Need to pass an argument to the function without temporary variable

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Hi everyone!
I need to call a function in this way:

nmsp.Operate( Nmsp::Derived{}.SetName( "New Name") );

So, I can't understand how to realize this.

Thank u for your help in advance.

P.S.
if I use unique_ptr I get an error:

Visual studio compiler: Error C2259 'Nmsp::Base': cannot instantiate abstract class

What I have tried:

C++

#include <iostream>

namespace Nmsp
{
	class Base
	{
	public:
		virtual Base& SetName(const std::string& c) = 0;
		virtual ~Base() = default;
		virtual void Func(std::ostream& out) const = 0;

		double param = 1;
	};

	class Derived : public Base
	{
	public:
		Derived()
		{
			std::cout << "Derived Ctor" << ", name=" << name << std::endl;
		}

		Base& SetName(const std::string& c) override
		{
			name = c;
			param = 2;
			std::cout << "SetName" << ", New name=" << name << std::endl;
			std::cout << "SetName" << ", New param=" << param << std::endl;

			return *this;
		}
		void Func(std::ostream& out) const override
		{
			out << "Derived param = " << param << std::endl;
			out << "Derived caption = " << name << std::endl;
		}
		~Derived()
		{
			std::cout << "Derived Dtor" << ", name=" << name << std::endl;
		}
		std::string name = "Default Name";
	};

	class Holder //: public Base
	{
	public:
		~Holder()
		{
			if (basePtr)
				delete basePtr;
		}
		template <typename T>
		void Operate(T& bPtr)
		{
			basePtr = &bPtr;
			//basePtr = std::make_unique<T>(std::move(bPtr));
		}

		void Go(std::ostream& out)
		{
			basePtr->Func(out);
		}
	private:
		Base* basePtr = nullptr;
		// std::unique_ptr<Base> basePtr;
	};
}

int main()
{
	Nmsp::Holder nmsp;

	Nmsp::Base* pBase = new Nmsp::Derived;
	*pBase = Nmsp::Derived{}.SetName("New Name");
	nmsp.Operate( *pBase );

	std::cout << "before go" << std::endl;

	// Need to call the function Operate() in this way
	//nmsp.Operate( Nmsp::Derived{}.SetName( "New Name") );

	nmsp.Go( std::cout );

	return 0;
}

Posted 8-Oct-20 23:18pm

bartello

Updated 9-Oct-20 9:59am

v3

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[no name] 9-Oct-20 5:50am

Your question is not clear. What is the actual problem?

bartello 9-Oct-20 16:16pm

Here is the actual problem: if I use the commented line in function main() instead of three lines (where the function Operate() is called) - the code will work not properly.

[no name] 10-Oct-20 3:56am

"the code will work not properly."
What does that mean?

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KarstenK · Answer 1 · 2020-10-09T02:20:00

Solution 1

That is what you have implemented. Maybe you can remove the Nmsp to make live easier or write

C++

using namespace Nmsp;

Your Operate function needs a Derived object reference as input, so an overriden

C++

void Operate(T *bPtr)
{
	// reactivated code (why did you use it?)
	basePtr = std::make_unique<T>(std::move(bPtr));
}

should work.

Posted 9-Oct-20 2:20am

KarstenK

Comments

bartello 9-Oct-20 12:03pm

KarstenK, thank u for answer, but I commented that code, because it does not work!
Visual studio compiler: Error C2259 'Nmsp::Base': cannot instantiate abstract class