The University of Iowa
The College of Liberal Arts and Sciences
Department of Computer Science
Programming Languages and Tools:
CS:3210:0001
Lecture/Lab #21
Programming with C++
shared_ptr, pointers and run-time polymorphism
Warm up
-
What will be the output of the following program?
struct car
{
car( std::string const& make_and_model )
: make_and_model( make_and_model ) {}
~car() {
std::cout << "Destroying " << make_and_model << std::endl;
}
std::string const make_and_model;
};
int main() {
std::vector<std::unique_ptr<car>> cars;
cars.push_back( std::make_unique<car>( "Tesla Model X" ) );
cars.push_back( std::make_unique<car>( "Honda Accord" ) );
for ( auto const& c : cars )
std::cout << c->make_and_model << std::endl;
cars[0] = std::make_unique<car>( "Hyundai Sonata" );
for ( auto const& c : cars )
std::cout << c->make_and_model << std::endl;
}shared_ptr
Shared object ownership
auto w = std::make_shared<widget>();
std::cout << "use_count: " << w.use_count() << std::endl;
if ( true ) {
auto w2 = w;
std::cout << "use_count: " << w.use_count() << std::endl;
std::cout << "use_count: " << w2.use_count() << std::endl;
}
std::cout << "use_count: " << w.use_count() << std::endl;
shared_ptr (cont.)
T* ptr
[control block]*
T object
reference count
weak count
other data (custom deleter, etc.)
Control block
- Twice the size of a raw pointer
- Reference counter is dynamically allocated
Primary characteristics:
- Reference counter increments/decrements are atomic
- Supports custom deleters
shared_ptr<T>
Heap
Exercise 1
Use smart pointers to simplify memory management in the following program
-
Open the exercise template
-
Write your code, press Run to test
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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 "Lab21 exercises" poll in #general
Pointers and run-time polymorphism
struct car
{
virtual void print( std::ostream& out ) const
{ out << make_and_model; }
std::string const make_and_model;
};
struct truck : car { // truck is-a car
void print( std::ostream& out ) const override {
car::print( out );
out << ", capacity: " << capacity;
}
double capacity;
};
Containers of polymorphic objects
std::vector<car> cars;
cars.push_back( car( "Tesla Model X" ) );
cars.push_back( car( "Honda Accord" ) );
cars.push_back( truck( "Toyota Tacoma", 1620 ) );
for ( auto const& c : cars ) {
c.print( std::cout );
std::cout << std::endl;
}
Will this work?
Programming with C++, Fall 2019, Lecture #21
By Aleksey Gurtovoy
Programming with C++, Fall 2019, Lecture #21
shared_ptr, pointers and run-time polymorphism
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