如何复制std :: vector <boost::shared_ptr<T> &gt;到std :: list <T>

Question

std::list<KinBody::Link::Geometry> geometries = link->GetGeometries();

The type of link->GetGeometries() is std::vector<boost::shared_ptr<Geometry>> and I got the following error with the above code.

error: conversion from ‘const std::vector<boost::shared_ptr<OpenRAVE::KinBody::Link::Geometry> >’ to     non-scalar type ‘std::list<OpenRAVE::KinBody::Link::Geometry>’ requested
std::list<KinBody::Link::Geometry> geometries = link->GetGeometries();

What should I do ?

Answer 1

std::list<KinBody::Link::Geometry> geometries;

for (auto const & p : link->GetGeometries())
    geometries.push_back(*p);

For the for (auto const & p : ...) part you need C++11 support enabled (it uses automatic type deduction and range-based for loop).

Pre-C++11 equivalent is

std::list<KinBody::Link::Geometry> geometries;

typedef std::vector<boost::shared_ptr<KinBody::Link::Geometry>> geometries_vector_t;

geometries_vector_t const & g = link->GetGeometries();

for (geometries_vector_t::const_iterator i = g.begin(); i != g.end(); ++i)
    geometries.push_back(**i); // dereferencing twice: once for iterator, once for pointer

NB: all this looks very unnatural. Objects being returned as shared pointers means that KinBody::Link::Geometry is actually a base class or interface, or that objects of this type are large and the interface is designed to avoid heavy copying, or something else. I advise not to copy the objects and store them as shared pointers as the interface suggests, unless you actually know you need the copies.

Answer 2

Since you mentioned boost, let me show you some Boost Range sugar.

link->getgeometries() | adaptors::indirected

this will result in a range that contains the Geometry& elements. Fill the list with copy_range :

geometries = boost::copy_range<std::list<link::geometry>>(
        link->getgeometries() | adaptors::indirected
    );

See a fully working demo:

Live On Coliru

#include <boost/range/algorithm.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/make_shared.hpp>
#include <iostream>

namespace KinBody {

    struct Link {
        struct Geometry {};

        std::vector<boost::shared_ptr<Geometry> > GetGeometries() const {
            return {};
        }
    };
}

int main() {
    using namespace boost;
    using namespace KinBody;

    auto link = make_shared<Link>();
    auto geometries = boost::copy_range<std::list<Link::Geometry>>(
            link->GetGeometries() | adaptors::indirected
        );
}

Answer 3

The line

std::list<KinBody::Link::Geometry> geometries = link->GetGeometries();

tries to assign a vector to a list. The compiler tells you that you cannot do that. What you have to do is iterate the vector and insert each element to the list.

Note that the vector contains shared pointers to geometries whereas the list is declared to contain instances. If you want to store a pointer in the list you have to declare it as:

std::list<KinBody::Link::Geometry*>  // note the *