First element of a pointer is polluted C++ - c++

Question

I have this piece of code that puts a random string in an char pointer array:
char * str[100] = {0};
const int elems = sizeof(str)/sizeof(str[0]),size=5;
srand(time(0));
for(int i=0;i<elems;i++){
char rString[size] = {0};
for(int j = 0;j<size;j++){
rString[j] = 97+rand()%26;
}
rString[size] = '\0';
str[i] = new char[size];
strcpy(str[i],rString);
}
for(int i=0;i<elems;i++){
cout<<str[i]<<endl;
}
But str[0] is polluted or empty.

Answer 1

Your rString[size] = '\0'; writes to an element out of bounds! The last element is rString[size-1]. Anything after this will be undefined behaviour.
To fix this (but keeping 5-character strings), you need to increase size to 6 and change your loop limits and null-terminator index, as follows:
int main()
{
char* str[100] = { 0 };
const int elems = sizeof(str) / sizeof(str[0]), size = 6; // Add space for nul-terminator
srand(time(0));
for (int i = 0; i < elems; i++) {
char rString[size] = { 0 };
for (int j = 0; j < size-1; j++) { // End BEFORE last character
rString[j] = 97 + rand() % 26;
}
rString[size-1] = '\0'; // Last element is at [size-1] NOT [size]
str[i] = new char[size];
strcpy(str[i], rString);
}
for (int i = 0; i < elems; i++) {
std::cout << str[i] << std::endl;
}
// And, for good measure, don't forget to free the allocated strings:
for (int i = 0; i < elems; i++) {
delete[] str[i];
}
return 0;
}
Alternatively (if you insist on using raw pointers and C-style strings), you can just change your rString declaration and str[i] allocation lines to use size+1 (less typing):
//...
char rString[size+1] = { 0 };
//...
str[i] = new char[size+1];
But you really should consider using more modern C++ techniques, like std::string and std::vector, in place of your dynamic arrays. Here's a version using those STL containers:
int main()
{
std::vector<std::string> str(100);
const int elems = str.size(), size = 5;
srand(time(0));
for (int i = 0; i < elems; i++) {
std::string rString {""};
for (int j = 0; j < size; j++) {
rString += static_cast<char>(97 + rand() % 26);
}
str[i] = rString;
}
for (int i = 0; i < elems; i++) {
std::cout << str[i] << std::endl;
}
return 0;
}
Feel free to ask for further clarification and/or explanation.