Consider the following program:
#include <iostream>
#include <ostream>
#include <string>
#include <utility>
using namespace std;
struct Name { string s; Name(string s) : s(move(s)) { } };
struct A : virtual Name { A(string s) : Name(move(s)) { } };
struct B : virtual Name { B(string s) : Name(move(s)) { } };
struct C : A, B { C(string s) : A(string()), B(string()), Name(move(s)) { } };
C f() { return C("abcdefghijklmnopqrstuvwxyz"); }
int main()
{
C c1("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
C c2("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
C ff = f();
c1 = f();
c2 = ff;
cout << "C1 = " << c1.s << " " << "C2 = " << c2.s << "\n";
return 0;
}
gcc (4.9.2) (http://ideone.com/G7uzCQ) and clang++ both print different values for C1 and C2 whereas Visual Studio 2013 and 2015 consistently print the lower case alphabet sequence for both C1 and C2.
Who is right? or is it just some loop hole in the standard?
From §12.8/28 [class.copy]:
It is unspecified whether subobjects representing virtual base classes are assigned more than once by the
implicitly-defined copy/move assignment operator. [ Example:
struct V { };
struct A : virtual V { };
struct B : virtual V { };
struct C : B, A { };
It is unspecified whether the virtual base class subobject V is assigned twice by the implicitly-defined copy-/move assignment operator for C. — end example ]
The behavior of your program is unspecified according to the standard. Visual Studio decides to call the assignment operator once while GCC and Clang do so twice. The result of a move operation leaves the move-from object in a valid but unspecified state, so subsequently moving from the temporary again will leave s with an unspecified value, but per the above clause this is allowed to happen. So to answer your question, there is no correct behavior for this program.
Related
I am depending on a library, the authors of which extensively used the brace-notation for invoking all constructors, as has been fondly advertised and recommended by a number of parties in recent years.
The library is mostly developed on Linux using gcc but aims at being cross-platform compatible and in my case is used on Windows using Visual Studio 2015.
If I try to build the library, I get a C2447 compiler error which arises when templates of templates use this brace-notation.
I tried to illustrate my case with the following MWE.
#include <iostream>
template <typename T>
class A
{
public:
A(T x);
virtual ~A() = default;
T getX();
private:
T x;
};
template <typename T>
class B : public A<T>
{
public:
B(T x);
};
template <typename T>
class C : public A<T>
{
public:
C(T x);
};
template <typename T>
class D : public A<T>
{
public:
D(T x);
};
int main(int argn, char** argc)
{
A<int> a(42);
B<int> b(42);
C<int> c(42);
D<int> d(42);
std::cout << "A: " << a.getX() << std::endl
<< "B: " << b.getX() << std::endl
<< "C: " << c.getX() << std::endl
<< "D: " << d.getX() << std::endl;
return 0;
}
template<typename T>
A<T>::A(T x) : x(x) {}
template<typename T>
T A<T>::getX() { return x; }
template<typename T>
B<T>::B(T x) : A{ x / 2 } {} // does not compile in gcc [1]
template<typename T>
C<T>::C(T x) : A<T>(x * 2) {} // compiles fine in both
template<typename T>
D<T>::D(T x) : A<T>{ x*x } {} // does not compile in MSVC 2015 [2]
/*
[1]: error: class 'B<T>' does not have any field named 'A'
B<T>::B(T x) : A{ x / 2 } {}
[2]: error C2447: '{': missing function header (old-style formal list?)
*/
My online-search to figure out whether this is a compiler-bug or if it is invalid notation according to the standard remained fruitless. Can anybody please elucidate which notations used in B, C, and D are to be considered correct, please? Obviously both compilers agree on C, but naively I would consider the notations used in B and D valid as well.
In notation B (below), base class template A is missing the template parameter (the constructor of A refers to it), so GCC (and Clang as well) fail to compile it.
template<typename T>
B<T>::B(T x) : A{ x / 2 } {} // does not compile in gcc [1]
Changing it like this works.
template<typename T>
B<T>::B(T x) : A<T>{ x / 2 } {} // does not compile in gcc [1]
In case of notation D, GCC (and Clang as well) are correct because there is a matching constructor for initialization of A<int>. MSVC is wrong to reject it.
