C + + "using namespace std;" The understanding

If you need to use the C + + input (CIN) or output (cout) tool in your program, you must provide two lines of code:

1 #include <iostream>2usingnamespace//can use the space name for something inside STD

This code appears when I build a C + + project called Helloword:

1 using namespace //You can use the space name for something inside the Helloword

The purpose of introducing namespace namespace in C + + is to prevent multiple programmers from having a function or variable duplicate name when writing the same project. The following code can be compiled by:

1#include <iostream>2 using namespacestd;3 4 namespaceX5 {6       intA=2;7 }8 9 namespaceYTen { One       intA=3; A }         -  - voidMian () //Although the program defines three variable a within the same file, they belong to different spaces and can be compiled.  the { -     intA=1; -     using namespaceA; -     using namespaceB; +cout<<"m,a="<<a<<Endl; Output: A=1 -cout<<"x,a="<<A::a<<Endl; Output: a=2 +cout<<"y,a="<<B::a<<Endl; Output: A=3 A} 

C + + "using namespace std;" The understanding