test2.h:5:17: error: ‘string’ is not a member of ‘std’
aries@ubuntu:~/aries/Makefile/Demo$ g++ -o main main.cpp 
In file included from main.cpp:1:0:
test1.h:5:17: error: variable or field ‘test1_cout’ declared void
test1.h:5:17: error: ‘string’ was not declared in this scope
In file included from main.cpp:2:0:
test2.h:5:17: error: variable or field ‘test2_cout’ declared void
test2.h:5:17: error: ‘string’ was not declared in this scope
aries@ubuntu:~/aries/Makefile/Demo$ 

ok！遇到问题我首先来一个个的排除问题

#ifndef _ERROR_1_
#define _ERROR_1_
#include<cstring>

void cout_1(string a1);

#endif

error.c

#include<error.h>
#include<iostream>
using namespace std;
void cout_1(string str)
{
        cout << str<< endl;
}

int main()
{

        cout_1("test");
        return 0;

}

运行效果：

aries@ubuntu:~/aries/Makefile/Demo$ g++ -o error error.cpp 
aries@ubuntu:~/aries/Makefile/Demo$ ./error 
test

2 把 main单独提出来看又会怎么样呢？
error_h.cpp

#include<error.h>

void cout_1(string str)
{
        cout << str<< endl;
}

#include"error.h"
#include"error_h.cpp"
#include<iostream>
using namespace std;

int main()
{

	cout_1("test");
	return 0;
	
}

运行效果如下：

aries@ubuntu:~/aries/Makefile/Demo$ g++ -o error error.cpp 
In file included from error.cpp:1:0:
error.h:5:13: error: variable or field ‘cout_1’ declared void
error.h:5:13: error: ‘string’ was not declared in this scope
In file included from error.cpp:2:0:
error_h.cpp:3:13: error: variable or field ‘cout_1’ declared void
error_h.cpp:3:13: error: ‘string’ was not declared in this scope
error.cpp: In function ‘int main()’:
error.cpp:9:15: error: ‘cout_1’ was not declared in this scope

OK ！又出现上面的问题了。

1.变量、函数、或者类未声明或者定义。这是最简单的情况。

2.头文件相互#include时，导致了依赖关系错误。比如，头文件形成了一个环形依赖，

  /***file a ****/
  #ifndef FILE_A_
  #define FILE_A_
  #include <file b>
  #endif

  /****file b ***/
   #ifndef FILE_B_
  #define FILE_B_
  #include <file a>
  #endif

  如果在file b中用到了file a中的变量、函数、类，那么由于#ifndef和#define的作用，file b中的#include <file a>语句将失去效果。

3.我遇到的情况：我在给头文件起名字的时候不小心和某个库中的头文件重名了，而在程序中又用来了这个库的这个头文件。这样， #ifndef XXXX 和 #ifndef XXXX中的宏重名了，我自己写的头文件就失去了效果。别的文件自然找不到这个头文件中的声明，就提示 was not decleared in this scope了。

#include<error.h>
#include<cstring>
#include<iostream>
using namespace std;

void cout_1(string str)
{
        cout << str<< endl;
}

运行效果如下：

aries@ubuntu:~/aries/Makefile/Demo$ g++ -o error error.cpp 
In file included from error.cpp:1:0:
error.h:5:13: error: variable or field ‘cout_1’ declared void
error.h:5:13: error: ‘string’ was not declared in this scope

还好错误减少了！不过还是error.h 报错。