I have a boost graph with multiples weights for each edges (imagine one set of weights per hour of the day). Every one of those weights values is stored in a propretyEdge class :
class propretyEdge {
std::map<std::string,double> weights; // Date indexed
}
I created a graph with those properties, and then filled it with the right values.
The problem is now that I want to launch the Dijkstra algorithm over a particular set of weight on the graph : for example a function that could be :
void Dijkstra (string date, parameters ... )
That would use the
weights[date]
value for each Edge of the graph.
I read over and over the documentation, and I couldn't have a clear picture of what I have to do. I surely need to write something like this, but I have no idea were to start :
boost::dijkstra_shortest_paths (
(*graph_m),
vertex_origin_num_l,
// weight_map (get (edge_weight, (*graph_m)))
// predecessor_map(boost::make_iterator_property_map(predecessors.begin(), get(boost::vertex_index, (*graph_m)))).
// distance_map(boost::make_iterator_property_map(distances.begin (), get(vertex_index,(*graph_m) )))
predecessor_map(predecessorMap).
distance_map(distanceMap)
);
Thank you for your help.
Edit
Thanks to the wonderful Answer of Sehe, I was able to do exactly what I wanted on MacOS and on Ubuntu.
But when we tried to compile this piece of code on Visual Studio 2012, it appeared that VS wasn't very good at understanding pointer function of boost. So we modified the part of Sehe :
auto dated_weight_f = [&](Graph::edge_descriptor ed) {
return g[ed].weights.at(date);
};
auto dated_weight_map = make_function_property_map<Graph::edge_descriptor, double>(dated_weight_f);
by :
class dated_weight_f {
public:
dated_weight_f(Graph* graph_p,std::string date_p){
graph_m=graph_p;
date_m=date_p;
}
typedef double result_type;
result_type operator()(Edge edge_p) const{
return (*graph_m)[edge_p].weights.at(date_m);
}
private:
Graph* graph_m;
std::string date_m;
};
const auto dated_weight_map = make_function_property_map<Edge>(dated_weight_f(graph_m,date_l));
Which had the advantage of not using a pointer function.
Since it's apparently not immediately clear that this question is answered in the other answer, I'll explain.
All you really need is a custom weight_map parameter that is "stateful" and can select a certain value for a given date.
You can make this as complicated as you wish ¹, so you could even interpolate/extrapolate a weight given an unknown date ², but let's for the purpose of this demonstration keep it simple.
Let's define the graph type (roughly) as above:
struct propretyEdge {
std::map<std::string, double> weights; // Date indexed
};
using Graph = adjacency_list<vecS, vecS, directedS, no_property, propretyEdge>;
Now, let's generate a random graph, with random weights for 3 different dates:
int main() {
Graph g;
std::mt19937 prng { std::random_device{}() };
generate_random_graph(g, 8, 12, prng);
uniform_real<double> weight_dist(10,42);
for (auto e : make_iterator_range(edges(g)))
for (auto&& date : { "2014-01-01", "2014-02-01", "2014-03-01" })
g[e].weights[date] = weight_dist(prng);
And, jumping to the goal:
for (std::string const& date : { "2014-01-01", "2014-02-01", "2014-03-01" }) {
Dijkstra(date, g, 0);
}
}
Now how do you implement Dijkstra(...)? Gleaning from the documentation sample, you'd do something like
void Dijkstra(std::string const& date, Graph const& g, int vertex_origin_num_l = 0) {
// magic postponed ...
std::vector<Graph::vertex_descriptor> p(num_vertices(g));
std::vector<double> d(num_vertices(g));
std::vector<default_color_type> color_map(num_vertices(g));
boost::typed_identity_property_map<Graph::vertex_descriptor> vid; // T* property maps were deprecated
dijkstra_shortest_paths(g, vertex_origin_num_l,
weight_map(dated_weight_map).
predecessor_map(make_iterator_property_map(p.data(), vid)).
distance_map(make_iterator_property_map(d.data(), vid)).
color_map(make_iterator_property_map(color_map.data(), vid))
);
Now the only unclear bit here should be dated_weight_map.
