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分享一段简单的C++代码在VC6.0没问题，但是在VC2017出错，求分析原因！
如图，在VC6.0中没有问题，在VC2017编译出错，试了一下，只要把public CExpetcion 改成 protected CExpetcion，或者将throw BatchException("~ServiceHandle"); 改为 throw new BatchException("~ServiceHandle"); 就没有这个link报错的问题，为什么throw普通对象的时候会去触发CObject的复制构造方法？求分析原因！
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pstrunner 2019-03-05
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是的，我用你的代码试了，这里只要加上throw tst;就出错，可是为什么呢，你前main函数里面两句代码分别测试了构造方法和赋值构造方法，都没有问题，甚至我在你代码的基础上，“throw tst; ”前加上一句CMyException tst3(tst2);再编译也是没有问题的，说明单独调用拷贝构造函数也不会错。[/quote]
我在上面已经回答了，估计是VC问题，你参考下面的说明：
The throw of an instance of a class object causes a copy to be generated when the object is copied into the C++ runtime memory location that holds the exception object.
It may be possible for some compilers to "elide" (get rid of) this call to the copy-constructor in a similar way to which they can get rid of the call to a copy-constructor when returning an instance of a class object from a function (the RVO optimization) - but the compiler still needs to check that the copy-constructor is callable: so if the copy-constructor is private it should still cause an error even though the compiler would not actually generate a call to it.
uouo88 2019-03-05
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是的，我用你的代码试了，这里只要加上throw tst;就出错，可是为什么呢，你前main函数里面两句代码分别测试了构造方法和赋值构造方法，都没有问题，甚至我在你代码的基础上，“throw tst; ”前加上一句CMyException tst3(tst2);再编译也是没有问题的，说明单独调用拷贝构造函数也不会错。
uouo88 2019-03-05
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补充测试代码如下
#include "pch.h"
#include "afx.h"
#include <iostream>
using namespace std;
class BaseException
{
public:
BaseException() throw() {cout << "Constructor BaseException() is called" << endl;}
~BaseException() throw() { cout << "Destructor ~BaseException() is called" << endl; }
//virtual ~BaseException() = 0;
private:
BaseException(const BaseException& be) throw() // = delete;
{
cout << "Copy-Constructor BaseException(const BaseException& be) is called" << endl;
}
};
class BatchException : public BaseException //public CException
{
private:
CString m_cdMsg = "";
public:
BatchException() throw() { cout << "Constructor BatchException() is called" << endl; }
BatchException(const char* p_szMes) throw()
{
m_cdMsg = p_szMes;
cout << "Constructor BatchException(const char* p_szMes) is called" << endl;
}
BatchException(const BatchException& be) throw()
{
this->m_cdMsg = be.m_cdMsg;
cout << "Copy-Constructor BatchException(const BatchException& be) is called" << endl;
}
const BatchException& operator = (const BatchException& be) throw()
{
this->m_cdMsg = be.m_cdMsg; return *this;
cout << "Assign-Constructor BatchException(const BatchException& be) is called" << endl;
}
virtual ~BatchException() throw() { cout << "Destructor ~BatchException() is called" << endl; }
virtual const char *what() const throw() { return m_cdMsg; }
};
int main()
{
try
{
//BatchException be("BatchException occurs");
//throw be;
throw BatchException("BatchException occurs");
//throw new BatchException("BatchException occurs");
}
catch (BatchException& e)
{
cout << "e.what() = " <<e.what() << endl;
}
//catch (CException* e)
//{
// cout << "e->what() = " << ((BatchException*)e)->what() << endl;
// e->Delete();
//}
//pause:
getchar();
}
#include "pch.h"
#include "afx.h"
#include <iostream>
using namespace std;
class BaseException
{
public:
BaseException() throw() {cout << "Constructor BaseException() is called" << endl;}
~BaseException() throw() { cout << "Destructor ~BaseException() is called" << endl; }
//virtual ~BaseException() = 0;
private:
BaseException(const BaseException& be) throw() // = delete;
{
cout << "Copy-Constructor BaseException(const BaseException& be) is called" << endl;
}
};
class BatchException : public BaseException //public CException
{
private:
CString m_cdMsg = "";
public:
BatchException() throw() { cout << "Constructor BatchException() is called" << endl; }
