Heap corruption when using delete - c++

Question

I have a MultiString class that has some methods in it.
I am strugling with deleting my array of strings in the destructor, I get heap corruption.
Here is my class (some methods are cut)
class MultiString {
public:
//constructor destructor
MultiString(int);
~MultiString();
//methods
void Setat(int nindex, const char* str);
//attributes
char** buf;//pointer to vector
int str_nmb;//strings number
};
Constructor code:
MultiString::MultiString(int number)
{
str_nmb = number;
buf = new char* [number];
for (int i = 0; i < number; i++) buf[i] = NULL;
}
Setat code (used to set strings in array):
void MultiString::Setat(int nindex, const char* str)
{
if (nindex<0 || nindex>str_nmb || str == NULL) {
std::cout << "gg";
return;
}
char* tmp = new char[strlen(str)+1];
if (tmp == NULL) return;
if (buf[nindex] != NULL) delete buf[nindex]; //here delete works fine
buf[nindex] = tmp;
strcpy_s(buf[nindex], strlen(buf[nindex]), str);
std::cout << buf[nindex]<< std::endl;
}
Destructor code:
MultiString::~MultiString()
{
for (int i = 0; i < str_nmb; i++)
delete buf[i]; // heap corruption here
delete buf;
}
And my main():
int main()
{
MultiString* ms = new MultiString(3);
ms->etat(0, "qwerty");
ms->Setat(1, "asdfgh");
ms->Setat(2, "zxcvbn");
delete ms;
return 0;
}

Answer 1

char* tmp = new char[strlen(str)+1];
// ...
buf[nindex] = tmp;
strcpy_s(buf[nindex], strlen(buf[nindex]), str);
buf[nindex] points to the freshly allocated, but uninitialized memory area. Calling strlen on that is undefined behavior and likely what corrupts your heap. You probably want to call strlen(str) instead.
Apart from that you're making a few other mistakes:
Rule of 0/3/5 broken.
delete where delete[] would be necessary
Wrong bounds check (nindex>str_nmb)
What about Multistring(-5) ?
if (tmp == NULL) return; ... no, you shouldn't mute errors. Throw an exception or something.
Also .. why strcpy_s ? Use std::strncpy ... that's at least guaranteed to be available! Make sure the copied C string is null terminated!