首页 \ 问答 \ 在c ++对象上使用extern时的未定义引用，但不是整数类型(Undefined reference when using extern on a c++ object, but not integral type)

在c ++对象上使用extern时的未定义引用，但不是整数类型(Undefined reference when using extern on a c++ object, but not integral type)

 尝试在c ++对象上使用extern时，我得到未定义的引用错误。 整体类型似乎不会发生这种情况。 我错过了什么？！ 下面的代码复制了这个问题：  
 file1.cpp：  
#include <string>

const std::string s("test");
int i = 99;

int main()
{
        extern void Test();
        Test();
}
 
 file2.cpp：  
#include <iostream>
#include <string>

extern const std::string s;
extern int i;

void Test()
{
        std::cout << s << std::endl;
        std::cout << i << std::endl;
}
 
 如果我注释掉's'std :: string变量的用法，那么链接错误就会消失。  
 关于SO的其他问题与此类似，但它们似乎都与未定义变量的人有关，我就是这样！ 

I'm getting undefined reference errors when trying to use extern on a c++ object. It doesn't appear to happen with integral types. What am I missing?! This code below replicates the problem: 
file1.cpp: 
#include <string>

const std::string s("test");
int i = 99;

int main()
{
        extern void Test();
        Test();
}
 
file2.cpp: 
#include <iostream>
#include <string>

extern const std::string s;
extern int i;

void Test()
{
        std::cout << s << std::endl;
        std::cout << i << std::endl;
}
 
if i comment out the usage of the 's' std::string variable, the linking errors go away. 
There are other questions on SO similar to this, but they all seem to be related to people not defining the variable, which I am!

原文：https://stackoverflow.com/questions/12701130

更新时间：2024-03-27 11:03

最满意答案

 您的try / except块没有正确布局以处理KeyError 。 它可能发生在三个地方，但你只能抓住一个。 逻辑有点疯狂，但只是编辑你有什么，我认为这是你试图达到的目的：  
try:
     if datetime.now() - request.session['last_touch'] > timedelta( 0, settings.AUTO_LOGOUT_DELAY * 60, 0):
          logout(request)
          del request.session['last_touch']
          return self.get_response(request) 
     else:
          request.session['last_touch'] = datetime.now()
          return self.get_response(request)  
except KeyError: #KeyError thrown if last touch doesn't exist, so set it. 
     request.session['last_touch'] = datetime.now()

Your try/except block isn't laid out correctly to catch and deal with the KeyError. It can occur in three places, but you're only catching one. The logic is a bit janky, but just editing what you have, I think this is what you're trying to achive:  
try:
     if datetime.now() - request.session['last_touch'] > timedelta( 0, settings.AUTO_LOGOUT_DELAY * 60, 0):
          logout(request)
          del request.session['last_touch']
          return self.get_response(request) 
     else:
          request.session['last_touch'] = datetime.now()
          return self.get_response(request)  
except KeyError: #KeyError thrown if last touch doesn't exist, so set it. 
     request.session['last_touch'] = datetime.now()

在c ++对象上使用extern时的未定义引用，但不是整数类型(Undefined reference when using extern on a c++ object, but not integral type)

最满意答案

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