* How to avoid errors in programs?

* If an error happen, how can we find it?

* What can go wrong with the program below?

#include <iostream>
unsigned myStrLen(char* s) {
  int len = 0;
  char* p = s;
  while (*p != '\0') {
    p++;
    len++;
  }
  return len;
}
int main(int argc, char** argv) {
  std::cout << "Len " << argv[1] << " = " << myStrLen(argv[1]) << std::endl;
  return 0;
}

* How can you try to identify the error?

#include <iostream>
#include <assert.h>
unsigned myStrLen(char* s) {
  assert(s != NULL);
  int len = 0;
  char* p = s;
  while (*p != '\0') {
    p++;
    len++;
  }
  return len;
}
int main(int argc, char** argv) {
  assert(argc == 2);
  std::cout << "Len " << argv[1] << " = " << myStrLen(argv[1]) << std::endl;
  return 0;
}

* What are assertions good for?

* How can you see the code that is produced by the assertion?

* How to disable the assertion?

#define NDEBUG
#include <assert.h>

* What are common errors in C/C++?

* What will be printed by the program below?

#include <iostream>
struct Data {
  int x;
};
Data foo(Data d) {
  return d;
}
int main() {
  Data d0;
  d0.x = 1;
  Data d1 = foo(d0);
  std::cout << "Data = " << d1.x << std::endl;
  d0.x = 2;
  std::cout << "Data = " << d1.x << std::endl;
}

- Remember: in C++, parameters and return values are passed by copy!

* What will be printed by the program below?

#include <iostream>
void arithProg(int* limit, int* sum) {
  *sum = 0;
  for (int i = 0; i < *limit; i++) {
    *sum += i;
  } 
}
int main() {
  int S, N = 10;
  arithProg(&N, &S);
  std::cout << S << std::endl;
}

* And now, what will be printed by this new 'main'?

int main() {
  int S, N = 10;
  arithProg(&N, &S);
  std::cout << S << std::endl;
  arithProg(&N, &N);
  std::cout << N << std::endl;
}

* Could you use assertions to flag this problem?

#include <assert.h>
void arithProg(int* limit, int* sum) {
  assert(limit != sum);
  *sum = 0;
  for (int i = 0; i < *limit; i++) {
    *sum += i;
  } 
}

* Why doesn't the program below do what you want?

#include <assert.h>
void arithProg(int* limit, int* sum) {
  assert(&limit != &sum);
  *sum = 0;
  for (int i = 0; i < *limit; i++) {
    *sum += i;
  } 
}

- Remember: updates in a pointer might change other pointers!

* What will be printed by the program below?

#include <iostream>
struct Data {
  int x;
};
Data* getData(int value) {
  Data d;
  d.x = value;
  return &d;
}
int main() { 
  Data *d0 = getData(0);
  Data *d1 = getData(1); 
  std::cout << "Data = " << d0->x << std::endl;
  std::cout << "Data = " << d1->x << std::endl;
}

* What is the problem with this program?

* What is the meaning of the warning? E.g.:

  "dangling0.cpp:10:11: warning: address of stack memory associated with local
   variable 'd' returned [-Wreturn-stack-address]"

* How could you fix it?

#include <iostream>
struct Data {
  int x;
};
Data* getData(int value) {
  Data *d = new Data();
  d->x = value;
  return d;
}
int main() {
  Data *d0 = getData(0);
  Data *d1 = getData(1);
  std::cout << "Data = " << d0->x << std::endl;
  std::cout << "Data = " << d1->x << std::endl;
}

- Remember: you cannot access local variables after the function where they
  exist returns!

* What is the value that the program below prints?

#include <iostream>
struct Data {
  int x;
};
int main() {
  Data *d0 = new Data();
  d0->x = 0;
  std::cout << d0->x << std::endl;
  delete d0;
  Data *d1 = new Data();
  d1->x = 1;
  d0->x = 2;
  std::cout << d1->x << std::endl;
}

- Remember: you cannot access the contents of a pointer after deletion.

* What is the value that this program below prints?

#include <iostream>
int main() {
  int a[1] = {17};
  int b[1] = {19};
  b[1] = 3;
  std::cout << a[0] << std::endl;
}

* What is the problem with this program?

* Can you check the addresses of the arrays?

#include <iostream>
#include <stdio.h>
int main() {
  int a[1] = {17};
  int b[1] = {19};
  printf("%p\n%p\n%p\n%p\n", (void*)&b, (void*)&b[1], (void*)&a, (void*)&a[1]);
  b[1] = 3;
  std::cout << a[0] << std::endl;
}

* What is the meaning of the warning that we get?

"array.cpp:7:3: warning: array index 1 is past the end of the array (which
 contains 1 element) [-Warray-bounds]"

- Remember: arrays cannot be accessed outside their bounds!

* What does the program below print?

#include <iostream>
int searchArray(int to_search[], int len, int to_find) {
  int i;
  for( i = 0; i < len; ++i) { 
    if ( to_search[i] == to_find) {
      return i;
    }
  }
}
#define N 6
int main() {
  int x[N] = {2, 3, 5, 7, 11, 13};
  std::cout << searchArray(x, N, 5) << std::endl;
}

* What about the following main function?

int main() {
  int x[N] = {2, 3, 5, 7, 11, 13};
  std::cout << searchArray(x, N, 8) << std::endl;
}

* What is the problem with the code above?

