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ECE 39595 Homework 8

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Homework 8
Homework 8 will involve converting a program from using raw C pointers, i.e., the C pointers you
learned in C++, to using unique_ptr and shared_ptr. In the homework directory you will find the code for
a Duck simulator using the Strategy pattern from last semester. You don’t need to understand the
strategy pattern to work this assignment, however.
You should convert the supplied program into three forms:
1. A program that only uses unique_ptr
2. A program that only uses shared_ptr and does initialization using
std::shared_ptr<Type1>(new Type2(args))
where Type2 ISA Type1
3. A program that only uses shared_ptr and does initialization using
std::make_shared<Type1>(Type2(…))
where Type1 is not abstract and Type2 ISA Type1. More on why Type1 should not be abstract
below under “Some useful information”.
Some useful information.
First bit of useful information:
When you have a statement that looks like an assignment but is doing an initialization, e.g.,
Type1 var = std::move(unique_ptr<Type1>(new Type2(args)));
or
var = std::move(unique_ptr<Type1>(new Type2(args)));
or
Type1( ) : var(std::move(unique_ptr<Type1>(new Type2(args)))) {. . .}
where var is an object field and the statement is an initialization being performed in a constructor, C++ is
actually doing an initialization and not an assignment. In these cases you do not have to use
std::move(…), and the statements above could be correctly written as:
Type1 var = unique_ptr<Type2>(new Type2(args));
var = unique_ptr<Type2>(new Type2(args));
Type1( ) : var(unique_ptr<Type1>(new Type2(args))) {. . .}
However, if you use std::move it will not be an error.
Second bit of useful information:
C++ tries to make the semantics of shared_ptr match the semantics of raw C pointers as closely as
possible. It also tries to make the semantics of make_shared mimic the semantics of new as much as
possible. As a result, if you code:
std::shared<AbstractBase> sp = std::make_shared<AbstractBase>(DerivedNotAbstract( ));
this mimicking new you will cause you to get an error since std::make_shared<AbstractBase> will be
treated as an attempt to create a new AbstractBase object. The correct way to do this is to do:
std::shared<AbstractBase> sp = std::make_shared<DerivedNotAbstract>(DerivedNotAbstract());
For those of you that remember last semester when I talked about the desire to program to interfaces,
the use of the concrete DerivedNotAbstract twice should be bothersome. This can be avoided by having
a factory method, std::shared_ptr<AbstractBase> factory(…) {…} and use it to do the initialization, e.g.,
std::shared<AbstractBase> sp = factory(“Derived”);
but this is not necessary for this homework.
What to turn in:
Turn in a directory userid with three subdirectories: Unique, Shared, MakeShared. Unique contains your
program using only unique_ptr, Shared contains your program using only shared_ptr, and not using
std::make_shared<Type>, and MakeShared contains the program that uses only
std::make_shared<Type>(…) to make the shared pointers. If executing
g++ *.cpp
will compile your program, you do not need to include a make file.
Points:
1 points for compiling and running correctly
3 points for the Unique program to only use unique pointers
3 points for the Shared program to only use shared pointers and not use make_shared
3 points for the MakeShared program to only use shared pointers and to always use make_shared to
create a new shared pointer.

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