for_each and functors

**KIBO** · 08-10-2009

I want to use for_each to iterate over a series of sprites based on a predicate and then draw them. But I forgot how to use predicates. The only way I could think of was the following:

Code:

struct OpaqueTest
{
	void operator()(CSprite &sprite) 
	{ 
		if (sprite.GetOpacity() == ALPHA_OPAQUE)
		{
			sprite.Draw();
		}
	}
};

struct NonOpaqueTest
{
	void operator()(CSprite &sprite) 
	{ 
		if (sprite.GetOpacity() != ALPHA_OPAQUE)
		{
			sprite.Draw();
		}
	}
};

std::for_each(m_Sprites, m_Sprites + Size, OpaqueTest);
std::for_each(m_Sprites, m_Sprites + Size, NonOpaqueTest);

Can I compact the two functors into one and use some predicate in the for_each calls? Because as it is now I couldve just written out a for loop. The only thing different is the == and != in the functors.

**anon** · 08-10-2009

You can parametrize the difference. Something like this:

Code:

#include <iostream>
#include <algorithm>
#include <functional>

const int ALPHA_OPAQUE = 1;

struct CSprite
{
    int op; 
    int GetOpacity() const { return op; }
    void Draw() const {
        std::cout << op << '\n';
    }
};

template <class Func>
struct OpaqueTest
{
	void operator()(CSprite &sprite) 
	{ 
		if (Func()(sprite.GetOpacity(), ALPHA_OPAQUE))
		{
			sprite.Draw();
		}
	}
};


int main()
{
    const int Size = 4;
    CSprite m_Sprites[Size] = {{1}, {0}, {1}, {3}};
    std::for_each(m_Sprites, m_Sprites + Size, OpaqueTest<std::equal_to<int> >());
    std::for_each(m_Sprites, m_Sprites + Size, OpaqueTest<std::not_equal_to<int> >());
}

Personally I find that for_each does so little for you, that the trouble is not often worth it. (C++0x has auto keyword which makes writing the manual loop easier, as well as range-based for loop to begin with. So far I mostly use BOOST_FOREACH in more complicated cases.)

**KIBO** · 08-10-2009

seems complex indeed

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