C++ std::begin方法代码示例

int main()
{
	vector<int> ivec;
#ifdef LIST_INIT
	vector<int> v = {0,1,2,3,4,5,6,7,8,9};
#else
	int temp[] = {0,1,2,3,4,5,6,7,8,9};
	vector<int> v(begin(temp), end(temp));
#endif

	// range variable must be a reference so we can write to the elements
	for (auto &r : v)   // for each element in v
		r *= 2;         // double the value of each element in v
	
	// print every element in v
	for (int r : v)
		cout << r << " "; // print the elements in v
	cout << endl;

	return 0;
}

overload_set resolve_overloads(Set const& overloads, Args...) {
    overload_set candidates;
    
    using std::begin;
    using std::end;

    std::copy_if(begin(overloads), end(overloads), std::back_inserter(candidates), [](auto potential_candidate) -> bool {
        using boost::fusion::all;
        using boost::fusion::transform;
        boost::phoenix::arg_names::arg1_type identity;

        return potential_candidate.size() == sizeof...(Args) && all(
                transform(
                    detail::fusion_tie<sizeof...(Args)>::call(potential_candidate), 
                    std::tie(typeid(Args)...),
                    std::equal_to<std::type_index>()
                ), identity);
        });

    return candidates;
}

void findAndInsert(C& container,         // in container, find
                   const V& targetVal,   // first occurrence
                   const V& insertVal)   // of targetVal, then
{                                        // insert insertVal
//  using std::cbegin;                     // there
//  using std::cend;

//  auto it = std::find(cbegin(container),  // non-member cbegin
//                      cend(container),    // non-member cend
//                      targetVal);

  // Support of `std::cbegin()`in C++14
  // url: http://stackoverflow.com/a/31038315
  using std::begin;                     // there
  using std::end;

  auto it = std::find(begin(container),  // non-member cbegin
                      end(container),    // non-member cend
                      targetVal);

  container.insert(it, insertVal);
}

            bool check(T&& t) {
                using std::begin;
                std::regex r{ s_ };
                std::match_results<decltype(begin(t))> results;

                if (std::regex_match(std::forward<T>(t), results, r)) {
                    // The first sub_match is the whole string; the next
                    // sub_match is the first parenthesized expression.
                    if (results.size() != len + 1) {
                        return false;
                    }
                    else{
                        for (std::size_t i = 1; i <= ar_.size();++i) {
                            ar_[i - 1] = results[i].str();
                        }
                        return match_check(get_tuple(), t_);
                    }
                }
                else {
                    return false;
                }
            }

  bool Json_Plugin::poll_request(net::req::Request& req) noexcept
  {
    io_->step();
    if(!bufs_.size()) return false;

    Json::Value val;

    Json::Reader read(Json::Features::strictMode());

    using std::begin; using std::end;
    // While the queued buffer is failing to compile.
    while(!read.parse(std::string(begin(bufs_.front()), end(bufs_.front())),
                      val))
    {
      // Ignore it and continue on.
      bufs_.pop();
      if(!bufs_.size()) return false;
    }

    bufs_.pop();
    req = FORMATTER_TYPE(net::req::Request)::parse(val);
    return true;
  }

void SaveUnweightedDistances(const StudentNetwork& network,
							 const string& filename,
							 FilterFunc filter) {
	ofstream bfs_file{filename};
	for (auto vertex_d : network.GetVertexDescriptors()) {
		auto student_id = network[vertex_d];

		// Determine whether or not we should process this student.
		if (!filter(student_id)) { continue; }

		bfs_file << network[vertex_d] << "\t";

		// unweighted distance stats
		auto unweighted_distances = network.FindUnweightedDistances(vertex_d);

		// get the second member of the pair
		auto distance_values = vector<string>{};
		transform(begin(unweighted_distances), end(unweighted_distances),
				back_inserter(distance_values),
				[](pair<Student::Id, double> elt)
				{ return to_string(elt.second); });
		bfs_file << join(distance_values, "\t") << endl;
	}
}

namespace detail {
using std::begin;
using std::end;

template<typename Container, typename Compare>
inline void sort_(Container& container, Compare&& comp, ...) {
    std::sort(begin(container), end(container), std::forward<Compare>(comp));
}

template<typename Container, typename Compare>
inline auto sort_(Container& container, Compare&& comp, int) ->
  decltype(container.sort(std::forward<Compare>(comp))) {
    return container.sort(std::forward<Compare>(comp));
}

template<typename Container, typename Compare = std::less<
  typename std::decay<
    decltype(*begin(std::declval<Container&>()))
  >::type
>>
inline void sort(Container& container, Compare&& comp = {}) {
    sort_(container, std::forward<Compare>(comp), 0);
}
} // namespace detail

int main()
{
	int ia[] = {0,1,2,3,4,5,6,7,8,9};

	int *p = ia; // p points to the first element in ia
	++p;           // p points to ia[1]

	int *e = &ia[10]; // pointer just past the last element in ia
	for (int *b = ia; b != e; ++b)
		cout << *b << " "; // print the elements in ia
	cout << endl;

	const size_t sz = 10;
	int arr[sz];  // array of 10 ints

	for (auto &n : arr) // for each element in arr
		cin >> n;  // read values from the standard input

	for (auto i : arr)
		cout << i << " ";
	cout << endl;

