Why do we need virtual functions in C++? (轉貼)

Why do we need virtual functions in C++?

1989 down voteaccepted

I'm a C++ newbie myself, but here is how I understood not just what virtual functions are, but why they're required: Let's say you have these two classes: class Animal { public: void eat() { std::cout << "I'm eating generic food."; } }; class Cat : public Animal { public: void eat() { std::cout << "I'm eating a rat."; } }; In your main function: Animal *animal = new Animal; Cat *cat = new Cat; animal->eat(); // Outputs: "I'm eating generic food." cat->eat(); // Outputs: "I'm eating a rat." So far so good, right? Animals eat generic food, cats eat rats, all without virtual. Let's change it a little now so that eat() is called via an intermediate function (a trivial function just for this example): // This can go at the top of the main.cpp file void func(Animal *xyz) { xyz->eat(); } Now our main function is: Animal *animal = new Animal; Cat *cat = new Cat; func(animal); // Outputs: "I'm eating generic food." func(cat); // Outputs: "I'm eating generic food." Uh oh... we passed a Cat into func(), but it won't eat rats. Should you overload func() so it takes a Cat*? If you have to derive more animals from Animal they would all need their own func(). The solution is to make eat() from the Animal class a virtual function: class Animal { public: virtual void eat() { std::cout << "I'm eating generic food."; } }; class Cat : public Animal { public: void eat() { std::cout << "I'm eating a rat."; } }; Main: func(animal); // Outputs: "I'm eating generic food." func(cat); // Outputs: "I'm eating a rat." Done.

大量PNG圖去背轉GIF圖檔

1. 安裝 ImageMagick
(2.a 單轉檔, MJt1.png轉MJt1.gif)
>convert "MJt1.png" -transparent #ffffff "output\MJt1.gif"

2.b *.png去背(白色去背#ffffff)轉gif

>FOR %G IN ("*.png") DO convert "%G" -transparent #ffffffff "output\%G.gif"

==>會生成許多*.png.gif

3.用Windows PowerShell，將*.png.gif轉成*.gif
>Dir | Rename-Item -NewName { $_.name –eplace "png.gif", "gif" }

Note:在cmd下用ren *.png.gif *.gif無効…

Python 3超新手常犯的錯誤

##注意parameter的值是variable時，不會跟著variable的值來改變

t = 0

def p(a=t):
    print("a=%d"%a)

def main():
    global t
    t = 100
    p()
    print("t=%d"%t)

if __name__ == "__main__":
    main()

##############Output#############
a=0
t=100

Python3 "from m import *" 超新手常犯的錯誤

#在python 3, 請注意s的值並不會共用
# a.py
from b import *

def main():
    global s
    print(s)
    add()
    print(s)
    s += 6
    print(s)
    add()
    print(s)

if __name__ == "__main__":
    main()

# b.py
s =10

def add():
    global s
    s += 1
    print("s=%d"%s)

###
# Execute a.py
###
10
s=11
10
16
s=12
16

#如果要s的值共用，可以如下修改：
# a.py
import b

def main():
    print(b.s)
    b.add()
    print(b.s)
    b.s += 6
    print(b.s)
    b.add()
    print(b.s)

if __name__ == "__main__":
    main()

# b.py (不變)
s =10

def add():
    global s
    s += 1
    print("s=%d"%s)

###
# Execute a.py
###
10
s=11
11
17
s=18
18

Can't log in to ubuntu 12.04 (轉貼)

Can't log in to ubuntu 12.04

up vote2down votefavorite 1

I can't log in to my ubuntu 12.04. Once I login I got returned back to the original screen after a trial to enable X11 Forwarding. Can you help me with this. login shareimprove this question edited Jun 26 '12 at 19:44 Thomas W. 18.3k1357101 asked May 19 '12 at 20:40 Ahmed 16112

Are you trying to login remotely? What did you do to try to enable X11 forwarding? –  izx May 19 '12 at 21:40 Does this only happen when trying to log in remotely? Or are you having this problem logging in locally as well? –  Eliah Kagan Mar 27 '13 at 1:07 Check your .xsession-errors in your home directory, I found an error over there, fixed it, then I can login from desktop. I did try to change the permission and ownership of .Xauthority but it doesn't help in my case. – user288195 Jun 3 '14 at 23:25 add a comment

9 Answers

activeoldestvotes

up vote7down vote

I haven't tried to enable X11Forwarding, but I used to have the same problem. You can fix this in a few steps: When you get to the graphical login screen, hit Ctrl+Alt+F2, thus you get to a textual login. Put in your name and password (password is not displayed when you type, neither as stars or bullets). Type sudo apt-get install gdm and hit enter, give your password and wait to get a line ending with a dollar sign again. Type sudo dpkg-reconfigure gdm. Hit Enter at the first screen. Select gdm in the list you get in the second screen with the arrow keys. Hit the Tab button (above the caps lock one). Now you see the word Ok highlighted. Hit Enter. Type sudo reboot and hit enter. You'll get another graphical login screen, but you'll be able to log in! P.S.: If you are used to running some programs like ifconfig or others which are administrative ones from a terminal emulator (e.g. gnome-terminal) you'd better add a line like PATH="$PATH:/sbin:/usr/sbin" at the end of the .bashrc file in your home directory. You can do it with echo 'PATH="$PATH:/sbin:/usr/sbin"' >> ~/.bashrc. If you want to change back to the other graphical login screen from time to time and see if it was bug-fixed and it works again you can run sudo dpkg-reconfigure gdm and select lightdm and reboot (doesn't look like it works if you just run sudo pkill X). shareimprove this answer edited Sep 11 '13 at 14:42 jokerdino♦ 23.3k1888166 answered May 23 '12 at 10:53 Lorra 1841

Excel 2010中某些行隱藏的解法

常用->儲存格->格式->隱藏及取消隱藏->取消隱藏欄