3.15 引用类型
C++中值传递实现的是值得单向传递，而引用传递实现的是值的双向传递。
应用类型的语法规定
C++
引用（&）是标识符的别名
定义一个引用时，必须同时对它进行初始化，是他执行一个已存在的对象;定义引用以后马上就要跟一个已经存在的变量联系起来，得说清楚是谁的引用，因此定义引用时必须初始化，必须要用一个已经存在的对象或者变量初始化这个引用。
一旦一个引用被初始化后，就不能改为指向其它对象。
C++中引用大多用来作为函数的形参出现的，进而实现函数参数的双向传递
int i,j;
int &ri=i;//定义int引用ri，并初始化为变量i的引用例：输入两个整数并交换（值传递）
C++
#include <iostream>
using namespace std;
void swap(int a, int b);
int main() {
int x;
int y;
cout << "请输入要交换的整数值" << endl;
cin >> x >> y;
swap(x, y);
cout << "x的值为" << x << ",y的值为" << y;
return 0;
}
void swap(int a, int b) {
int i;
i = a;
a = b;
b = i;
cout << "a的值为" << a << ",b的值为" << b;
}其对应的汇编
assembly
--- E:\C++\C++\3_13.cpp --------------------------------------------------------
1: #include <iostream>
2: using namespace std;
3: void swap (int a, int b);
4: int main() {
00007FF60DC86960 40 55 push rbp
00007FF60DC86962 57 push rdi
00007FF60DC86963 48 81 EC 28 01 00 00 sub rsp,128h
00007FF60DC8696A 48 8D 6C 24 20 lea rbp,[rsp+20h]
00007FF60DC8696F 48 8D 7C 24 20 lea rdi,[rsp+20h]
00007FF60DC86974 B9 12 00 00 00 mov ecx,12h
00007FF60DC86979 B8 CC CC CC CC mov eax,0CCCCCCCCh
00007FF60DC8697E F3 AB rep stos dword ptr [rdi]
00007FF60DC86980 48 8B 05 79 66 00 00 mov rax,qword ptr [__security_cookie (07FF60DC8D000h)]
00007FF60DC86987 48 33 C5 xor rax,rbp
00007FF60DC8698A 48 89 85 F8 00 00 00 mov qword ptr [rbp+0F8h],rax
00007FF60DC86991 48 8D 0D F0 C6 00 00 lea rcx,[__CEA39ACC_3_13@cpp (07FF60DC93088h)]
00007FF60DC86998 E8 69 AA FF FF call __CheckForDebuggerJustMyCode (07FF60DC81406h)
00007FF60DC8699D 90 nop
5: int x;
6: int y;
7: cout << "请输入要交换的整数值" << endl;
00007FF60DC8699E 48 8D 15 43 43 00 00 lea rdx,[string "\xc7\xeb\xca\xe4\xc8\xeb\xd2\xaa\xbd\xbb\xbb\xbb\xb5\xc4\xd5\xfb\xca\xfd\xd6\xb5" (07FF60DC8ACE8h)]
00007FF60DC869A5 48 8B 0D EC A7 00 00 mov rcx,qword ptr [__imp_std::cout (07FF60DC91198h)]
00007FF60DC869AC E8 DB A6 FF FF call std::operator<<<std::char_traits<char> > (07FF60DC8108Ch)
00007FF60DC869B1 48 8D 15 84 A6 FF FF lea rdx,[std::endl<char,std::char_traits<char> > (07FF60DC8103Ch)]
00007FF60DC869B8 48 8B C8 mov rcx,rax
00007FF60DC869BB FF 15 BF A7 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF60DC91180h)]
00007FF60DC869C1 90 nop
8: cin >> x >> y;
00007FF60DC869C2 48 8D 55 04 lea rdx,[x]
00007FF60DC869C6 48 8B 0D 83 A7 00 00 mov rcx,qword ptr [__imp_std::cin (07FF60DC91150h)]
00007FF60DC869CD FF 15 8D A7 00 00 call qword ptr [__imp_std::basic_istream<char,std::char_traits<char> >::operator>> (07FF60DC91160h)]
00007FF60DC869D3 48 8D 55 24 lea rdx,[y]
00007FF60DC869D7 48 8B C8 mov rcx,rax
00007FF60DC869DA FF 15 80 A7 00 00 call qword ptr [__imp_std::basic_istream<char,std::char_traits<char> >::operator>> (07FF60DC91160h)]
00007FF60DC869E0 90 nop
9: swap(x, y);
00007FF60DC869E1 8B 55 24 mov edx,dword ptr [y]
00007FF60DC869E4 8B 4D 04 mov ecx,dword ptr [x]
00007FF60DC869E7 E8 83 AA FF FF call swap (07FF60DC8146Fh)
