C++ swap two numbers using 11 chars of code [closed]

问题

My friend sent me a exercise that he can't do:
(C++)

int main()
{
    unsigned int x = 0xB0FF14a5;
    unsigned int y = 0x7340c00e;
    // enter code here
    if(x==0x7340c00e && y==0xB0FF14a5) victory();
    return 0;
}

The main goal is to run victory() function.
Assumptions:
-max 11 chars
-You can't use: "main", "victory", "asm", "&", "*", "(", "/"
-You can use only one semicolon

I tried with #define and some other things, but nothing (I'm not C++ master) :/
I have no idea how to solve this; thanks for helping!

回答1:

Use the XOR swap algorithm:

x^=y^=x^=y;

This is equivalent (usually, see below) to:

          //x==A,   y==B
x ^= y;   //x==A^B, y==B
y ^= x;   //x==A^B, y==A
x ^= y;   //x==B,   y==A

It works because XORing by the same number twice gives you the original number.

In C++03 the single-expression version is undefined behavior so may not work correctly on all compilers/platforms. This is because there is no sequence point between modification and use of variables.

In C++11, it is well-defined. The standard says (5.17.1):

In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression.

回答2:

Undefined behaviour, but Works On My Computer:

x^=y^=x^=y;

UPDATE: apparently, this is well-defined since 2011; see interjay's answer.

回答3:

13 characters and violates other rules, but gets the job done and too cute not to post:

#include<iostream>

void victory()
{
   std::cout << "Yes we can\n";
}

int main()
{
    unsigned int x = 0xB0FF14a5;
    unsigned int y = 0x7340c00e;
    #define if(x)
    if(x==0x7340c00e && y==0xB0FF14a5) victory();
    return 0;
}

Output on Ideone

回答4:

Look at this algorithm: XOR swap algorithm But you will get a compile warning like:

warning: operation on ‘x’ may be undefined

if you use this algorithm in just one line

x ^= y ^= x ^= y;

来源：https://stackoverflow.com/questions/11989319/c-swap-two-numbers-using-11-chars-of-code

标签

c++

obfuscation