Vector of shared pointers to templated classes - c++

Question

I have a templated class TaskRunner that takes a polymorphic type Task and I want to create a container of shared pointers to them.
class Task {
virtual void run() = 0;
};
class LoudTask : Task {
void run() {
std::cout << "RUNNING!" << std::endl;
}
};
class QuietTask : Task {
void run() {
std::cout << "running!" << std::endl;
}
};
template<typename T> class TaskRunner {
public:
TaskRunner<T>() {
task = std::make_unique<T>();
}
private:
std::unique_ptr<T> task;
};
using Runner = std::shared_ptr<TaskRunner<Task>>;
However I get error: no matching member function for call to 'push_back' with:
std::vector<Runner> runners;
runners.push_back(std::make_shared<TaskRunner<QuietTask>>());
runners.push_back(std::make_shared<TaskRunner<LoudTask>>());
Due to:
note: candidate function not viable: no known conversion from 'shared_ptr<TaskRunner>' to 'const shared_ptr<TaskRunner>' for 1st argument

Answer 1

Implemented IgorTandetnik's suggestion, and it works for me:
#include <iostream>
#include <memory>
#include <vector>
class Task {
virtual void run() = 0;
};
class LoudTask : Task {
public:
void run() {
std::cout << "RUNNING!" << std::endl;
}
};
class QuietTask : Task {
public:
void run() {
std::cout << "running!" << std::endl;
}
};
class TaskRunnerBase
{
public:
virtual void run() =0;
};
template <class T>
class TaskRunner: public TaskRunnerBase {
public:
TaskRunner():
task(std::make_unique<T>()) {
}
void run() override
{
task->run();
}
private:
std::unique_ptr<T> task;
};
int main()
{
using Runner = std::shared_ptr<TaskRunnerBase>;
std::vector<Runner> runners;
runners.push_back(std::make_shared<TaskRunner<QuietTask>>());
runners.push_back(std::make_shared<TaskRunner<LoudTask>>());
runners[0]->run();
runners[1]->run();
}
Output:
running!
RUNNING!
Note however that TaskRunner doesn't need to be a template; as it is currently implemented above, it has a kind of double role: (1) task factory, and (2) container and runner of tasks.
paolo's answer separates this out nicely, there, the factory aspect is moved to the main function.

Answer 2

If TaskRunner is supposed to run tasks that inherit from Task only, you may consider de-templatizing it:
#include <iostream>
#include <memory>
#include <vector>
class Task {
public:
virtual void run() = 0;
};
class LoudTask : public Task {
public:
void run() override { std::cout << "RUNNING!" << std::endl; }
};
class QuietTask : public Task {
public:
void run() override { std::cout << "running!" << std::endl; }
};
class TaskRunner {
public:
explicit TaskRunner(std::unique_ptr<Task>&& task_)
: task(std::move(task_)) {}
void run() {
if (this->task) this->task->run();
}
private:
std::unique_ptr<Task> task;
};
int main() {
using Runner = std::shared_ptr<TaskRunner>;
std::vector<Runner> runners;
runners.push_back(
std::make_shared<TaskRunner>(std::make_unique<QuietTask>()));
runners.push_back(
std::make_shared<TaskRunner>(std::make_unique<LoudTask>()));
for (auto& runner : runners) runner->run();
}

Answer 3

This is another implementation that eliminates the need for another inheritance hierarchy and vtable since we already accomplished that with the Task hierarchy:
#include <iostream>
#include <vector>
class Task {
public:
virtual void run() = 0;
};
class LoudTask : public Task {
public:
void run() {
std::cout << "RUNNING!" << std::endl;
}
};
class QuietTask : public Task {
public:
void run() {
std::cout << "running!" << std::endl;
}
};
class TaskRunner {
public:
TaskRunner(std::unique_ptr<LoudTask> task) : m_task{ std::unique_ptr<Task>(task.release()) } {}
TaskRunner(std::unique_ptr<QuietTask> task) : m_task{ std::unique_ptr<Task>(task.release()) } {}
void run()
{
m_task->run();
}
private:
std::unique_ptr<Task> m_task;
};
using Runner = std::shared_ptr<TaskRunner>;
int main()
{
std::vector<Runner> runners;
runners.push_back(std::make_shared<TaskRunner>(std::make_unique<QuietTask>()));
runners.push_back(std::make_shared<TaskRunner>(std::make_unique<LoudTask>()));
runners[0]->run();
runners[1]->run();
}