c++ - Confused by Tuple example in TC++PL 4th Edition -


i'm having trouble working out how code works.

i have shorter version of example here, think qualifies fair use:

#include <typeinfo> #include <iostream>  using namespace std;  struct nil{};  // primary template. template<typename t1=nil, typename t2=nil> struct tuple : tuple<t2> {     t1 x;      using base = tuple<t2>;     base* base() { return static_cast<base*>(this); }     const base* base() const { return static_cast<const base*>(this); }      tuple(const t1& t1, const t2& t2) : base{t2}, x{t1} { } };  // specialization. template<> struct tuple<> { tuple() {} };  // specialization. template<typename t1> struct tuple<t1> : tuple<> {     t1 x;      using base = tuple<>;     base* base() { return static_cast<base*>(this); }     const base* base() const { return static_cast<const base*>(this); }      tuple(const t1& t1) : base{}, x{t1} { } };  template<typename t1, typename t2> void print_elements(ostream& os, const tuple<t1,t2>& t) {     os << t.x << ", ";     print_elements(os,*t.base()); }  template<typename t1> void print_elements(ostream& os, const tuple<t1>& t) {     os << t.x; }  template<> void print_elements(ostream& os, const tuple<>& t) {     os << " "; }  template<typename t1, typename t2> ostream& operator<<(ostream& os, const tuple<t1,t2>& t) {     os << "t1: " << typeid(t1).name() << ", t2: " << typeid(t2).name() << "\n";     os << "{ ";     print_elements(os,t);     os << " }";     return os; }  int main() {     cout << tuple<double, int>{1.1, 22} << "\n";     cout << tuple<double>{1.1} << "\n";     cout << tuple<>{} << "\n"; }

the output of program (from stacked-crooked)

t1: d, t2: { 1.1, 22 } t1: d, t2: 3nil { 1.1 } t1: 3nil, t2: 3nil {   }

there 1 definition of operator<< (for primary template).

this can called tuple<> , tuple. way understand means call operator<< tuple , tuple.

secondly, when invoked tuple defaulted (nil) types, calls correct print_elements. why not call print_elements output nil values?

i added operator<< nil , doesn't change behaviour.

thanks!

that how works print_elements. there overload tuple of 2 elements, tuple 1 element (second nil) , overload tuple 0 elements (both nil).

in operator <<:

when t tuple<t1, t2>, print_elements<tuple<t1, t2> > called;

when t tuple<t1> (really tuple<t1, nil>), print_elements<tuple<t1> > called, since version better, print_elements<tuple<t1, t2> >, t2 nil;

when t tuple<> (really tuple<nil, nil>), print_elements<tuple<> > called, since version better, versions.


Comments

Post a Comment

Popular posts from this blog

php - failed to open stream: HTTP request failed! HTTP/1.0 400 Bad Request -

this code i'm using find latitude , longitude of place: function coords($address){ echo $url="http://maps.googleapis.com/maps/api/geocode/json?address=".$address; $json = file_get_contents(urlencode($url)); $data = json_decode($json, true); print_r($data['results'][0]['geometry']['location']['lat']); print_r($data['results'][0]['geometry']['location']['lng']); } however returns warning: failed open stream: http request failed! http/1.0 400 bad request if use above code in simple procedure works fine not in function. note: have tried curl_init()... method produced same result? instead of encoding whole url encode address part. following code work fine $add='jamshoro phase 2'; $url="http://maps.googleapis.com/maps/api/geocode/json?address=".urlencode($add); $json = file_get_contents($url); $data = json_decode($json, true); print_r($data['r...
Read more

command line - Use qwinsta in PowerShell ISE -

i trying use qwinsta , rwinsta powershell code. when try run error "the term 'qwinsta' not recognized name of cmdlet, function, script file, or operable program." need able remotely on several machines without having add .dll's or modules other machines. tried terminal services powershell module, need able put on remote machines , not able that. within powershell ise how run qwinsta? not looking parse information gather it. following code have tried (which had found on site also): function get-tssessions { param( $computername = "localhost" ) qwinsta /server:$computername | #parse output foreach-object { $_.trim() -replace "\s+","," } | #convert objects convertfrom-csv } get-tssessions -computername "localhost" | ft -autosize best have running powershell ise (x86). when run command qwinsta there same error you. of course assuming have 64-bit os. th...
Read more

java - Incorrect order of records in M-M relationship in hibernate -

Image
i have 4 records in bridge table in mysql database. using @manytomany in hibernate, accurate order of employees incorrect incorrect of projects. following required material figure out doing mistake. this source. @entity @table(name="project") public class projectbean { @id @generatedvalue @column(name="project_id") private int projectid; @column(name="title") private string projecttitle; @manytomany(mappedby="projects") private collection<employeebean> employees; public projectbean() { employees = new arraylist<employeebean>(); } //getters & setters } @entity @table(name="employee") public class employeebean { @id @generatedvalue @column(name="employee_id") private int employee_id; @column(name="first_name") private string ...
Read more