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how to implement a template class with c++ and export in dll/so

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questions

模板类必须在header中实现，而不能在cpp中实现，否则作为dll调用进行链接的时候回出错。

common solutions(Recommend)

implement template functions in header.

ThreadPool.h 

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class  SHARED_EXPORT ThreadPool {
public:
	static ThreadPool* Instance(size_t max_thread_pool_size);
	~ThreadPool(); 

	// Add new work item to the pool.
	template<class F> 
	inline void Enqueue(F f)
	{
		io_service_.post(f);//sync, return immediately
	}

	void Free();

private:
	static std::shared_ptr<ThreadPool> m_pInstance;
	bool bfree;

	ThreadPool(size_t size);
	DISABLE_COPY_AND_ASSIGN(ThreadPool);

	boost::thread_group workers_;
	boost::asio::io_service io_service_;
	boost::asio::io_service::work work_;
};

Seperate from headers

solutions 1

A common solution to this is to write the template declaration in a header file, then implement the class in an implementation file (for example .tpp), and include this implementation file at the end of the header.

Foo.h

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template <typename T>
struct Foo
{
    void doSomething(T param);
};

#include "Foo.cpp" // here

Foo.cpp

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template <typename T>
void Foo<T>::doSomething(T param)
{
    //implementation
}

solutions 2

Another solution is to keep the implementation separated, and explicitly instantiate all the template instances you’ll need:

Foo.h

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// no implementation
template <typename T> struct Foo { ... };

Foo.cpp

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#include "Foo.h"

// implementation of Foo's methods

// explicit instantiations
template class Foo<int>;
template class Foo<float>;
// You will only be able to use Foo with int or float

// template void TestClass::templateFunction<int, int>(int, int);

Reference

History

  • 20191012: created.