00007FF60DC869EC 90 nop
10: cout << "x的值为" << x << ",y的值为" << y;
00007FF60DC869ED 48 8D 15 24 43 00 00 lea rdx,[string "x\xb5\xc4\xd6\xb5\xce\xaa" (07FF60DC8AD18h)]
00007FF60DC869F4 48 8B 0D 9D A7 00 00 mov rcx,qword ptr [__imp_std::cout (07FF60DC91198h)]
00007FF60DC869FB E8 8C A6 FF FF call std::operator<<<std::char_traits<char> > (07FF60DC8108Ch)
00007FF60DC86A00 8B 55 04 mov edx,dword ptr [x]
00007FF60DC86A03 48 8B C8 mov rcx,rax
00007FF60DC86A06 FF 15 6C A7 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF60DC91178h)]
00007FF60DC86A0C 48 8D 15 F5 42 00 00 lea rdx,[string ",y\xb5\xc4\xd6\xb5\xce\xaa" (07FF60DC8AD08h)]
00007FF60DC86A13 48 8B C8 mov rcx,rax
00007FF60DC86A16 E8 71 A6 FF FF call std::operator<<<std::char_traits<char> > (07FF60DC8108Ch)
00007FF60DC86A1B 8B 55 24 mov edx,dword ptr [y]
00007FF60DC86A1E 48 8B C8 mov rcx,rax
00007FF60DC86A21 FF 15 51 A7 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF60DC91178h)]
00007FF60DC86A27 90 nop
11: return 0;
00007FF60DC86A28 33 C0 xor eax,eax
12: }
00007FF60DC86A2A 8B F8 mov edi,eax
00007FF60DC86A2C 48 8D 4D E0 lea rcx,[rbp-20h]
00007FF60DC86A30 48 8D 15 09 42 00 00 lea rdx,[__xt_z+1A0h (07FF60DC8AC40h)]
00007FF60DC86A37 E8 5C A9 FF FF call _RTC_CheckStackVars (07FF60DC81398h)
00007FF60DC86A3C 8B C7 mov eax,edi
00007FF60DC86A3E 48 8B 8D F8 00 00 00 mov rcx,qword ptr [rbp+0F8h]
00007FF60DC86A45 48 33 CD xor rcx,rbp
00007FF60DC86A48 E8 C5 A7 FF FF call __security_check_cookie (07FF60DC81212h)
00007FF60DC86A4D 48 8D A5 08 01 00 00 lea rsp,[rbp+108h]
00007FF60DC86A54 5F pop rdi
00007FF60DC86A55 5D pop rbp
00007FF60DC86A56 C3 ret
--- 无源文件 -----------------------------------------------------------------------例：输入两个整数并交换（引用传递）
c++
#include <iostream>
using namespace std;
void swap(int& a, int& b);
int main() {
int x = 5;
int y = 6;
cout << "x=" << x << ",y=" << y;
swap(x, y);
cout << "x=" << x << ",y=" << y;
return 0;
}
void swap(int& a, int& b) {
int i;
i = a;
a = b;
b = i;
}其对应的汇编
assembly
--- E:\C++\C++\3_14.cpp --------------------------------------------------------
1: #include <iostream>
2: using namespace std;
3: void swap(int& a, int& b);
4: int main() {
00007FF7146F65A0 40 55 push rbp
00007FF7146F65A2 57 push rdi
00007FF7146F65A3 48 81 EC 28 01 00 00 sub rsp,128h
00007FF7146F65AA 48 8D 6C 24 20 lea rbp,[rsp+20h]
00007FF7146F65AF 48 8D 7C 24 20 lea rdi,[rsp+20h]
00007FF7146F65B4 B9 12 00 00 00 mov ecx,12h
00007FF7146F65B9 B8 CC CC CC CC mov eax,0CCCCCCCCh
00007FF7146F65BE F3 AB rep stos dword ptr [rdi]
00007FF7146F65C0 48 8B 05 39 6A 00 00 mov rax,qword ptr [__security_cookie (07FF7146FD000h)]
00007FF7146F65C7 48 33 C5 xor rax,rbp
00007FF7146F65CA 48 89 85 F8 00 00 00 mov qword ptr [rbp+0F8h],rax
00007FF7146F65D1 48 8D 0D B1 CA 00 00 lea rcx,[__7C8346DC_3_14@cpp (07FF714703089h)]
00007FF7146F65D8 E8 15 AE FF FF call __CheckForDebuggerJustMyCode (07FF7146F13F2h)
00007FF7146F65DD 90 nop
