The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science

Programming Languages and Tools:

CS:3210:0001

Lecture/Lab #16

Programming with C++

Destructors, std::vector

Warm up

  • In your own words, define run-time polymorphism

  • How do we mark a member function as polymorphic?

  • How do we override a polymorphic function in a derived class?

Destructors

https://repl.it/@agurtovoy/lab16-destructors

struct car
{
    car() { std::cout << "car constructed\n"; }
    ~car() { std::cout << "car destructed\n"; }
};

int main() {
    car c;
}

  • Called right before the object is destroyed (e.g. when the object goes out of scope)

  • The primary purpose is to free the resources that the object may have acquired during its lifetime

  • The "inverse" of a constructor

  • Has a predefined name and signature: the name of the class prefixed by a tilde (~); has no return value and takes no parameters

Destructors (cont.)

  • There is always only one destructor for a given class (i.e. destructor cannot be overloaded)

  • The body of the destructor is executed right before the object is destroyed.

  • Data members and base classes are destroyed in reverse order from the order in which they were initialized.

  • Members of user-defined types are destroyed by running the member’s own destructor (if any).

  • Data members of the built-in types are left as-is.

Destructors are called when:

  • Variables are destroyed when they go out of scope.

  • Members and base objects (if any) of an instance of a user-defined type are destroyed when the object of which they are a part is destroyed.

  • Temporary objects are destroyed at the end of the full expression in which the temporary was created.

if ( true ) {
    car c;
} // c is destroyed

draw( point(0, 0), point(10, 10) );
// both temporary points are 
// destroyed now

std::vector

https://repl.it/@agurtovoy/lab16-vector

  • A linear collection of objects, all of which have the same type

  • Often referred to as a container because it “contains” other objects. 

  • Similar to list in Python / ArrayList in Java

  • Every object in vector has an associated index, which gives access to that object.

v[0]

v[1]

v[v.size()-1]

...

v.capacity()

v.size()

std::vector operations

#include <vector>

int main() {
    std::vector<int> v;
    assert( v.empty) );
    assert( v.size() == 0 );
    // v[0] = 100; // uh-oh, undefined behavior
    v.push_back( 101 );
    std::cout << v[0] << "\n"; // prints 101
}

v.empty()

v.size()

v.push_back( x )

v[n]

for ( auto e: v ) ...

Returns true if v is empty, false otherwise

Returns number of elements in v

Adds a (copy of) element x to the end of v

Returns a reference to the n-th element in v

Iterates through v's elements in linear order

std::vector vs Python's list

numbers = [ 1, 5, -7, 0, 15, 99 ]
print( 'size:', len( numbers ) )

numbers.append( 110 )
print( 'size:', len( numbers ) )

numbers[0] = -100
for n in numbers:
    print( n )

    std::vector<int> numbers = 
        { 1, 5, -7, 0, 15, 99 };
        
    std::cout << "size: " 
        << numbers.size() << "\n";

    numbers.push_back( 110 );
    std::cout << "size: " 
        << numbers.size() << "\n";

    numbers[0] = -100;
    for ( auto n: numbers )
        std::cout << n << "\n";

Python

C++

Exercise 1

Write a program that asks user for a number N, then asks user to enter N integers, then prints the space-separated sequence of integers to the console

https://repl.it/@agurtovoy/lab16-exercise1

  • Open the exercise template

  • Write your code, press Run to test

  • When you're done, grab your Repl's link and send it as a direct message to me (agurtovoy)

  • Click on the corresponding option in the "Lab16 exercises" poll in #general

Programming with C++, Spring 2020, Lecture #16

By Aleksey Gurtovoy

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Programming with C++, Spring 2020, Lecture #16

Destructors, std::vector

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