Getting Help

• Compilation errors — "my C++ won't compile"
• Run-time errors — "my program crashes"
• Logic mistakes — "my program runs but doesn't work"
• You're doing it wrong! — code so bad that you'll lose marks if we see it

Compilation errors

Active the Error List window

Click 1st error, then press F1 for help.

void main() {
    x = 5;           // ERROR: 'x': undeclared identifier; need to declare x.
    int y = yearr(); // ERROR: 'yearr': identifier not found; maybe year()?
}

struct point { int x,y; };
int x = point.x;     // ERROR: 'point' is not a variable, it's a variable *type*

struct point { int x,y; };
Point p;     // ERROR: compiler doesn't know what 'Point' is; should be 'point'

int   x = 0; // OK: 0 is understood to be of type 'int'
int*  y = 0; // OK: 0 is understood to be of type 'int*'  (null)
char* z = 0; // OK: 0 is understood to be of type 'char*' (null)
x = y;       // ERROR: cannot convert from 'int*'  to 'int'
y = x;       // ERROR: cannot convert from 'int'   to 'int*'
y = z;       // ERROR: cannot convert from 'char*' to 'int*'

int year() {
    return "2010"; // ERROR: cannot convert from 'const char [4]' to 'int'
}

void draw_point(point p);
void main()
{
    int x = 50;
    draw_point(x); // ERROR: cannot convert parameter 1 from 'int' to 'point'
}

int y = year;      // ERROR: cannot convert from 'int (*)()' to 'int'

struct point { int x,y; }; // OK
struct point { int x,y; }  // ERROR: declaration of 'point' must end with ;

struct point { int x,y  }; // ERROR: declaration of 'x,y' must end with ;
int five() { return 5 }    // ERROR: forgot to put ; after 5
if (ok) { cout << "ok" }   // ERROR: forgot to put ; after "ok"

int min(int a, int b) {
    if (a < b) { 
        return a;
    return b;
}              // compiler sees '}' as matching (i.e. closing) the 'if' block

void main() {  // ERROR C2601: compiler sees main() as though inside min()
}      
               // ERROR C1705: end of file before all '{' were matched

int min(int a, int b) {
    if (a < b)
        return a;
    }            // oops! this '}' closes the function!
    return b;    // ERROR: syntax error 'return'
}                // ERROR: syntax error '}'

x = 5;                // ERROR: should be declared as "int x = 5;"
five() { return 5; }  // ERROR: should be declared as "int five()"

point p;
p->x = 0;     // ERROR: 'p' is not a pointer, so use 'p.x'

point* q = &p;
q.x = 0;      // ERROR: 'q' is a pointer, so use 'q->x'

void hello();     // Declare 'hello' function (often in a .h file)

void main() {
    hello();      // ERROR: unresolved external symbol 'void hello(void)'
}

void hallo() {         // Rename this 'hello' and it will work, as long as 
    cout << "hello";   // it appears in any .cpp file in to your project
}

Run-time errors

• Access violation — accessed memory that doesn't belong to you (detected immediately)
• Heap corruption — wrote memory that doesn't belong to you (detected after the fact)
• Stack corruption — wrote memory that doesn't belong to you (detected after the fact)

Oy! Writing to address 0 is an "Access violation."

• Loop goes out of range
• Uninitialized variable
• Uninitialized data member
• NULL pointer dereference
• Dangling pointer

Code that crashes on delete. Can you spot the bug?

int grades[80];           // grades[0..79] = ???
int i = 0, j;             // i = 0, j = ???
int grade_i = grades[i];  // DANGER: grade_i = ???
int grade_j = grades[j];  // CRASH:  grade_j = grades[???]

int* ptr;        // ptr = ???
if (ptr != 0)    // Useless test, since ptr is probably not 0
    *ptr = 123;  // CRASH: *(???) = 123

class array_of_int {
public:
    void push_back(int item) { m_items[m_size++] = item; }
private:
    int m_size;
    int m_items[50];             // Stores up to 50 integers
};

void main() {
    array_of_int temperatures;
    temperatures.push_back(22);  // CRASH: grades.m_items[???] = 22
}

class array_of_int {
public:
    array_of_int() { m_size = 0; }  // Fixed the bug!
    ...

ptr1 points to x, ptr2 is NULL.

int* ptr = 0;  
*ptr = 123;     // CRASH: tried to write '123' to memory location 0

void main()
{
    int* grades = new int[50];    // 'grades' becomes valid pointer.
    delete [] grades;             // 'grades' becomes dangling pointer! Careful...

    for (int i = 0; i < 50; ++i)  // You can accidentally read/write a dangling pointer
        grades[i] = 0;            
}                                 // Until program exits -- heap corruption detected!

Logic mistakes

• Using = when you meant ==
• Modifying a list within a loop
• Copied object when you should've referenced it

int speed = 0;
if (speed = 0)
    cout << "speed is zero!";   // This line never runs!

void erase_perfect_grades(array_of_int& grades)
{
    for (int i = 0; i < grades.size(); ++i) // Buggy loop
        if (grades[i] == 100)
            grades.erase(i);
}

void erase_perfect_grades(array_of_int& grades)
{
    for (int i = 0; i < grades.size(); )    // Correct loop
        if (grades[i] == 100)
            grades.erase(i);
        else
            ++i;    // Only do ++i if item is not erased
}

struct rectangle { int x,y,wd,ht; };

void grow_rect(rectangle rect, int amount)  // Grow size of a rectangle by 'amount'
{
    rect.wd += amount;
    rect.ht += amount;
}

void main()
{
    rectangle mario_hitbox = { 0,0,50,100 };
    grow_rect(mario_hitbox,10);   // Doesn't modify mario_hitbox!
    draw_rect(mario_hitbox);      // Draws rect of size 50x100, not 60x110.
}

You're doing it wrong!

{
    T obj;
    obj.~T(); // Destructed once.
}             // Destructed twice!

{
    T obj;
}             // Destruct once.

T* p = new T[5]; // p[0..4] constructed.
delete p;        // Only p[0] destructed!

T* p = new T[5];
delete[] p;      // p[0..4] destructed.

ptr = 0;      // Memory leak!
delete ptr;   // Does nothing.

delete ptr;   // Free memory.