Accessing and modifying 2D dynamically allocated arrays - c++

Question

I'm trying to create a 2D array of c-strings (for a specific school exercise; I'm forced to use c-strings for practice) using dynamic memory allocation. However, it seems that when accessing and writing to the second index of the array (second sub-array), the actual memory location that's used is the same of the first index.
The code:
#include <iostream>
#include <string>
int main()
{
int n_names; std::string current; const int SPACE_FOR_EACH_NAME = 100;
std::cout << "How many names to input? "; std::cin >> n_names; std::cin.ignore();
//dynamically allocate a multi-dimensional array
char** names;
names = new char* [n_names];
for (int i = 0; i < n_names; i++)
names[i] = new char[SPACE_FOR_EACH_NAME];
int count = 0;
while (count < n_names) {
std::cout << "Name " << ++count << ": ";
std::getline(std::cin, current);
names[count-1] = (char*) current.c_str(); //THE TROUBLE SEEMS TO BE HERE
}
for (int i = 0; i < n_names; ++i) {
for (int j = 0; j < SPACE_FOR_EACH_NAME; ++j) {
if (names[i][j] == '\0') break; //termination of the current name
std::cout << names[i][j];
}
std::cout << "\n";
}
//free allocated memory
for (int i = 0; i < n_names; ++i)
delete[] names[i];
delete[] names;
}
What the debugger shows when modifying the 'names' array (consider user inputs 2 names):
+ names[count-1] 0x00affbe8 "dude" char * //here count is 1
+ names[count-1] 0x00affbe8 "noice" char * //here count is 2
And the console just prints "noice" twice.
What's wrong?

Answer 1

The result of current.c_str() should never be stored for any length of time. Doing so is undefined behaviour, but it is likely, as here that that memory pointed to will be reused.
You put the char* from current.c_str() into names[0], then you put a new value into current and put current.c_str() into names[1]. But because you have changed current you also change names[0].
Also, in both cases, you are discarding the pointer you already created with names[i] = new char[SPACE_FOR_EACH_NAME];
This allocates a block of memory and puts it's address into names[0] (0 as a specific example). The next thing that happens with names[0] is names[0] = (char*) current.c_str(); which puts a different address into the variable. The address returned from the new is lost completely, causing a memory leak.
Instead of
names[count-1] = (char*) current.c_str();
try
std::strcpy(names[count-1],current.c_str())