template\<class Arg1, class Arg2, class Result>
struct equal_to : public std::binary_function<Arg1, Arg2, Result>
{
bool operator() (const Arg1& arg1, const Arg2& arg2)
{
return (arg1.getID()) == arg2;
}
};
vec.erase(remove_if(vec.begin(), vec.end(), std::bind2nd(equal_to<Object, int, bool>(), NUM) vec.end())
Function Object Question
Author
Hello, I don't think my issue is too complicated - I just don't know what i'm doing.
I have a vector of objects. I want to perform an operation on the vector to do this: if any object in this vector has a variable named id equal to NUM delete object from vector and resize the vector so that there are no discontiguous elements.
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To illustrate:
vector abcd = {a, b, c, d}
abcd.erase(element_2 if element_2.ID == NUM)
Lets say element_2.ID does equal NUM.
I then want my vector to look like this: abcd = {a,c,d}.
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I believe the way to do it is something like this:
I've never used templates or function objects before though, so I need some guidance. Thanks.
Since you're using the STL, try using:
Skizz
remove_copy_if (src_begin, src_end, dest_begin, predicate);Skizz
Author
Thanks, but I don't know how to make a predicate to do what I want to do. That is my problem. And I know the predicate doesn't have to be a function object but I'd like it to be :D
Author
One of the problems with my above code is that I'm deriving from a binary function object and i think remove_if needs a unary_function...
Quote:Original post by Entheoremove_if() does require a unary function (specifically a predicate), but the use of bind2nd meets this requirement.
One of the problems with my above code is that I'm deriving from a binary function object and i think remove_if needs a unary_function...
Here's a working example of what (I think) you're trying to do:
#include <iostream>#include <vector>#include <algorithm>#include <iterator>using namespace std; // Just to make writing this example go a little faster :)struct Test { Test() {} Test(int value) : value(value) {} int Value() const { return value; }private: int value;};ostream& operator<<(ostream& os, const Test& test) { return os << test.Value(); }void print(const vector<Test>& v) { copy(v.begin(), v.end(), ostream_iterator<Test>(cout, " ")); cout << endl;}struct generator { generator() : count(0) {} Test operator()() { return Test(count++); }private: size_t count;};struct value_matches : public binary_function<Test, int, bool> { bool operator()(const Test& test, int value) const { return test.Value() == value; }};struct value_greater : public binary_function<Test, int, bool> { bool operator()(const Test& test, int value) const { return test.Value() > value; }};int main(){ vector<Test> v(10); generate(v.begin(), v.end(), generator()); print(v); v.erase(remove_if(v.begin(), v.end(), bind2nd(value_matches(), 3)), v.end()); print(v); v.erase(remove_if(v.begin(), v.end(), bind2nd(value_greater(), 6)), v.end()); print(v);}It may not be perfect, stylistically speaking, but it does compile and appears to work correctly.
I didn't examine your code carefully, but it looks like you have the right general idea. Some more information would be useful though (e.g. error messages if it's a compile-time error, or a description of the program's behavior if it's a run-time error).
boost::bind is very useful here:
#include <boost/bind.hpp>/* ... */int main(){ using boost::bind; vector<Test> v(10); generate(v.begin(), v.end(), generator()); print(v); v.erase(remove_if(v.begin(), v.end(), bind(&Test::Value, _1) == 3, v.end()); print(v); v.erase(remove_if(v.begin(), v.end(), bind(&Test::Value, _1) > 6, v.end()); print(v);}This topic is closed to new replies.
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