How to combine a function and a predicate in for_each? - c++

Question

How can you call a Function over some part of a container, using for_each() ?
I have created a for_each_if() to do a
for( i in shapes )
if( i.color == 1 )
displayShape(i);
and the call looks like
for_each_if( shapes.begin(), shapes.end(),
bind2nd( ptr_fun(colorEquals), 0 ),
ptr_fun( displayShape ) );
bool colorEquals( Shape& s, int color ) {
return s.color == color;
}
However, I feel immitating STL-like algorithms is not something that I should be doing.
Is there a way to use only existing STL keywords to produce this ?
I did not want to do a
for_each( shapes.begin(), shapes.end(),
bind2nd( ptr_fun(display_shape_if_color_equals), 0 ) );
because, in a more complicated case, the functor name would be misleading with respect to what the functor
*Is there a way to access a struct's member (like colorEquals) for functions like for_each without having to create a function ? *

Answer 1

Imitating STL-like algorithms is exactly what you should be doing. That's why they're in the STL.
Specifically, you can use a functor instead of creating an actual function and binding it. This is much neater, really.
template<typename Iterator, typename Pred, typename Operation> void
for_each_if(Iterator begin, Iterator end, Pred p, Operation op) {
for(; begin != end; begin++) {
if (p(*begin)) {
op(*begin);
}
}
}
struct colorequals {
colorequals(int newcol) : color(newcol) {}
int color;
bool operator()(Shape& s) { return s.color == color; }
};
struct displayshape {
void operator()(Shape& s) { // display the shape }
};
for_each_if(shapes.begin(), shapes.end(), colorequals(0), displayshape());
This is usually considered the idiomatic way to go.

Answer 2

Using boost range adaptors is much neater.
using boost::adaptor::filtered;
using boost::bind;
class Shape {
int color() const;
};
void displayShape(const Shape & c);
bool test_color(const Shape & s, int color ){
return s.color() == color;
}
boost::for_each
( vec | filtered(bind(&test_color, _1, 1)
, bind(&displayShape, _1)
)
Note the use of the new range library to abstract away
iterators in favor of ranges and the range adaptors
library to compose a pipeline of operations.
All the standard stl iterator based algorithms have
been ported to range based algorithms.
Imagine this
typedef boost::unordered_map<int, std::string> Map;
Map map;
...
using boost::adaptor::map_keys;
using boost::bind
using boost::ref
using boost::adaptor::filtered;
bool gt(int a, int b)
{ return a > b };
std::string const & get(const Map & map, int const & a)
{ return map[a] }
// print all items from map whose key > 5
BOOST_FOREACH
( std::string const & s
, map
| map_keys
| filtered(bind(&gt, _1, 5))
| transformed(bind(&get, ref(map), _1))
)
{
cout << s;
}
Read Range Adaptors and Range Algorithm.

Answer 3

You can use the C++20 ranges. Here an example where we add one to all even numbers of a std::vector
#include <ranges>
#include <algorithm>
#include <vector>
namespace ranges = std::ranges;
std::vector<int> vec = {1, 2, 3, 4, 5};
const auto even = [](int i) { return 0 == i % 2; };
ranges::for_each(vec | std::views::filter(even), [](int& i){ i+=1;});
You can find a living example on compiler explorer here

Answer 4

To use a regular for_each with an if you would need a Functor that emulates an if condition.
#include <algorithm>
#include <vector>
#include <functional>
#include <iostream>
#include <boost/bind.hpp>
using namespace std;
struct incr {
typedef void result_type;
void operator()(int& i) { ++i; }
};
struct is_odd {
typedef bool return_type;
bool operator() (const int& value) {return (value%2)==1; }
};
template<class Fun, class Cond>
struct if_fun {
typedef void result_type;
void operator()(Fun fun, Cond cond, int& i) {
if(cond(i)) fun(i);
}
};
int main() {
vector<int> vec;
for(int i = 0; i < 10; ++i) vec.push_back(i);
for_each(vec.begin(), vec.end(), boost::bind(if_fun<incr, is_odd>(), incr(), is_odd(), _1));
for(vector<int>::const_iterator it = vec.begin(); it != vec.end(); ++it)
cout << *it << " ";
}
Unfortunately my template hackery isn't good enough to manage this with bind1st and bind2nd as it somehow gets confusing with the binder being returned being a unary_function but it looks pretty good with boost::bind anyhow. My example is no means perfect as it doesn't allow the Func passed into if_fun to return and I guess somebody could point out more flaws. Suggestions are welcome.