char buff[50];
char str;
for (int i = 0; i < strlen(buff); i++){
    if (buff[i] >= '0' && buff[i] <= '9')
         strcat( str,buff[i]);
}
cout << str;

char buff[50];

// Надеюсь, код для заполнения  buff у вас есть? :)

char str[50] = { 0 };
for (int i = 0, j = 0; i < strlen(buff); i++)
{
    if (buff[i] >= '0' && buff[i] <= '9')
        str[j++] = buff[i];
}
cout << str << endl;

char buff[50];
std::string str;
for (int i = 0; i < strlen(buff); i++){
    if (buff[i] >= '0' && buff[i] <= '9')
      str += buff[i]
}
cout << str;