5: int x = 5;
00007FF7146F65DE C7 45 04 05 00 00 00 mov dword ptr [x],5
6: int y = 6;
00007FF7146F65E5 C7 45 24 06 00 00 00 mov dword ptr [y],6
7: cout << "x=" << x << ",y=" << y;
00007FF7146F65EC 48 8D 15 F5 46 00 00 lea rdx,[string "x=" (07FF7146FACE8h)]
00007FF7146F65F3 48 8B 0D 9E AB 00 00 mov rcx,qword ptr [__imp_std::cout (07FF714701198h)]
00007FF7146F65FA E8 8D AA FF FF call std::operator<<<std::char_traits<char> > (07FF7146F108Ch)
00007FF7146F65FF 8B 55 04 mov edx,dword ptr [x]
00007FF7146F6602 48 8B C8 mov rcx,rax
00007FF7146F6605 FF 15 9D AB 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7147011A8h)]
00007FF7146F660B 48 8D 15 D2 46 00 00 lea rdx,[string ",y=" (07FF7146FACE4h)]
00007FF7146F6612 48 8B C8 mov rcx,rax
00007FF7146F6615 E8 72 AA FF FF call std::operator<<<std::char_traits<char> > (07FF7146F108Ch)
00007FF7146F661A 8B 55 24 mov edx,dword ptr [y]
00007FF7146F661D 48 8B C8 mov rcx,rax
00007FF7146F6620 FF 15 82 AB 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7147011A8h)]
00007FF7146F6626 90 nop
8: swap(x, y);
00007FF7146F6627 48 8D 55 24 lea rdx,[y]
00007FF7146F662B 48 8D 4D 04 lea rcx,[x]
00007FF7146F662F E8 21 AA FF FF call swap (07FF7146F1055h)
00007FF7146F6634 90 nop
9: cout << "x=" << x << ",y=" << y;
00007FF7146F6635 48 8D 15 AC 46 00 00 lea rdx,[string "x=" (07FF7146FACE8h)]
00007FF7146F663C 48 8B 0D 55 AB 00 00 mov rcx,qword ptr [__imp_std::cout (07FF714701198h)]
00007FF7146F6643 E8 44 AA FF FF call std::operator<<<std::char_traits<char> > (07FF7146F108Ch)
00007FF7146F6648 8B 55 04 mov edx,dword ptr [x]
00007FF7146F664B 48 8B C8 mov rcx,rax
00007FF7146F664E FF 15 54 AB 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7147011A8h)]
00007FF7146F6654 48 8D 15 89 46 00 00 lea rdx,[string ",y=" (07FF7146FACE4h)]
00007FF7146F665B 48 8B C8 mov rcx,rax
00007FF7146F665E E8 29 AA FF FF call std::operator<<<std::char_traits<char> > (07FF7146F108Ch)
00007FF7146F6663 8B 55 24 mov edx,dword ptr [y]
00007FF7146F6666 48 8B C8 mov rcx,rax
00007FF7146F6669 FF 15 39 AB 00 00 call qword ptr [__imp_std::basic_ostream<char,std::char_traits<char> >::operator<< (07FF7147011A8h)]
00007FF7146F666F 90 nop
10: return 0;
00007FF7146F6670 33 C0 xor eax,eax
11: }
00007FF7146F6672 8B F8 mov edi,eax
00007FF7146F6674 48 8D 4D E0 lea rcx,[rbp-20h]
00007FF7146F6678 48 8D 15 C1 45 00 00 lea rdx,[__xt_z+1A0h (07FF7146FAC40h)]
00007FF7146F667F E8 00 AD FF FF call _RTC_CheckStackVars (07FF7146F1384h)
00007FF7146F6684 8B C7 mov eax,edi
00007FF7146F6686 48 8B 8D F8 00 00 00 mov rcx,qword ptr [rbp+0F8h]
00007FF7146F668D 48 33 CD xor rcx,rbp
00007FF7146F6690 E8 73 AB FF FF call __security_check_cookie (07FF7146F1208h)
00007FF7146F6695 48 8D A5 08 01 00 00 lea rsp,[rbp+108h]
00007FF7146F669C 5F pop rdi
00007FF7146F669D 5D pop rbp
00007FF7146F669E C3 ret
--- 无源文件 -----------------------------------------------------------------------