BatchException(const char* p_szMes) throw()
{
m_cdMsg = p_szMes;
cout << "Constructor BatchException(const char* p_szMes) is called" << endl;
}
BatchException(const BatchException& be) throw()
{
this->m_cdMsg = be.m_cdMsg;
cout << "Copy-Constructor BatchException(const BatchException& be) is called" << endl;
}
const BatchException& operator = (const BatchException& be) throw()
{
this->m_cdMsg = be.m_cdMsg; return *this;
cout << "Assign-Constructor BatchException(const BatchException& be) is called" << endl;
}
virtual ~BatchException() throw() { cout << "Destructor ~BatchException() is called" << endl; }
virtual const char *what() const throw() { return m_cdMsg; }
};
int main()
{
try
{
//BatchException be("BatchException occurs");
//throw be;
throw BatchException("BatchException occurs");
//throw new BatchException("BatchException occurs");
}
catch (BatchException& e)
{
cout << "e.what() = " <<e.what() << endl;
}
//catch (CException* e)
//{
// cout << "e->what() = " << ((BatchException*)e)->what() << endl;
// e->Delete();
//}
//pause:
getchar();
}
pstrunner 2019-03-05
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void main()
{
CMyException tst("hello"), tst2;
// 没有问题
tst2 = tst;
// 报错
throw tst;
}
pstrunner 2019-03-05
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我也写了测试代码：
class CMyException : public CException
{
public:
CMyException() {}
CMyException(const CMyException& x) {
*this = x;
}
explicit CMyException(const char* szMsg) {}
~CMyException() {}
CMyException& operator=(const CMyException& x) {
return *this;
}
private:
};
uouo88 2019-03-05
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可是throw对象的时候为啥是去看CObject的private的拷贝构造函数呢？难道不是应该调用BatchException这个类的拷贝构造函数吗？[/quote]
在affx.h文件里，CException是继承CObject的。
class AFX_NOVTABLE CException : public CObject我写了一个测试类A继承测试类B，且都加上了拷贝构造方法和cout测试语句，在main函数throw A的对象实例，然后B类的拷贝构造方法测试结果显示并没有调用到.
pstrunner 2019-03-05
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可是throw对象的时候为啥是去看CObject的private的拷贝构造函数呢？难道不是应该调用BatchException这个类的拷贝构造函数吗？[/quote]
在affx.h文件里，CException是继承CObject的。
class AFX_NOVTABLE CException : public CObjectuouo88 2019-03-04
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可是throw对象的时候为啥是去看CObject的private的拷贝构造函数呢？难道不是应该调用BatchException这个类的拷贝构造函数吗？
pstrunner 2019-03-04
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我查了下，估计这是VC的一个问题，msdn论坛解释如下:【https://social.msdn.microsoft.com/Forums/vstudio/en-US/a7dfa49c-6b81-4ce1-86a3-a926c700d626/yet-another-c2248-error?forum=vcgeneral】
The throw of an instance of a class object causes a copy to be generated when the object is copied into the C++ runtime memory location that holds the exception object.
It may be possible for some compilers to "elide" (get rid of) this call to the copy-constructor in a similar way to which they can get rid of the call to a copy-constructor when returning an instance of a class object from a function (the RVO optimization) - but the compiler still needs to check that the copy-constructor is callable: so if the copy-constructor is private it should still cause an error even though the compiler would not actually generate a call to it.
If you throw a pointer to an object, as in your code, then you need to catch a pointer. For example:
class X { };
void f()
{
throw new X();
}
void g()
{
try {
f();
}
catch (X* pX) {
delete pX;
}
}
Note: as object to be thrown is dynamically allocated you need to delete it otherwise you will have a memory leak.
uouo88 2019-03-04
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我看了一下，CException类的public的构造函数是存在的，但是CException继承了CObject类，CObject的拷贝构造方法是private并且标注未实现的。
然后我对比了一下VC6.0的CException和VC2017的CException，虽然都标注为抽象类，但是VC6.0的CException是可以用构造方法初始化的，但是VC2017的CException不能用它的构造方法初始化，错误提示CException是一个抽象类，不能被实例化。
pstrunner 2019-03-04
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你要看看是否存在public的CException的构造函数，我看到报错是因为private的Cobject构造函数不存在。
