???學(xué)習(xí)數(shù)據(jù)結(jié)構(gòu)，用到了輸入輸出運(yùn)算符重載，結(jié)果編譯之下，錯(cuò)誤重重，，隨即停止學(xué)習(xí)進(jìn)度，追查禍源，在花費(fèi)巨大腦力與時(shí)間成本之后，終于知自己錯(cuò)誤之所在，定位于名字空間之困擾。為牢記教訓(xùn)，寫一簡(jiǎn)化版本記錄
debug
過(guò)程如下，警示自己。
問(wèn)題：
???
???有如下兩個(gè)代碼文件：
???// 20060816_operator.cxx
?1?#include?<iostream>
?2?using?namespace?std;
?3?#include?"20060816_operator.h"
?4?int?main(int?argc,?char*?argv[])
?5?{
?6?????const?std::string?GHH("GuoHonghua");
?7?????Honghua?ghh(GHH);
?8?????cout?<<?ghh?<<?endl;
?9?????return?0;
10?}
???
// 20060816_operator.h
?1?#ifndef?_20060816_OPERATOR_GHH_
?2?#define?_20060816_OPERATOR_GHH_????1
?3?
?4?//?Author?:?GuoHonghua
?5?//?Date?:?2006.08.16
?6?//?File?:?20060816_operator.h
?7?
?8?#include?<iostream>
?9?#include?<string>
10?class?Honghua
11?{
12?????friend?std::ostream&?operator<<(std::ostream&?os,?const?Honghua&?ghh);
13?public:
14?????Honghua(const?std::string&?ghh)?:?_ghh(ghh)
15?????{
16?????}
17?
18?private:
19?????std::string?_ghh;
20?};
21?
22?std::ostream&?operator<<(std::ostream&?os,?const?Honghua&?ghh)
23?{
24?????os?<<?ghh._ghh;
25?????return?os;
26?}
27?
28?#endif?//?end?of?_20060816_OPERATOR_GHH_
用上面兩個(gè)文件建立工程，vc6.0下編譯會(huì)出錯(cuò)。報(bào)告如下：
--------------------Configuration: 20060816_operator - Win32 Debug--------------------
Compiling...
20060816_operator.cxx
f:\ghh_project\cxxdetail\20060816_operator.h(24) : error C2248: '_ghh' : cannot access private member declared in class 'Honghua'
??????? f:\ghh_project\cxxdetail\20060816_operator.h(19) : see declaration of '_ghh'
F:\ghh_project\CxxDetail\20060816_operator.cxx(8) : error C2593: 'operator <<' is ambiguous
Error executing cl.exe.
?
20060816_operator.exe - 2 error(s), 0 warning(s)
解決:
???
1．? 調(diào)整名字空間聲明和自定義頭文件的次序，如下：
1?#include?<iostream>
2?#include?"20060816_operator.h"
3?using?namespace?std;
4?int?main(int?argc,?char*?argv[])
5?
2．? 把類定義于標(biāo)準(zhǔn)名字之內(nèi)，20060816_operator.h文件修改如下（不推薦）
?1?#ifndef?_20060816_OPERATOR_GHH_
?2?#define?_20060816_OPERATOR_GHH_????1
?3?
?4?//?Author?:?GuoHonghua
?5?//?Date?:?2006.08.16
?6?//?File?:?20060816_operator.h
?7?
?8?#include?<iostream>
?9?#include?<string>
10?namespace?std
11?{
12?????class?Honghua
13?????{
14?????????friend?std::ostream&?operator<<(std::ostream&?os,?const?Honghua&?ghh);
15?????public:
16?????????Honghua(const?std::string&?ghh)?:?_ghh(ghh)
17?????????{
18?????????}
19?????????
20?????private:
21?????????std::string?_ghh;
22?????};
23?????
24?????
25?????std::ostream&?operator<<(std::ostream&?os,?const?Honghua&?ghh)
26?????{
27?????????os?<<?ghh._ghh;
28?????????return?os;
29?????}
30?}
31?#endif?//?end?of?_20060816_OPERATOR_GHH_?
3．? 使用頭文件iostream.h代替iostream,比較落后的方式（不推薦!）
?1?//?20060816_operator.cxx
?2?//?#include?<iostream>
?3?#include?<iostream.h>
?4?#include?<iostream.h>
?5?#include?"20060816_operator.h"
?6?int?main(int?argc,?char*?argv[])
?7?{
?8?????const?std::string?GHH("GuoHonghua");
?9?????Honghua?ghh(GHH);
10?????cout?<<?ghh?<<?endl;
11?????return?0;
12?}?
?1?//?20060816_operator.h
?2?#ifndef?_20060816_OPERATOR_GHH_
?3?#define?_20060816_OPERATOR_GHH_????1
?4?
?5?//?Author?:?GuoHonghua
?6?//?Date?:?2006.08.16
?7?//?File?:?20060816_operator.h
?8?//?#include?<iostream>
?9?#include?<iostream.h>
10?#include?<string>
11?
12?class?Honghua
13?{
14?????friend?ostream&?operator<<(ostream&?os,?const?Honghua&?ghh);
15?public:
16?????Honghua(const?std::string&?ghh)?:?_ghh(ghh)
17?????{
18?????}
19?????
20?private:
21?????std::string?_ghh;
22?};
23?
24?
25?ostream&?operator<<(ostream&?os,?const?Honghua&?ghh)
26?{
27?????os?<<?ghh._ghh;
28?????return?os;
29?}
30?
31?#endif?//?end?of?_20060816_OPERATOR_GHH_
4．? 任何時(shí)候都不使用using namespace 聲明！這是只需要修改20060816_operator.cxx文件如下：
?1?#include?<iostream>
?2?#include?"20060816_operator.h"
?3?int?main(int?argc,?char*?argv[])
?4?{
?5?????//?const?string?GHH("GuoHonghua");
?6?????const?std::string?GHH("GuoHonghua");
?7?????Honghua?ghh(GHH);
?8?????//?cout?<<?ghh?<<?endl;
?9?????std::cout?<<?ghh?<<?std::endl;
10?????return?0;
11?}
體會(huì)：
???名字空間本來(lái)就是為了解決名字沖突問(wèn)題而引入，以使標(biāo)準(zhǔn)庫(kù)不受外界影響。在這個(gè)意義上來(lái)說(shuō)，using namespace std;不是可以隨意使用的語(yǔ)句，應(yīng)該考慮到它的位置對(duì)程序的可能影響，而最徹底的杜絕錯(cuò)誤的解決辦法是在任何時(shí)候任何情況下都不使用此語(yǔ)句！