一句话概括,不能同时使用using 和include ***.h; 详细传送门:https://blog.csdn.net/m0_37876745/article/details/78565315
using namespace std let u access to standard library in cpp such string or vector , if u dont , for example to use tge cout , u have to access to it using :: , std::cout , so to make it simple , u use std namespace and just call cout 24th Nov 2017, 8:16 AM Maher Zaido...
Why is it bad to use namespace std? I know you can use using std::cout or anything else for that matter or just not use anything like and just have std::(whatever) however why would be bad just to use the whole namespace?
It's quite common among beginners to put int ausingnamespacestd;so they wouldn't have to type the fully qualified name for anything included in headers. To prevent naming conflicts, though, some people don't use it and type in an std:: before everything in headers without the .h extens...
using namespace std; 然后编译时出现 error C2871: 'std' : does not exist or is not a namespace 查了一下,原来 C++有两个不同版本号的头文件。引入名字空间这个概念曾经编译器用的是#include <iostream.h>, 而引入名字空间的概念以后std名字空间的头文件名称字变成了<iostream>。
using namespace std; 然后编译时出现 error C2871: 'std' : does not exist or is not a namespace 查了一下,原来 C++有两个不同版本的头文件。引入名字空间这个概念以前编译器用的是#include <iostream.h>, 而引入名字空间的概念以后std名字空间的头文件名字变成了<iostream>。
也就是早期的c++实现。3、头文件<iostream>则没有定义全局命名空间，使用时必须使用namespace std才能正确使用cout。4、vc6.0支持这个两个版本.，不过现在都用<iostream>了，所以只需将以上语句改为：include <iostream> using namespace std，或者是 #include <iostream.h>即可。应该...
using namespace std; 然后编译时出现 error C2871: 'std' : does not exist or is not a namespace 查了一下,原来 C++有两个不同版本号的头文件。引入名字空间这个概念曾经编译器用的是#include <iostream.h>, 而引入名字空间的概念以后std名字空间的头文件名称字变成了<iostream>。
问题也已经解决了，只要在头文件中加上using namespace std; 再编译就没有错误了。因为student_info 中...
// std__functional__is_placeholder.cpp// compile with: /EHsc#include<functional>#include<iostream>usingnamespacestd::placeholders;template<classExpr>voidtest_for_placeholder(constExpr&) {std::cout<<std::is_placeholder<Expr>::value <<std::endl; }intmain(){ test_for_placeholder(3.0); test_...