Enter Boost Property Maps
As I showed in the linked Is it possible to have several edge weight property maps for one graph BOOST?, you can have all kinds of property maps ³, including invocation of user-defined functions. This is the missing piece:
auto dated_weight_f = [&](Graph::edge_descriptor ed) {
return g[ed].weights.at(date);
};
auto dated_weight_map = make_function_property_map<Graph::edge_descriptor, double>(dated_weight_f);
Voilà: done
I hope that by now, the correspondence in the question as well as the answer of the linked question is clear. All that's left to do is post the full live sample and the outcome in a pretty picture:
Live On Coliru
#include <boost/property_map/property_map.hpp>
#include <boost/property_map/function_property_map.hpp>
#include <boost/property_map/property_map_iterator.hpp>
#include <random>
#include <boost/graph/random.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/dijkstra_shortest_paths.hpp>
#include <fstream>
using namespace boost;
struct propretyEdge {
std::map<std::string, double> weights; // Date indexed
};
using Graph = adjacency_list<vecS, vecS, directedS, no_property, propretyEdge>;
void Dijkstra(std::string const& date, Graph const& g, int vertex_origin_num_l = 0) {
auto dated_weight_f = [&](Graph::edge_descriptor ed) {
return g[ed].weights.at(date);
};
auto dated_weight_map = make_function_property_map<Graph::edge_descriptor, double>(dated_weight_f);
std::vector<Graph::vertex_descriptor> p(num_vertices(g));
std::vector<double> d(num_vertices(g));
std::vector<default_color_type> color_map(num_vertices(g));
boost::typed_identity_property_map<Graph::vertex_descriptor> vid; // T* property maps were deprecated
dijkstra_shortest_paths(g, vertex_origin_num_l,
weight_map(dated_weight_map).
predecessor_map(make_iterator_property_map(p.data(), vid)).
distance_map(make_iterator_property_map(d.data(), vid)).
color_map(make_iterator_property_map(color_map.data(), vid))
);
std::cout << "distances and parents for '" + date + "':" << std::endl;
for (auto vd : make_iterator_range(vertices(g)))
{
std::cout << "distance(" << vd << ") = " << d[vd] << ", ";
std::cout << "parent(" << vd << ") = " << p[vd] << std::endl;
}
std::cout << std::endl;
std::ofstream dot_file("dijkstra-eg-" + date + ".dot");
dot_file << "digraph D {\n"
" rankdir=LR\n"
" size=\"6,4\"\n"
" ratio=\"fill\"\n"
" graph[label=\"shortest path on " + date + "\"];\n"
" edge[style=\"bold\"]\n"
" node[shape=\"circle\"]\n";
for (auto ed : make_iterator_range(edges(g))) {
auto u = source(ed, g),
v = target(ed, g);
dot_file
<< u << " -> " << v << "[label=\"" << get(dated_weight_map, ed) << "\""
<< (p[v] == u?", color=\"black\"" : ", color=\"grey\"")
<< "]";
}
dot_file << "}";
}
int main() {
Graph g;
std::mt19937 prng { std::random_device{}() };
generate_random_graph(g, 8, 12, prng);
uniform_real<double> weight_dist(10,42);
for (auto e : make_iterator_range(edges(g)))
for (auto&& date : { "2014-01-01", "2014-02-01", "2014-03-01" })
g[e].weights[date] = weight_dist(prng);
for (std::string const& date : { "2014-01-01", "2014-02-01", "2014-03-01" }) {
Dijkstra(date, g, 0);
}
}
Output, e.g.
¹ As long as you keep the invariants required by the algorithm you're invoking. In particular, you must return the same weight consistently during the execution, given the same edge. Also, some algorithms don't support negative weight etc.
