This is part of an assignment, I am stuck at this instruction:
Sort your randomly generated pool of schedules.
Use std::stable_sort,
passing in an object of type schedule_compare as the custom comparison
operator.
UPDATE: I was checking cppreference stable_srot(), see method definition below:
void stable_sort ( RandomAccessIterator first, RandomAccessIterator
last,Compare comp );
, and it seems from what I understood is that you can only pass functions to the last argument (Compare comp) of the stable_sort() i.e:
However, in the instructions, it says that you need to pass an object of type schedule_compare. How is this possible ?
This is my code below:
struct schedule_compare
{
explicit schedule_compare(runtime_matrix const& m)
: matrix_{m} { }
bool operator()(schedule const& obj1, schedule const& obj2) {
if (obj1.score > obj2.score)
return true;
else
return false;
}
private:
runtime_matrix const& matrix_;
};
auto populate_gene_pool(runtime_matrix const& matrix,
size_t const pool_size, random_generator& gen)
{
std::vector<schedule> v_schedule;
v_schedule.reserve(pool_size);
std::uniform_int_distribution<size_t> dis(0, matrix.machines() - 1);
// 4. Sort your randomly generated pool of schedules. Use
// std::stable_sort, passing in an object of type
// schedule_compare as the custom comparison operator.
std::stable_sort(begin(v_schedule), end(v_schedule), ???)
return; v_schedule;
}
For algorithm functions that accepts a "function" (like std::stable_sort) you can pass anything that can be called as a function.
For example a pointer to a global, namespace or static member function. Or you can pass a function-like object instance (i.e. an instance of a class that has a function call operator), also known as a functor object.
This is simply done by creating a temporary object, and passing it to the std::stable_sort (in your case):
std::stable_sort(begin(v_schedule), end(v_schedule), schedule_compare(matrix));
Since the schedule_compare structure have a function call operator (the operator() member function) it can generally be treated like any other function, including being "called".
Related
I have a set of non-orthogonal policies, all of them implementing a common named method, the policies add safety checks.
I want users to be able to combine the policies to allow more complex validation without creating policies for each combination case by hand.
My approach is creating a new policy class to combine others.
The simplified example below shows C as the combining class, here the method id is combined. The expected result is, when calling id on C, to sequentially call the id of each base class.
#include <iostream>
using namespace std;
struct A
{
void id() { cout << "A ";}
};
struct B
{
void id() { cout << "B ";}
};
template<class A, class... As>
struct C : public A, public As...
{
void id()
{
A::id();
As...::id(); // This line does not work, it is illustrative.
}
};
int main()
{
C<A, B> c;
c.id();
//expected: result A B
}
The question is: Is it possible to expand As... somehow to do this without using a recursive approach, just using the ... operator?
Sure. You need a context that permits pack expansion - a simple one is a braced initializer list, which also has the benefit of guaranteeing left-to-right evaluation:
using expander = int[];
(void) expander { 0, ((void) As::id(), 0)... };
... expands a pattern to its left; in this case the pattern is the expression ((void) As::id(), 0).
The , in the expression is the comma operator, which evaluates the first operand, discards the result, then evaluates the second operand, and returns the result.
The (void) cast on As::id() exists to guard against overloaded operator,, and can be omitted if you are sure that none of the As::id() calls will return something that overloads the comma operator.
0 on the right hand side of the comma operator is because expander is an array of ints, so the whole expression (which is used to initialize an element of the array) must evaluate to an int.
The first 0 ensures that we don't attempt to create an illegal 0-sized array when As is an empty pack.
Demo.
In C++17 (if we are lucky), the entire body of C::id can be replaced with a binary fold expression: (A::id(), ... , (void) As::id()); Demo.
I am trying to get more familiar with the C++11 standard by implementing the std::iterator on my own doubly linked list collection and also trying to make my own sort function to sort it.
I would like the sort function to accept a lamba as a way of sorting by making the sort accept a std::function, but it does not compile (I do not know how to implement the move_iterator, hence returning a copy of the collection instead of modifying the passed one).
template <typename _Ty, typename _By>
LinkedList<_Ty> sort(const LinkedList<_Ty>& source, std::function<bool(_By, _By)> pred)
{
LinkedList<_Ty> tmp;
while (tmp.size() != source.size())
{
_Ty suitable;
for (auto& i : source) {
if (pred(suitable, i) == true) {
suitable = i;
}
}
tmp.push_back(suitable);
}
return tmp;
}
Is my definition of the function wrong? If I try to call the function, I recieve a compilation error.
