#include<iostream>
using namespace std;
int main()
{
	int a = 0;
	int b = 0;
	int c = ++a;
	int d = b++;
	cout << "c = " << c << endl;
	cout << "d = " << d << endl;
	return 0;
}

a先自增再賦值給c,所以輸出c為1。
b先賦值給d再自增，所以輸出d為0。

從這個方面來看，++i與i++的區(qū)別(尤其是性能方面)沒有什么差別，很多同學(xué)也并沒有思考過這個問題。

2.++i與i++哪個效率更高

下面是兩段源碼及其通過vs反匯編得到的匯編代碼:

使用++i

#include<iostream>
using namespace std;
int main()
{
	for (int i = 0; i < 100; ++i)
	{
		cout << "hello world" << endl;
	}
	return 0;
}

#include<iostream>
using namespace std;
int main()
{
00552540 push ebp
00552541 mov ebp,esp
00552543 sub esp,0CCh
00552549 push ebx
0055254A push esi
0055254B push edi
0055254C lea edi,[ebp-0Ch]
0055254F mov ecx,3
00552554 mov eax,0CCCCCCCCh
00552559 rep stos dword ptr es:[edi]
0055255B mov ecx,offset _57B8321F_源@cpp (055F029h)
00552560 call @__CheckForDebuggerJustMyCode@4 (055137Fh)
   for (int i = 0; i < 100; ++i)
00552565 mov dword ptr [ebp-8],0
0055256C jmp __$EncStackInitStart+2Bh (0552577h)
0055256E mov eax,dword ptr [ebp-8]
00552571 add eax,1
00552574 mov dword ptr [ebp-8],eax
00552577 cmp dword ptr [ebp-8],64h
0055257B jge __$EncStackInitStart+5Ch (05525A8h)
   {
       cout << "hello world" << endl;
0055257D mov esi,esp
0055257F push offset std::endl<char,std::char_traits<char> > (055103Ch)
00552584 push offset string "hello world" (0559B30h)
00552589 mov eax,dword ptr [__imp_std::cout (055D0D4h)]
0055258E push eax
0055258F call std::operator<<<std::char_traits<char> > (05511A9h)
00552594 add esp,8
00552597 mov ecx,eax
00552599 call dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (055D0A0h)]
0055259F cmp esi,esp
005525A1 call __RTC_CheckEsp (055128Fh)
   }
005525A6 jmp __$EncStackInitStart+22h (055256Eh)
   return 0;
005525A8 xor eax,eax
}

使用i++

#include<iostream>
using namespace std;
int main()
{
	for (int i = 0; i < 100; i++)
	{
		cout << "hello world" << endl;
	}
	return 0;
}

#include<iostream>
using namespace std;
int main()
{
008B2540 push ebp
008B2541 mov ebp,esp
008B2543 sub esp,0CCh
008B2549 push ebx
008B254A push esi
008B254B push edi
008B254C lea edi,[ebp-0Ch]
008B254F mov ecx,3
008B2554 mov eax,0CCCCCCCCh
008B2559 rep stos dword ptr es:[edi]
008B255B mov ecx,offset _57B8321F_源@cpp (08BF029h)
008B2560 call @__CheckForDebuggerJustMyCode@4 (08B137Fh)
   for (int i = 0; i < 100; i++)
008B2565 mov dword ptr [ebp-8],0
008B256C jmp __$EncStackInitStart+2Bh (08B2577h)
008B256E mov eax,dword ptr [ebp-8]
008B2571 add eax,1
008B2574 mov dword ptr [ebp-8],eax
008B2577 cmp dword ptr [ebp-8],64h
008B257B jge __$EncStackInitStart+5Ch (08B25A8h)
   {
       cout << "hello world" << endl;
008B257D mov esi,esp
008B257F push offset std::endl<char,std::char_traits<char> > (08B103Ch)
008B2584 push offset string "hello world" (08B9B30h)
008B2589 mov eax,dword ptr [__imp_std::cout (08BD0D4h)]
008B258E push eax
008B258F call std::operator<<<std::char_traits<char> > (08B11A9h)
008B2594 add esp,8
008B2597 mov ecx,eax
008B2599 call dword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (08BD0A0h)]
008B259F cmp esi,esp
008B25A1 call __RTC_CheckEsp (08B128Fh)
   }
008B25A6 jmp __$EncStackInitStart+22h (08B256Eh)
   return 0;
008B25A8 xor eax,eax
}

哈哈哈哈哈，有的同學(xué)已經(jīng)發(fā)現(xiàn)了，好像并沒有什么區(qū)別。

之前的說法是++i比i++的效率更高，但隨著編譯器的不斷優(yōu)化，兩者簡單應(yīng)用時并沒有什么區(qū)別。

但是，真是如此嗎？

i++使用時，要先用將自身數(shù)據(jù)拷貝到臨時變量中，再自增，最后傳輸臨時變量。

而++i并不需要這般麻煩，直接自增再傳輸即可。一些追求壓縮空間和時間的嵌入式工程師往往喜歡使用++i。

下面通過運算符重載自實現(xiàn)++i和i++來解釋:

#include<iostream>
using namespace std;
class MyInt
{
	friend ostream& operator<<(ostream& cout, MyInt& a);//友元
public:
	MyInt();
	MyInt& operator++();//前置++
	MyInt& operator++(int);//使用占位參數(shù)區(qū)別前后置++，使之可以發(fā)生函數(shù)重載
private:
	int m_num;
};
MyInt::MyInt()
{
	this->m_num = 0;
}
MyInt& MyInt::operator++()
{
	this->m_num++;
	return *this;
}
MyInt& MyInt::operator++(int)
{
	static MyInt temp = *this;
	this->m_num++;
	return temp;
}
ostream& operator<<(ostream& cout, MyInt& a)
{
	cout << a.m_num;
	return cout;
}

通過上面的代碼顯而易見其區(qū)別，且后置++難以實現(xiàn)鏈?zhǔn)骄幊獭?/p>