² I'd highly suggest using a Boost ICL interval_map in such a case but I digress
³ see also map set/get requests into C++ class/structure changes
Is it possible to loop over sub range using range based for loop ?
std::vector <std::string> inputs={"1","abaaaa","abc","cda"};
for (auto &it : new_vector(inputs.begin()+1, inputs.end()))
{
// …
}
You could use Boost's iterator_range:
for (auto &it : boost::make_iterator_range(inputs.begin()+1, inputs.end()))
{
cout << it << endl;
}
demo
Alternatively you could write your own wrapper.
Unfortunately, there is no such thing in the C++ standard library. However, you can define your own wrapper like this (requires at least C++ 11 - which should not be problem in 2021):
template<typename Iter>
struct range
{
Iter b, e;
Iter begin() const { return b; }
Iter end() const { return e; }
};
template<typename T>
auto slice(const T& c, std::size_t from, std::size_t to = -1) -> range<decltype(c.begin())>
{
to = (to > c.size() ? c.size() : to);
return range<decltype(c.begin())>{c.begin() + from, c.begin() + to};
}
And then you can use it:
std::vector<int> items(100);
// Iterates from 4th to 49th item
for (auto x: slice(items, 4, 50))
{
}
// Iterates from 15th to the last item
for (auto x: slice(items, 15))
{
}
tl;dr
Long story short, you #include <range/v3/view/subrange.hpp> and change your new_vector to ranges::subrange. And that's it. Demo on Compiler Explorer.
So
Given the name you imagine for this function, new_vector, maybe you think you need the entity on the right of : to be a std::vector or at least some type of container.
If that's the case, then change your mind, it's not needed. All that : wants from its "right hand side" is that it have begin and end defined on them, member or non member. For instance, this compiles and runs just fine:
struct A {};
int* begin(A);
int* end(A);
struct B {
int* begin();
int* end();
};
int main()
{
for (auto it : A{}) {}
for (auto it : B{}) {}
}
I made a simple example to test boost bind's interaction with derived classes.
I created two subclasses with different getarea functions. I expected
g1 = boost::bind(boost::mem_fn(&Shape::getarea), Rec)
to print the area of Rectangle(10,20) but instead it printed '1'. I get the same when I instead write Rectangle::getarea. It prints the same even when I input other functions eg. member of Rectangle
double sum(double h,double w){return h+w; }
and use
g1 = boost::bind(boost::mem_fn(&Rectangle::sum), Rec,2,3)
Question 1: Why does it return '1'?Is that a default response for error?
My second problem is to do the same of printing g2 but now Rec is replaced by **iter, i.e. an object of some derived class type from a list of objects. Since getarea is a virtual fcn, once I get the above working it should be fine to just write:
g2= boost::bind(boost::mem_fn(& Shape::getarea , &(**iter));
Question 2: However, I was wondering if there is a way to return the classtype of **iter eg. classof(**iter) and then put it in g2 i.e.
g2= boost::bind(boost::mem_fn(& classof(**iter)::getarea , &(**iter));
When I ran g2 by writing Shape::getarea, I got '1' again for all iter.
#include <memory>
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <boost/bind.hpp>
using namespace std;
class Shape {
public:
Shape(double h, double w) :height(h), width(w) {};
virtual double getarea() = 0;
double height;
double width; };
class Rectangle: public Shape {
public:
Rectangle(double h, double w): Shape(h,w) {};
double getarea() override { return height*width; } };
class Triangle : public Shape {
public:
Triangle(double h, double w) :Shape(h,w) {};
double getarea() { return height*width*0.5; }};
int main() {
//create objects
Rectangle Rec(10, 20);
Triangle Tri(2, 3);
//create boost bind function
boost::function<double(double, double)> g1;
g1 = boost::bind(boost::mem_fn(&Shape::getarea), Rec);
//print area and g
cout << Rec.getarea()<<" should be equal to " << g1<< '\n';
//create list
vector<shared_ptr<Shape>> Plist;
Plist.push_back(make_shared<Rectangle>(Rec));
Plist.push_back(make_shared<Triangle>(Tri));
//print each element from the vector list
for (auto iter = Plist.begin(); iter != Plist.end(); iter ++ ) {
boost::function<double(double, double)> g2;
g2= boost::bind(boost::mem_fn(& .... , &(**iter));
//where in dots we need Classtype_of_**iter::getarea
cout << (**iter).getarea()<<"should be equal to " << g2<< '\n';
}
}
You... forget to invoke the functions...
