c++ - Passing const reference leads to paradox in code..? -


ok have function within class named vector_ref

class vector_ref {     public:        int* data() const { return m_data }        void retarget(std::vector<int> const& _t)         { m_data = _t.data();           m_count = _t.size() }     private:        int* m_data;        size_t m_count; }

i trying retarget vector_ref object existing vector object named v populated ints , call data(). 

vector_ref<int> tmp; tmp.retarget(const_cast<std::vector<int> const&>(v)); tmp.data(); // error here

the pass retarget compiles, calling data() yields error:

invalid conversion const int* int* { m_data = v.data() ... }

this makes sense me, member variable m_data isn't const, given class definition how can ever retarget existing vector. if have pass const reference retarget(), sets non-const member variable m_data, how can ever hope compile code retargets vector_ref instance ?

just declare m_data as

const int* m_data;

this doesn't mean m_data const. means cannot modify int m_data points to. can still this:

m_data = _t.data();

but can't this:

*m_data = x; m_data[5] = y; // or

by way, there's no need const_cast. need const_cast if removing const qualifier (i.e. passing const object function expects non-const reference). don't need in order pass non-const object function accepts const reference.


Comments

Post a Comment

Popular posts from this blog

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m...
Read more

python Tkinter Capturing keyboard events save as one single string -

is possible store keyboard events 1 string? code below stores 1 char , prints it. card reader or bar code reader, contains collection of character/ string not 1 character @ time. goal save char pressed text variable. from tkinter import * root = tk() def key(event): text= event.char text+= event.char print ("pressed", text) def callback(event): frame.focus_set() print ("clicked at", event.x, event.y) frame = frame(root, width=100, height=100) frame.bind("<key>", key) frame.bind("<button-1>", callback) frame.pack() root.mainloop() currently, creating text variable , everytime key() function gets called, , text stores last character typed. you can define text module level variable , , use module level text inside key function - text = '' def key(event): global text text+= event.char print("pressed", text)
Read more

sql server - Why does Linq-to-SQL add unnecessary COUNT()? -

here tables: create table customer ( cid int primary key, name varchar(32) ) create table ord ( oid int primary key, cid int foreign key references customer, address varchar(20) ) here's linq-to-sql statement: var aaa = c in db.customer select new { c, o = c.ord.tolist() }; and here's inexplicable query generated linq-to-sql: select [t0].[cid] [cid], [t0].[name] [name], [t1].[oid] [oid], [t1].[cid] [cid2], [t1].[address] [address], (select count(*) [dbo].[ord] [t2] [t2].[cid] = [t0].[cid]) [value] [dbo].[customer] [t0] left outer join [dbo].[ord] [t1] on [t1].[cid] = [t0].[cid] order [t0].[cid], [t1].[oid] i want understand how rid of count(*) part. it's entirely uncalled for! linq needs count(*) in order determine number of entries returned o = c.ord.tolist() .
Read more