LinkedList<std::string> strings{
"one",
"two",
"long string",
"the longest of them all"
};
auto sortedByLength = sort(strings, [](const std::string& a, const std::string& b){
return a.length() < b.length();
});
Error: no instance of function template "sort" matches the argument
list argument types are: (LinkedList, lambda []bool
(const std::string &a, const std::string &)->bool)
Additional info, the compilation also gives the following error:
Error 1 error C2784: 'LinkedList<_Ty> sort(const
LinkedList<_Ty> &,std::function)' : could not
deduce template argument for 'std::function<bool(_By,_By)>'
Update: I know the sorting algorithm is incorrect and would not do what is wanted, I have no intention in leaving it as is and do not have a problem fixing that, once the declaration is correct.
The problem is that _By used inside std::function like this cannot be deduced from a lambda closure. You'd need to pass in an actual std::function object, and not a lambda. Remember that the type of a lambda expression is an unnamed class type (called the closure type), and not std::function.
What you're doing is a bit like this:
template <class T>
void foo(std::unique_ptr<T> p);
foo(nullptr);
Here, too, there's no way to deduce T from the argument.
How the standard library normally solves this: it does not restrict itself to std::function in any way, and simply makes the type of the predicate its template parameter:
template <typename _Ty, typename _Pred>
LinkedList<_Ty> sort(const LinkedList<_Ty>& source, _Pred pred)
This way, the closure type will be deduced and all is well.
Notice that you don't need std::function at all—that's pretty much only needed if you need to store a functor, or pass it through a runtime interface (not a compiletime one like templates).
Side note: your code is using identifiers which are reserved for the compiler and standard library (identifiers starting with an underscore followed by an uppercase letter). This is not legal in C++, you should avoid such reserved identifiers in your code.
Scott Meyer's Effective STL showed an example of using sorted vector of pair<key, value> to simulate a map or multimap. In order to sort the vector and later look up the elements in the vector, he has to create a functor DataCompare where there are 3 overloaded function operator(): one that takes two pair object for sorting; two that take a pair and a key because in algorithms like lower_bound, equal_range, we don't know the order in which the key object and the pair object are passed to the custom comparison functor:
typedef pair<string, int> Data; // type held in the "map"
class DataCompare { // class for comparison
public:
bool operator()(const Data& lhs, const Data& rhs) const // comparison func for sorting
{
return keyLess(lhs.first, rhs.first);
}
bool operator()(const Data& Ihs, const Data::first_type& k) const // comparison func for lookups
{ //(form 1)
return keyLess(lhs.first, k);
}
bool operator()(const Data::first_type& k, const Data& rhs) const // comparison func for lookups
{ // (form 2)
return keyLessfk, rhs.first);
}
....
Now that we have C++11, we have lambda and std::bind and std::function to replace functors. But how do I use lambda or std::bind or std::function to deal with the uncertainty of the order the arguments are passed in (lookup) algorithms like lower_bound, upper_bound and equal_range?
In http://www.cplusplus.com/reference/algorithm/lower_bound/, the description for comp seems to suggest I can create a lambda whose first parameter can be the pair and the second parameter can be of the same type as key. However, inside the implementation of lower_bound, it can switch the order in calling the lambda. In g++ (my version is 4.8.4), I got the following error:
... else if (__comp(__val, *__middle))
...
no known conversion for argument 1 from ‘const std::basic_string’ to ‘const ms2dict& {aka const std::pair< std::basic_string< char>, std::basic_string< char> >&}’
Here the else if (__comp(__val, *__middle)) is using the lambda but passing the arguments in reverse order. Is there a way to create a universal callable object that deal with this problem?
I'm looking for a way of checking whether a std::function pointer is bound to a member function of a particular object. I'm aware that std::function itself has no '==' operator. I have however come across the std::function::target method which should be able, in principle, to give me the address of the function to which the pointer is pointing. My starting point was therefore this:
bool MyClass::isThePointerSetToMyMethod(std::function<void (const char*, string)> const& candidate)
{
// Create a pointer to the local reportFileError function using the same syntax that we did in the constructor:
std::function<void (const char *, string)> localFn = std::bind(&MyClass::theLocalMember, this,
std::placeholders::_1, std::placeholders::_2);
// Find the target
auto ptr1 = localFn.target< std::function<void (const char *, string)> >();
// Find the target of the candidate
auto ptr2 = candidate.target< std::function<void (const char *, string)> >();
// Compare the two pointers to see whether they actually point to the same function:
if (!ptr1 || !ptr2) return false;
if (*ptr1 == *ptr2)
return true;
else
return false;
}
This doesn't work, and the reason is that the values of 'ptr1' and 'ptr2' are always returned as null. According to the documentation for the std::function::target method, this must be because the type that I've specified for the target is not correct.
