for_each calls copy ctor on functor?

Malcolm Smart

This is simplified version of what I am doing but still shows the issue : The Copy Ctor is being called by the for_each algorithm. Whilst in this case it isn't an issue, but in my live app it is. Why is the copy ctor being called and can I prevent it..?

class ProcessVector
{
public:
ProcessVector(const ProcessVector &rhs)
{
cout << "copy ctor" << endl;
}
ProcessVector()
{
cout << "ctor" << endl;
}
~ProcessVector()
{
cout << "dtor" << endl;
}
void operator ()(int i)
{
cout << i << endl;
}

};

int main(int argc, char* argv[])
{
std::vector <int> test;
for (int i = 0 ; i< 5; i++)
test.push_back(i);

for\_each(test.begin(), test.end() , ProcessVector() );

return 0;

}

gives the following output

ctor
0
1
2
3
4
copy ctor
dtor
dtor

Regards Angel ********************************************* The sooner you fall behind, the longer you have to catch up.

Stuart Dootson

It's by design - the C++ Standard specifies that the function object is passed by value, not by reference and that a *copy* of the function object is returned by for_each. You'll have to work round whatever problems this causes you. Stuart Dootson 'Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p'

Andrew Walker

Easy, add an extra level of redirection to manage the lifetime. This issue is described as Stateful Predicates in the book Exceptional C++

#include <algorithm>
#include <vector>
#include <iostream>

using namespace std;

class ProcessVector
{
public:
	ProcessVector()
	{
		cout << "ctor" << endl;
	}
	~ProcessVector()
	{
		cout << "dtor" << endl;
	}
	void operator ()(int i)
	{
		cout << i << endl;
	}

    ProcessVector(const ProcessVector &rhs)
	{
		cout << "copy ctor" << endl;
	}
};

class ProcessVectorIndirect
{
public:
    ProcessVectorIndirect(ProcessVector* process)
        : pProcess(process)
    {
    }

	void operator ()(int i)
	{
		(*pProcess)(i);
	}

private:
    ProcessVector* pProcess;
};

int main(int argc, char* argv[])
{
	std::vector <int> test;
	for (int i = 0 ; i< 5; i++)
		test.push_back(i);
	
        ProcessVector vect;
	for_each(test.begin(), test.end() , ProcessVectorIndirect(&vect) );

	return 0;
}