C++ Memory Pointers and joy! January 07, 2019 Cinda Heeren / Will - - PowerPoint PPT Presentation

C++ Memory

Pointers and joy!

January 07, 2019 Cinda Heeren / Will Evans / Geoffrey Tien 1

Announcements

• HW1 corrections

– please check pinned Piazza post – Gradescope registrations will start today – check your ugrad e-mail for invitation

• Cinda's C++ mini-camps happening this week!

– Monday, Tuesday, Wednesday, 18:00-20:00, SWNG 122, 221, 122

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Code analysis

• The loop variables here does not increment in the usual way

– Then how many times is the loop body executed?

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Loop variables

void candyapple(int n) { for (int i = 1; i < n; i *= 3) cout << "iteration: " << i << endl; } void caramelcorn(int n) { for (int i = 0; i * i < 6 * n; i++) cout << "iteration: " << i << endl; }

A visual aid for loop executions

• Determine the range of your loop variable
• Determine how many elements within that range will be "hit"
• Complexities of nested loops are (usually) multiplied
• Complexities of separate loops are (usually) added

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int i, j; for (i = 1; i < 9*n; i = i*2) { for (j = n*n; j > 0; j--) { ... } } 1 9𝑜 1 𝑜2

Memory

Functions Dynamic memory and... Pointers!

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Calling functions in C++

• Actual parameter

– Value(s) or variable(s) specified by the function caller

• Formal parameter

– Variables found in the signature/header of the function itself

• Formal parameters must match with actual parameters in
• rder, number, and data type

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Parameters

int a = 5; int b = 7; int result = sum(a, b); int sum(int x, int y) { return x + y; } Actual parameters Formal parameters

Function parameters

• Unless written otherwise, (most) parameters in C++ are

passed by value ("call-by-value")

– the value of the actual parameter is copied to the formal parameter when the function is called

• The actual parameters and formal parameters are different

variables in memory, even if they are named the same

• If you change the value of the formal parameter, this does not

affect the value of the actual parameter back in the caller's memory

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Function parameters and the call stack

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Example

// ... int r = 3; double area = circleArea(r); // ... double circleArea(double radius){ double pi = 3.1415; double sq_r = square(radius); return sq_r * pi; } double square(double x){ return x * x; } main memory

3

r

3.0

radius

3.1415

pi

3.0

x

Function parameters and the call stack

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Example

// ... int r = 3; double area = circleArea(r); // ... double circleArea(double radius){ double pi = 3.1415; double sq_r = square(radius); return sq_r * pi; } double square(double x){ return x * x; } main memory

3

r

3.0

radius

3.1415

pi

9.0

sq_r

Function parameters and the call stack

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Example

// ... int r = 3; double area = circleArea(r); // ... double circleArea(double radius){ double pi = 3.1415; double sq_r = square(radius); return sq_r * pi; } double square(double x){ return x * x; } main memory

3

r

28.274

area

Call-by-value problems

• Do the values really get swapped in the calling function?

– What does the call stack look like?

• This can be fixed using call-by-reference

– add a '&' symbol after the parameter's data type in the signature – see mystery function from Jan.04 slides

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int a = 5; int b = 7; swap(a, b); cout << "a: " << a << endl; cout << "b: " << b << endl; void swap(int x, int y) { int temp = x; x = y; y = temp; }

Dynamic memory

• Variables declared in a function only exist within the scope of

that function (or code block enclosed by { } )

• The data structures we will learn about in this course require
• bjects and variables to persist beyond a function's lifetime

– cannot be allocated to call stack, need to put somewhere else – heap memory / dynamic memory

• We still need local variables that refer or point to the

dynamically allocated memory

– In C++ such variables are pointers

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aka heap memory

Addresses and pointers

• Every storage location in memory (RAM) has an address

associated with it

– The address is the location in memory where a given variable or identifier stores its data

• Can think of address in memory like a mailbox number

– Use the address to find where the mailbox is – Look inside the mailbox to access the contents/value

• A pointer is a special type of variable

– That stores an address rather than a value – The address is used to find a value elsewhere in memory

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Declaring pointers

• Pointer variables are declared as follows:

datatype* identifier – e.g. int* ptr; or int * ptr; or int *ptr;

• Note that the type of a pointer is not the same as the type it

points to

– e.g. ptr is a pointer to an int, but is itself not an int

• Warning! The declaration

int* var1, var2; – declares var1 as a pointer, but var2 as an integer!

• To declare both as pointers, either declare individually, or:

int *var1, *var2;

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Address operator and dereferencing

• Pointers can be assigned the address of an existing variable

– Using the address operator, &

• The value which a pointer points to can be accessed by

dereferencing the pointer

– Using the * operator

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… 1 220-1 int x = 23; 23 212 x int* p = &x; 4096 … p x = 47; 47 *p = 38; 38

Pointers as parameters

• Passing pointer parameters allows the function to access

and/or modify the actual parameter

int getArraySum(vector<int>& arr, int* pcount) { int sum = 0; for (int i = 0; i < arr.size(); i++) { if (arr[i] > 0) (*pcount)++; sum += arr[i]; } return sum; } int numpositive = 0; vector<int> numbers; // assume that 3, 7, -9, 5, -4 get push_back-ed into numbers int result = getArraySum(numbers, &numpositive); cout << "Array sum: " << result << endl; cout << "Number of positive elements: " << numpositive << endl;

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Pointers as parameters

Another example

void f1(int arg) { arg = 22; cout << "f1 arg: " << arg << "\n"; } void f2(int* arg) { *arg = 410; cout << "f2 arg: " << arg << "\n"; } int x = 45; f1(x); cout << "x after f1: " << x << "\n"; f2(&x); cout << "x after f2: << x << "\n";

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Pointer to a pointer

• "You can keep adding levels of pointers until your brain

explodes or the compiler melts – whichever happens soonest"

– stackoverflow user JeremyP

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...to a pointer to a pointer...

int main() { int x = 5; int* p = &x; *p = 6; int** q = &p; int*** r = &q; cout << "*p: " << *p << endl; cout << "*q: " << *q << endl; cout << "**q: " << *(*q) << endl; }

Pointers and dynamic memory

• The new keyword allocates space in dynamic memory and

returns the first address of the allocated space

• delete releases the memory at the address referenced by its

pointer variable

– delete[] is used to release memory allocated to array variables Beware of memory leaks!

int a = 5; int* b = new int; int* c = &a; *c = 4; int** d = &b; int* e = new int[a]; **d = 3; int* f = new int[*b]; delete b; delete e; // causes a memory leak delete[] f;

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allocates an array of size a in dynamic memory

Readings for this lesson

• Carrano & Henry

– Background: 1.4, C1.1-C1.4 – C2.3, C2.5 (Pointers, dynamic arrays)

• Next class

– Carrano & Henry, Chapter 4 (Linked lists)

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