for (auto iter = Plist.begin(); iter != Plist.end(); iter++) {
boost::function<double()> g2;
g2 = boost::bind(&Shape::getarea, iter->get());
cout << (*iter)->getarea() << " should be equal to " << g2() << '\n';
}
What you saw what the implicit conversion to bool (http://www.boost.org/doc/libs/1_60_0/doc/html/boost/function.html#idm45507164686720-bb)
Note also I fixed the signature of g1 and g2: Live On Coliru.
Some further improvements (remove the need for the g2 in the loop?):
auto getarea = boost::mem_fn(&Shape::getarea);
for (auto iter = Plist.begin(); iter != Plist.end(); iter++) {
cout << (*iter)->getarea() << " should be equal to " << getarea(**iter) << '\n';
}
Or, indeed in c++11:
for (auto& s : Plist)
cout << s->getarea() << " should be equal to " << getarea(*s) << '\n';
By this time, you'd wonder why you have this accessor when you can just use the member.
Assume you have many elements, and you need to keep track of the equivalence relations between them. If element A is equivalent to element B, it is equivalent to all the other elements B is equivalent to.
I am looking for an efficient data structure to encode this information. It should be possible to dynamically add new elements through an equivalence with an existing element, and from that information it should be possible to efficiently compute all the elements the new element is equivalent to.
For example, consider the following equivalence sets of the elements [0,1,2,3,4]:
0 = 1 = 2
3 = 4
where the equality sign denotes equivalence. Now we add a new element 5
0 = 1 = 2
3 = 4
5
and enforcing the equivalence 5=3, the data structure becomes
0 = 1 = 2
3 = 4 = 5
From this, one should be able to iterate efficiently through the equivalence set for any element. For 5, this set would be [3,4,5].
Boost already provides a convenient data structure called disjoint_sets that seems to meet most of my requirements. Consider this simple program that illustates how to implement the above example:
#include <cstdio>
#include <vector>
#include <boost/pending/disjoint_sets.hpp>
#include <boost/unordered/unordered_set.hpp>
/*
Equivalence relations
0 = 1 = 2
3 = 4
*/
int main(int , char* [])
{
typedef std::vector<int> VecInt;
typedef boost::unordered_set<int> SetInt;
VecInt rank (100);
VecInt parent (100);
boost::disjoint_sets<int*,int*> ds(&rank[0], &parent[0]);
SetInt elements;
for (int i=0; i<5; ++i) {
ds.make_set(i);
elements.insert(i);
}
ds.union_set(0,1);
ds.union_set(1,2);
ds.union_set(3,4);
printf("Number of sets:\n\t%d\n", (int)ds.count_sets(elements.begin(), elements.end()));
// normalize set so that parent is always the smallest number
ds.normalize_sets(elements.begin(), elements.end());
for (SetInt::const_iterator i = elements.begin(); i != elements.end(); ++i) {
printf("%d %d\n", *i, ds.find_set(*i));
}
return 0;
}
As seen above one can efficiently add elements, and dynamically expand the disjoint sets. How can one efficiently iterate over the elements of a single disjoint set, without having to iterate over all the elements?
Most probably you can't do that, disjoint_sets doesn't support iteration over one set only. The underlying data structure and algorithms wouldn't be able to do it efficiently anyway, i.e. even if there was support built in to disjoint_sets for iteration over one set only, that would be just as slow as iterating over all sets, and filtering out wrong sets.
Either I am missing something, you forgot to mention something, or maybe you were overthinking this ;)
Happily, equivalence is not equality. For A & B to be equivalent; they only need to share an attribute with the same value. this could be a scalar or even a vector. Anyway, I think your posted requirements can be achieved just using std::multiset and it's std::multiset::equal_range() member function.
