新手请教个 变量赋值的问题

#ifndef _TEST_H                                                                 
#define _TEST_H                                                                 
                                                                        
extern int a;                                                                                                                       
                                                                        
#endif                                                                          

#include <iostream>                                                            
#include "test1.h"                                                              
int a = 1;

#include "test1.h"                                                              
#include <iostream>                                                            
                                                                                
using namespace::std;                                                                                                                                          
a = 2;  // 如果这样写会出错， does not name a type   　将这行放在 main 中赋值就没有问题，这是为什么            
int main() {                                                                    
                                                                                
    cout << a << endl;                                                         
                                                                                
}