Classes

Week 3

A class is a new type

• new type
• interface in .h, implementation in .cpp file.

Interface:

• define what can be called -- operations
• define what a object stores inside of it -- data

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Example: a Pair class

Pair class

• data: stores two elements of the same type. Call them myFirst and mySecond
• operations:
• getFirst(), getSecond(), setFirst(), setSecond().
• default constructor
• explicit-value constructor
• (copy constructor -- eventually)
• toString() -- return a string representation of the object.

Example: a Pair class

How to hide the contents of an object so that outside code cannot manipulate it directly?

• use private:

For functions that users of the class should call:

• use public:

public: and private: are called access modifiers.

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class XYZ {

public:

void copyXTo(XYZ other) {

other.x = x; // legal?

}

private:

int x;

};

In the class XYZ, x is private.

Will the line "other.x = x;" compile?

1. Yes
2. No

Test-driven Development

TDD: philosophy of software development.

• implement your tests first.
• your tests help you understand what your code must do.
• your tests may help you refine your interface (what parameters are required, types, return type, etc.)
• helps you think about weird cases, etc.

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Pair implementation

Using tests.cpp/catch.hpp as the testing framework.

#include <string>

using namespace std;

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typedef int Item;

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class Pair {

public:

// default constructor

Pair();

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// explicit-value constructor

Pair(Item first, Item second);

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// getters

Item getFirst() const;

Item getSecond() const;

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// setters

void setFirst(Item newVal);

void setSecond(Item newVal);

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string toString() const;

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private:

Item myFirst;

Item mySecond;

};

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ostream &operator<<(ostream &out, const Pair &p);

/**

* @brief return a string with the format <myFirst, mySecond>.

*

* @return string

*/

string Pair::toString() const {

return "<" + to_string(myFirst) + ", " + to_string(mySecond) + ">";

}

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ostream &operator<<(ostream &out, const Pair &p) {

out << p.toString();

return out;

}

​

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main…

assert(p2.toString() == "<1, 2>");

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cout << p2 << endl;

cout << "Check that above output is <1, 2>\n";

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Pair p3 = testReturnPair(7, 10);

assert(p3.toString() == "<7, 10>");

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cout << "All tests passed!\n";

​

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Create an Enemy class

Enemy class

• color
• speed
• name
• health
• x
• y

Rectangle Class

With neighbor, define the class.

What data?

What operations?

What tests?

Exercise: practice for loop

Suppose we have an array, numbers, of size 8, containing integers.

Write code to declare a second array, n2, of size 8, and copy the 8 integers from numbers into n2.

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Exercise: practice for loop

Suppose we have an array, numbers, of size 20, containing integers.

Write code to move all the values "down" in the same array.

(The top value (at index 19) is left as it is. The 0th value is "lost".)

Exercise: practice for loop

Suppose we have an array, numbers, of size 8, containing integers.

Write code to create a second array, reversed, and copy the 8 integers from numbers into reversed, in reverse order.

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