//////////////////////////////////////
class E
{
//could be a GUID or something instead but the time complexity of
//std::multiset::equal_range with a simple int comparison should be logarithmic
static size_t _faucet;
public:
struct LessThan
{
bool operator()(const E* l, const E* r) const { return (l->eqValue() < r->eqValue()); }
};
using EL=std::vector<const E*>;
using ES=std::multiset<const E*, E::LessThan>;
using ER=std::pair<ES::iterator, ES::iterator>;
static size_t NewValue() { return ++_faucet; }
~E() { eqRemove(); }
E(size_t val) : _eqValue(val) {}
E(std::string name) : Name(name), _eqValue(NewValue()) { E::Elementals.insert(this); }
//not rly a great idea to use operator=() for this. demo only..
const E& operator=(const class E& other) { eqValue(other); return *this; }
//overriddable default equivalence interface
virtual size_t eqValue() const { return _eqValue; };
//clearly it matters how mutable you need your equivalence relationships to be,,
//in this implementation, if an element's equivalence relation changes then
//the element is going to be erased and re-inserted.
virtual void eqValue(const class E& other)
{
if (_eqValue == other._eqValue) return;
eqRemove();
_eqValue=other._eqValue;
E::Elementals.insert(this);
};
ES::iterator eqRemove()
{
auto range=E::Elementals.equal_range(this);
//worst-case complexity should be aprox linear over the range
for (auto it=range.first; it!=range.second; it++)
if (this == (*it))
return E::Elementals.erase(it);
return E::Elementals.end();
}
std::string Name; //some other attribute unique to the instance
static ES Elementals; //canonical set of elements with equivalence relations
protected:
size_t _eqValue=0;
};
size_t E::_faucet=0;
E::ES E::Elementals{};
//////////////////////////////////////
//random specialisation providing
//dynamic class-level equivalence
class StarFish : public E
{
public:
static void EqAssign(const class E& other)
{
if (StarFish::_id == other.eqValue()) return;
E el(StarFish::_id);
auto range=E::Elementals.equal_range(&el);
StarFish::_id=other.eqValue();
E::EL insertList(range.first, range.second);
E::Elementals.erase(range.first, range.second);
E::Elementals.insert(insertList.begin(), insertList.end());
}
StarFish() : E("starfish") {}
//base-class overrides
virtual size_t eqValue() const { return StarFish::_id; };
protected: //equivalence is a the class level
virtual void eqValue(const class E& other) { assert(0); }
private:
static size_t _id;
};
size_t StarFish::_id=E::NewValue();
//////////////////////////////////////
void eqPrint(const E& e)
{
std::cout << std::endl << "elementals equivalent to " << e.Name << ": ";
auto range=E::Elementals.equal_range(&e);
for (auto it=range.first; it!=range.second; it++)
std::cout << (*it)->Name << " ";
std::cout << std::endl << std::endl;
}
//////////////////////////////////////
void eqPrint()
{
for (auto it=E::Elementals.begin(); it!=E::Elementals.end(); it++)
std::cout << (*it)->Name << ": " << (*it)->eqValue() << " ";
std::cout << std::endl << std::endl;
}
//////////////////////////////////////
int main()
{
E e0{"zero"}, e1{"one"}, e2{"two"}, e3{"three"}, e4{"four"}, e5{"five"};
//per the OP
e0=e1=e2;
e3=e4;
e5=e3;
eqPrint(e0);
eqPrint(e3);
eqPrint(e5);
eqPrint();
StarFish::EqAssign(e3);
StarFish starfish1, starfish2;
starfish1.Name="fred";
eqPrint(e3);
//re-assignment
StarFish::EqAssign(e0);
e3=e0;
{ //out of scope removal
E e6{"six"};
e6=e4;
eqPrint(e4);
}
eqPrint(e5);
eqPrint(e0);
eqPrint();
return 0;
}
online demo
NB: C++ class inheritance also provides another kind of immutable equivalence that can be quite useful ;)