ACM--C++初学
对于一个存在着标准输入输出的C++控制台程序，一般会在#include <iostream>的下一行发现一句话，using namespace std。
这句话其实就表示了所有的标准库函数都在标准命名空间std中进行了定义。其作用就在于避免发生重命名的问题。
#include <iostream>
using namespace std;
namespace ZhangSan
{
int a=10; //张三把10赋值给了变量a
}
namespace LiSi
{
int a=5; //李四把10赋值给了变量a
}
void main()
{
int a=1;
cout<<"张三定义的a="<<ZhangSan::a<<endl;
cout<<"李四定义的a="<<LiSi::a<<endl;
cout<<"主函数定义的a="<<a<<endl;
}
上例中的“ZhangSan::a”和“LiSi::a”分别表示了调用张三命名空间中的a变量和李四命名空间中的a变量。这样的好处显而易见，那就是虽然张三和李四这两个程序员都定义了一个变量a，但是并不会出现重名的危险。
2. 关于using namespace *
顾名思义，using namespace * 就表示释放命名空间* 中间的东西。好处在于我们在程序里面就不用在每个函数的头上都加上*::来调用。比如说如果上面那个程序，如果我们不在using namespace std，那么我们就需要在主函数中的标准输出流cout函数前面加上std，写成
std::cout |
表示调用std空间里面的标准输出流cout。但是有些时候我们也不能图这个方便，比如说如果在主函数中将命名空间ZhangSan和LiSi的中所定义的变量释放出来，如下例1：
#include <iostream>
using namespace std;
namespace ZhangSan
{
int a=10; //张三把10赋值给了变量a
}
namespace LiSi
{
int a=5; //李四把10赋值给了变量a
}
void main()
{
int a=1;
using namespace ZhangSan;
using namespace LiSi;
cout<<a<<endl;
}
如果我们在主函数中把 int a=1给删除，如下例2：
#include <iostream>
using namespace std;
namespace ZhangSan
{
int a=10; //张三把10赋值给了变量a
}
namespace LiSi
{
int a=5; //李四把10赋值给了变量a
}
void main()
{
using namespace ZhangSan;
using namespace LiSi;
cout<<a<<endl;
}
会发现根本就不会通过编译，输出的错误信息为：
error C2872: “a”: 不明确的符号
分析可以看出，上面这个例2会引起歧义。因为ZhangSan中间的a被释放出来，同理LiSi中间的a也被释放出来了。那么编译器就不知道到底哪个才是需要输出的a，自然就会引起歧义了。同理，在例1中，编译器同样不知道到底哪个才是需要输出的a，于是它只采用了主函数中自己定义的a，这样程序也不会报错，但是只会输出1，自然结果就如上面的图所示了。
摘自：http://www.cnblogs.com/uniqueliu/archive/2011/07/10/2102238.html
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2022年8月22日 02:15
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