* What is the meaning of the warning?

"ret0.cpp:10:1: warning: control may reach end of non-void function [-Wreturn-type]"

* How could you fix the program above?

- Remember: if a function returns a value, it must do it through any path that
  the function contains.

* What about the following fix? Does it work?

#include <iostream>
int searchArray(int to_search[], int len, int to_find) {
  int i = -1;
  for( i = 0; i < len; ++i) { 
    if ( to_search[i] = to_find) {
      return i;
    }
  }
  return i;
}
#define N 6
int main() {
  int x[N] = {2, 3, 5, 7, 11, 13};
  std::cout << searchArray(x, N, 5) << std::endl;
}

* What is the meaning of the warning?

"ret0.cpp:6:23: warning: using the result of an assignment as a condition
 without parentheses [-Wparentheses]"

* What is actually that code above doing?

- Remember: the expression "if(a = x)" is valid in C/C++!

* What does the program below print?

#include <iostream>
char last(char* s) {
  int i, l;
  for(i = 0; s[i] != '\0'; ++i) {
    l = s[i];
  }
  return l;
}
int main(int argc, char** argv) {
  for (int i = 1; i < argc; i++) {
    std::cout << "Last char: " << last(argv[i]) << std::endl;
  }
}

* How could we deal with this problem?

#include <assert.h>
#include <iostream> 
char last(char* s) {
  assert(strlen(s) > 0);
  int i, l;
  for(i = 0; s[i] != '\0'; ++i) {
    l = s[i];
  }
  return l;
}
int main(int argc, char** argv) {
  for (int i = 1; i < argc; i++) {
    std::cout << "Last char: " << last(argv[i]) << std::endl;
  }
}

- Remember: every variable must be initialized before being used!

* What is the problem with this program below?

#include <iostream>
void logic1(int n) {
  if (n != 1 || n != 2) {
    std::cout << n << " is odd prime, or has non-trivial divisor\n";
  } 
} 
int main(int argc, char** argv) {
  logic1(argc);
}

* Can you write the truth table for n != 1 || n != 2

 n   n!=1   n!=2   ||
 1     0      1     1
 2     1      0     1
 ?     1      1     1

* What's the problem of the expression?
- It will never be false.

* Can you pin-point the problem with this function, that does the same thing?

void logic1(int n) {
  if (!(n == 1 && n == 2)) {
    std::cout << n << " is odd prime, or has non-trivial divisor\n";
  } 
} 

- Remember: there are different ways to combine ==, !=, && and ||, and several
  of them make no sense!

* What does this program below print?

#include <iostream> 
int prec(int n, int m) {
  return 2 + n > m ? n : m;
}
int main() {
  std::cout << prec(10, 11) << std::endl;
}

* How to fix this program?

#include <iostream> 
int prec(int n, int m) {
  return 2 + (n > m ? n : m);
}
int main() {
  std::cout << prec(10, 11) << std::endl;
}

- Remember: the ternary operator has very low priority!

* What is a debugger?

* Can you give me examples of debuggers?
- gdb, lldb, ipdb, pydb, etc

* Can you inspect every command of the program below in action?

#include<iostream>
int main(int argc, char** argv) {
  std::cout << "Hello World!\n";
  return 0;
}

* What does the '-g' flag does?
$> clang++ -g hello.cpp

* How can we use lldb to see the commands executed one-by-one?

$> lldb a.out
(lldb) process launch
...
(lldb) breakpoint set -name main
(lldb) process launch
...
(lldb) frame variable
...
(lldb) frame variable argv[0]
...
(lldb) thread step-out

* Can you explain what each statement above does?

* Consider the program below, which contains an index out of bounds error.
  Can you find the error using lldb?

#include <iostream>
#include <stdio.h>
int foo(int v) {
  int a[1];
  a[1] = 17;
  return a[1] + v;
}
int main() {
  int x = 25;
  std::cout << foo(x) << std::endl;
}

* How can you use lldb on the program above?

$> clang++ -g stack.cpp

$> lldb a.out
(lldb) breakpoint set -name foo
(lldb) process launch
(lldb) thread backtrace
(lldb) frame variable &a[1]
...
(lldb) frame variable &v
...

* So, what's going on with the program above?

* How can you set up an input file?

(lldb) setting set target.input-path t.txt
(lldb) process launch
- or -
(lldb) process launch -i t.txt

* How can you find the bug in buggy.cpp using lldb?

$> lldb buggy.exe
(lldb) process launch
(lldb) thread backtrace
(lldb) memory read --format x --count 1 0x7ffeefbff6a4
# "consider also:" (lldb) x/w 0x00007ffeefbff6a4
(lldb) breakpoint set --name LinkedList<int>::remove
(lldb) breakpoint modify -c "item_to_remove==1"
(lldb) frame variable item_to_remove
(lldb) thread step-over