array
    while (*(ptr + i) != '\0'){
        i = i + 1;
    }

    char revStr [i];
    char * revStrChar = &revStr[0];
    int revStrPos = 0;

    cout << *(ptr + 3) << endl;

}

Here i am just trying to copy it in normal order, but if I print the last letter of the input ("abcd"), nothings happens. It prints only an empty line.

But if I delete the declaration of a new char array:

void reversString(char* str){
    char* ptr = str;

    int i = 0;
    // getting length of str/ptr array
    while (*(ptr + i) != '\0'){
        i = i + 1;
    }

    //char revStr [i];
    //char * revStrChar = &revStr[0];
    //int revStrPos = 0;

    cout << *(ptr + 3) << endl;

}

Then it prints the last letter correctly, which is "d". I do not understand how declaring a new char array influences the output! (compiler is minGW, OS is Win10)