對於類的聲明的頭文件，對於include它的任何一個文件，都需要將這個類內部聲明的成員需要的頭文件include進去。代碼如下：

// hehe.h
#ifndef HEHE_H
#define HEHE_H
class Hehe{
public:
    Hehe() = default;
    void ShowSomthing();
    // do somthing 
private:
    std::vector<double> vec;
};
#endif  

　　// hehe.cpp
　　#include <iostream>
　　#include "hehe.h
 
"
　　void Hehe::ShowSomthing(){
   　　 std::cout << __func__ << std::endl;
　　}
　　// do somthing

// main.cpp
#include <iostream>
#include "hehe.h"
using namespace std;    
int main(){
    Hehe h;
    h.ShowSomething();
    return 0;
}

進行編譯，將會報出錯

In file included from hehe.cpp:2:0:
hehe.h:
 
9:7: error: ‘vector‘ in namespace ‘std‘ does not name a template type
  std::vector<double> vec;
       ^~~~~~
In file included from main.cpp:2:0:
hehe.h:9:7: error: ‘vector‘ in namespace ‘std‘ does not name a template type
  std::vector<double> vec;
       ^~~~~~
main.cpp: In function ‘int main()
 
‘:
main.cpp:6:5: error: ‘class Hehe‘ has no member named ‘ShowSomething‘; did you mean ‘ShowSomthing‘?
   h.ShowSomething();
     ^~~~~~~~~~~~~
     ShowSomthing

之後，對於main.cpp添加#include ,通過

// main.cpp
#include <iostream>
#include <vector>
#include "hehe.h"
using namespace std;
int main(){
    Hehe h;
    h.ShowSomething();
    return 0;
}

C++:in namespace 'std' does not name a template type