依旧是那个最简单的hello world !
程序是vs自动给新手建的，我自己没有写代码，但是手贱把 #include "pch.h" 放到了 #include <iostream> 后面：
编译失败：
error C2039: “cout”: 不是“std”的成员 error C2065: “cout”: 未声明的标识符 error C2039: “endl”: 不是“std”的成员 ...解决办法：换顺序
#include "pch.h" #include <iostream>错误原因：预编译头文件不是第一个，即"pch.h"被我放到后面去，那pch.h前面的iostream会被忽略，so报错
Example: precompiled header isn’t first This error can occur if you put any preprocessor directives, such as #include, #define, or #pragma, before the #include of a precompiled header file. If your source file uses a precompiled header file (that is, if it’s compiled by using the /Yu compiler option) then all preprocessor directives before the precompiled header file are ignored. This example fails to compile because cout and endl are defined in the <iostream> header, which is ignored because it is included before the precompiled header file. error详情见 vs官网：https://docs.microsoft.com/en-us/cpp/error-messages/compiler-errors-1/compiler-error-c2065?f1url=https://msdn.microsoft.com/query/dev15.query?appId=Dev15IDEF1&l=ZH-CN&k=k(C2065)&rd=true&view=vs-2019