	// pbeg points to the first and 
	// pend points just past the last element in arr
	int *pbeg = begin(arr),  *pend = end(arr);

	// find the first negative element, 
	// stopping if we've seen all the elements
	while (pbeg != pend && *pbeg >= 0)
		++pbeg;
	if (pbeg == pend)
		cout << "no negative elements in arr" << endl;
	else
		cout << "first negative number was " << *pbeg << endl;
	return 0;
}

 Container query(TQuery&& q, multiple_results_tag)
 {
   using std::to_string; using std::begin; using std::end;
   formatter fmt { };
   const std::string qry = to_string(fix::forward<TQuery>(q), fmt);
   if(0 != _context->query(qry.c_str())) {
     throw std::runtime_error("Could not query database");
   }
   mysql::basic_result res(*_context);
   if(res) {
     Container ret;
     ret.reserve(res.rows());
     std::transform(
       begin(res),
       end(res),
       std::back_inserter(ret),
       [](const basic_row& r) {
         return impl::make_record<TRecord>::make(r);
       }
     );
     return ret;
   }
   throw std::runtime_error("No valid context available");
 }

	bool compare( Range1&& range1, Range2&& range2, Pred&& pred, RangeCapacityCompare&& capacitypred ) {
		using std::begin;
		using std::end;
		return compare( begin( range1 ), end( range1 ), begin( range2 ), end( range2 ), std::forward<Pred>( pred ), std::forward<RangeCapacityCompare>( capacitypred ) );
	}

			proxy & operator =(Type && val)
			{
				using std::begin; using std::end;
				str->append(begin(val), end(val));
				return *this;
			}

int main(int, char**) {
    // This example will cover the read-only BSON interface.

    // Lets first build up a non-trivial BSON document using the builder interface.
    using builder::basic::kvp;
    using builder::basic::sub_array;

    auto doc = builder::basic::document{};
    doc.append(kvp("team", "platforms"), kvp("id", types::b_oid{oid()}),
    kvp("members", [](sub_array sa) {
        sa.append("tyler", "jason", "drew", "sam", "ernie", "john", "mark", "crystal");
    }));

    // document::value is an owning bson document conceptually similar to string.
    document::value value{doc.extract()};

    // document::view is a non-owning bson document conceptually similar to string_view.
    document::view view{value.view()};

    // Note: array::view and array::value are the corresponding classes for arrays.

    // we can print a view using to_json
    std::cout << to_json(view) << std::endl;

    // note that all of the interesting methods for reading BSON are defined on the view type.

    // iterate over the elements in a bson document
    for (document::element ele : view) {
        // element is non owning view of a key-value pair within a document.

        // we can use the key() method to get a string_view of the key.
        stdx::string_view field_key{ele.key()};

        std::cout << "Got key, key = " << field_key << std::endl;

        // we can use type() to get the type of the value.
        switch (ele.type()) {
        case type::k_utf8:
            std::cout << "Got String!" << std::endl;
            break;
        case type::k_oid:
            std::cout << "Got ObjectId!" << std::endl;
            break;
        case type::k_array: {
            std::cout << "Got Array!" << std::endl;
            // if we have a subarray, we can access it by getting a view of it.
            array::view subarr{ele.get_array().value};
            for (array::element ele : subarr) {
                std::cout << "array element: " << to_json(ele.get_value()) << std::endl;
            }
            break;
        }
        default:
            std::cout << "We messed up!" << std::endl;
        }

        // usually we don't need to actually use a switch statement, because we can also
        // get a variant 'value' that can hold any BSON type.
        types::value ele_val{ele.get_value()};
        // if we need to print an arbitrary value, we can use to_json, which provides
        // a suitable overload.
        std::cout << "the value is " << to_json(ele_val) << std::endl;
        ;
    }

    // If we want to search for an element we can use operator[]
    // (we also provide a find() method that returns an iterator)
    // Note, this does a linear search so it is O(n) in the length of the BSON document.
    document::element ele{view["team"]};
    if (ele) {
        // this block will execute if ele was actually found
        std::cout << "as expected, we have a team of " << to_json(ele.get_value()) << std::endl;
    }

    // Because view implements begin(), end(), we can also use standard STL algorithms.