If I look at what target_type(localFn) actually is (using Visual C++ 2013), it's a bit frightening:
class std::_Bind<1,void,struct std::_Pmf_wrap<void (__thiscall MyClass::*)(char const *, string),void,class MyClass,char const *,string>,class MyClass * const,class std::_Ph<1> &,class std::_Ph<2> &>
Nevertheless, target_type(candidate) gives the same result, so I thought I'd try a typedef:
bool MyClass::isThePointerSetToMyMethod(std::function<void (const char*, string)> const& candidate)
{
typedef class std::_Bind<1,void,struct std::_Pmf_wrap<void (__thiscall MyClass::*)(char const *, string),void,class MyClass,char const *,string>,class MyClass * const,class std::_Ph<1> &,class std::_Ph<2> &> wally;
// Create a pointer to the local reportFileError function using the same syntax that we did in the constructor:
std::function<void (const char *, string)> localFn = std::bind(&MyClass::theLocalMember, this,
std::placeholders::_1, std::placeholders::_2);
// Find the target
auto ptr1 = localFn.target< wally >();
// Find the target of the candidate
auto ptr2 = candidate.target< wally >();
// Compare the two pointers to see whether they actually point to the same function:
if (!ptr1 || !ptr2) return false;
if (*ptr1 == *ptr2)
return true;
else
return false;
}
Alas this gets me no further; the values of ptr1 and ptr2 are still null.
So for now I've run out of ideas. Is there anyone reading this who knows either:
(1) The appropriate form for a typedef for a std::function pointer to the member function of a class, or
(2) A better way to achieve my ultimate objective, which is to tell whether a std::function pointer is pointing to a particular object's member function or whether it isn't?
[Background, in case anyone is interested: the reason I'm doing this is that I have a callback table where different callbacks are set to different functions depending on the state that the system is in; this makes state control very simple, as it means that in a given context I can call a given callback and know that the actions taken by the function I've called will be appropriate for the current state, without having to know anything about what that state actually is. Usually, when an object is instantiated which will change the system state, it takes control of the relevant callback(s) and binds them to whatever local member functions are appropriate for whatever state it's in. Under these circumstances, however, the object's destructor ought to return the callbacks to their status quo ante so that they are not left pointing to nothing.
Very rarely, an object may bind the callbacks to its member functions in its constructor, but before its destructor is called another object may take control of the same callbacks itself, and re-bind them to member functions of its own. If this happens, then the first object's destructor needs to be able to recognise that this has happened, and exit without affecting the callbacks' assignment to the second object's methods. The obvious way to do this is for the destructor to be able to check whether the callbacks are still assigned to its own methods or not, and if they are not then to leave well alone.]
Flesh out the callback table into a class which manages the table. All modifications to the table should be done through this class's interface. Internally, you would maintain a stack-like structure which lets you undo the changes done to the callback table. Barebones interface would look something like:
class CallbackTable
{
public:
bool ApplyChanges(...)
{
//Push the old values of the entries that would be changed here into your change-tracker stack and modify the table
}
bool UnApplyChanges(...)
{
//Pop the change-tracker stack and restore the table to the state it was in before the most recent change was applied.
}
};
I have the boost::variant over set of non-default constructible (and maybe even non-moveable/non-copyable and non-copy/move constructible) classes with essentialy different non-default constructor prototypes, as shown below:
#include <boost/variant.hpp>
#include <string>
#include <list>
struct A { A(int) { ; } };
struct B { B(std::string) { ; } };
struct C { C(int, std::string) { ; } };
using V = boost::variant< A const, B const, C const >;
using L = std::list< V >;
int main()
{
L l;
l.push_back(A(1)); // an extra copy/move operation
l.push_back(B("2")); // an extra copy/move operation
l.push_back(C(3, "3")); // an extra copy/move operation
l.emplace_back(4);
l.emplace_back(std::string("5"));
// l.emplace_back(3, std::string("3")); // error here
return 0;
}
I expect, that std::list::emplace_back allows me to construct-and-insert (in single operation) new objects (of all the A, B, C types) into list, even if they have T & operator = (T const &) = delete;/T & operator = (T &&) = delete; and T(T const &) = delete;/T(T &&) = delete;. But what should I do, if constructor is a non-conversion one? I.e. have more, than one parameter. Or what I should to do if two different variant's underlying types have ambiguous constructor prototypes? In my opinion, this is the defect of implementation of the boost::variant library in the light of the new features of C++11 standard, if any at all can be applyed to solve the problem.
I specifically asked about std::list and boost::variant in superposition, because they are both internally implement the pimpl idiom in some form, as far as I know (say, boost::variant currently designed by means of temporary heap backup approach).
emplace can only call the constructors of the type in question. And boost::variant's constructors only take single objects which are unambiguously convertible to one of the variant's types.
variant doesn't forward parameters arbitrarily to one of its bounded types. It just takes a value. A single value that it will try to convert to one of the bounded types.
So you're going to have to construct an object and then copy that into the variant.
Assuming you can modify your "C" class, you could give it an additional constructor that takes a single tuple argument.