基类catch不捕获异常，即使它出现在派生类catch之前

Question

请参阅以下代码的预期流程

在这种情况下，基类异常由于其多态性而被捕获为预期行为。

#include <stdio.h>
#include<iostream.h>
using namespace std;
void main() {
try {
//throw CustomException();
throw bad_alloc();
}
///due to poly morphism base class reference will
catch(exception &ex) {
cout<<"Base :"<<ex.what()<<endl;
}
catch(bad_alloc &ex) {
cout<<"Derieved :"<<ex.what()<<endl;
}
}

输出

bad_alloc

但是，如果我像下面的代码中所示那样进行自定义异常，则派生类正在捕获异常，即使基类捕获首先出现在catch块中：

class CustomException : exception {
public :
CustomException() { }
const char * what() const throw() {
// throw new exception();
return "Derived Class Exception !";
}
};
void main() {
try {
throw CustomException();
//throw bad_alloc();
}
///due to poly morphism base class reffrence will
catch(exception &ex) {
cout<<"Base :"<<ex.what()<<endl;
}
catch(CustomException &ex) {
cout<<"Derived :"<<ex.what()<<endl;
}
} 

预期输出：

Base : Derived Class Exception !

实际输出：

Derived: Derived Class Exception !

Answer 1

使用class关键字声明的类的默认继承访问说明符是private。这意味着CustomException继承自exceptionprivately。使用private继承的派生类不能绑定到其父类的引用。

如果您继承publicly，它将正常工作：

class CustomException : public exception // <-- add public keyword here
{
public :
CustomException()
{
}
const char * what()
{
return "Derived Class Exception !";
}
};
int main()
{
try
{
throw CustomException();
}
catch(exception &ex)
{
cout<<"Base :"<<ex.what()<<endl;
}
catch(CustomException &ex)
{
cout<<"Derived :"<<ex.what()<<endl;
}
} 

实时演示