    // i.e. if we want to find the number of keys in a document we can use std::distance
    using std::begin;
    using std::end;

    auto num_keys = std::distance(begin(view), end(view));
    std::cout << "document has " << num_keys << " keys." << std::endl;

    // i.e. if we want a vector of all the keys in a document, we can use std::transform
    std::vector<std::string> doc_keys;
    std::transform(begin(view), end(view), std::back_inserter(doc_keys), [](document::element ele) {
        // note that key() returns a string_view
        return ele.key().to_string();
    });

    std::cout << "document keys are: " << std::endl;
    for (auto key : doc_keys) {
        std::cout << key << " " << std::endl;
    }
    std::cout << std::endl;
}

	AmfByteArray(const T& v) {
		using std::begin;
		using std::end;
		value = std::vector<u8>(begin(v), end(v));
	}

TEST(FilterTest, ConstructRangeSlotsHash) {
  const int elems[] = {2, 3, 4, 5, 1, 2, 3};
  const filter_t c(begin(elems), end(elems), 30, test_hash{29});
  expect_properties(c, sc_at_least(32), test_hash{29}, default_max_load_factor);
  expect_contents(c, {1, 2, 3, 4, 5});
}

int main()
{
#ifdef LIST_INIT
    // list initialization, articles has 3 elements
    vector<string> articles = {"a", "an", "the"};
#else
    string temp[] = {"a", "an", "the"};
    vector<string> articles(begin(temp), end(temp));
#endif

    vector<string> svec; // default initialization; svec has no elements
    vector<int> ivec;             // ivec holds objects of type int
    vector<Sales_item> Sales_vec; // holds Sales_items

    vector<vector<string>> file;  // vector whose elements are vectors
    vector<vector<int>> vecOfvec; // each element is itself a vector

    // all five vectors have size 0
    cout << svec.size() << " " << ivec.size() << " "
         << Sales_vec.size() << " "
         << file.size() << " " << vecOfvec.size() << endl;

    vector<int> ivec2(10);     // ten elements, each initialized to 0
    vector<int> ivec3(10, -1); // ten int elements, each initialized to -1
    vector<string> svec2(10);  // ten elements, each an empty string
    vector<string> svec3(10, "hi!"); // ten strings; each element is "hi!"
    cout << ivec2.size() << " " << ivec3.size() << " "
         << svec2.size() << " " << svec3.size() << endl;

    // 10 is not a string, so cannot be list initialization
    vector<string> v1(10); // construct v1 with ten value-initialized elements
#ifdef LIST_INIT
    vector<string> v2 {10}; // ten elements value-initialized elements
#else
    vector<string> v2(10);
#endif
    vector<string> v3(10, "hi");  // ten elements with value "hi"
#ifdef LIST_INIT
    // again list initialization is not viable, so ordinary construction
    vector<string> v4{10, "hi"};  // ten elements with values "hi"
#else
    vector<string> v4(10, "hi");  // ten elements with values "hi"
#endif

    // all four vectors have size ten
    cout << v1.size() << " " << v2.size()
         << " " << v3.size() << " " << v4.size() << endl;

#ifdef LIST_INIT
    vector<string> vs1 {"hi"}; // list initialization: vs1 has 1 element
    vector<string> vs2{10};   // ten default-initialized elements
    vector<string> vs3{10, "hi"}; // has ten elements with value "hi"
#else
    vector<string> vs1;
    vs1.push_back("hi"); // explicitly add the element; vs1 has 1 element
    vector<string> vs2(10); // don't use curlies;
    // vs2 has ten default-initialized elements
    vector<string> vs3(10, "hi"); // don't use curlies;
    // vs3 has ten elements with value "hi"
#endif
    cout << vs1.size() << " " << vs2.size() << " " << vs3.size() << endl;

    vector<int> v5(10, 1);  // ten elements with value 1
#ifdef LIST_INIT
    vector<int> v6 {10, 1}; // two elements with values 10 and 1
#else
    vector<int> v6;
    v6.push_back(10);
    v6.push_back(1);
#endif
    cout << v5.size() << " " << v6.size() << endl;

#ifdef LIST_INIT
    // intention is clearer
    vector<int> alt_v3 = {10};    // one element with value 10
    vector<int> alt_v4 = {10, 1}; // two elements with values 10 and 1
#else
    vector<int> alt_v3;
    alt_v3.push_back(10);    // one element with value 10

    vector<int> alt_v4;
    alt_v4.push_back(10);
    alt_v4.push_back(1); // two elements with values 10 and 1
#endif
    cout << alt_v3.size() << " " << alt_v4.size() << endl;

    return 0;
}