catch子句捕获异常时既可以按值传递,也可以按照引用传递,甚至按照指针传递,但推荐使用引用捕获异常。考察如下程序:
#include <iostream>
using namespace std;
class Base
{
public:
Base()
{
cout << "Base's constructor" << endl;
}
Base(const Base& rb)
{
cout << "Base's copy constructor" << endl;
}
virtual void print()
{
cout << "Base" << endl;
}
};
class Derived :public Base
{
public:
Derived()
{
cout << "Derived's constructor" << endl;
}
Derived(const Derived& rd):Base(rd)
{
cout << "Derived's copy constructor" << endl;
}
virtual void print()
{
cout << "Derived" << endl;
}
};
void throwFunc()
{
Derived d;
throw d;
}
int main()
{
try
{
throwFunc();
}
catch (Base b)
{
cout << "Base catched" << endl;
b.print();
}
catch (Derived d)
{
cout << "Derived catched" << endl;
d.print();
}
cout << "---------------" << endl;
try
{
throwFunc();
}
catch (Base& b)
{
cout << "Base catched" << endl;
b.print();
}
catch (Derived& d)
{
cout << "Derived catched" << endl;
d.print();
}
}
程序运行结果:
Base's constructor
Derived's constructor
Base's copy constructor
Derived's copy constructor
Base's copy constructor
Base catched
Base
---------------
Base's constructor
Derived's constructor
Base's copy constructor
Derived's copy constructor
Base catched
Derived
阅读以上程序,注意以下几点: (1)程序中在函数throwFunc()中构造对象d,先后分别调用基类Base和派生类Derived的构造函数完成对象d的初始化,分别输出Base’s constructor与Derived’s constructor; (2)C++标准要求被作为异常抛出的对象必须被拷贝复制,导致异常对象d在离开作用域时,触发一次临时对象的拷贝构造,程序输出从结果来看,先后调用了基类Base的拷贝构造函数和派生类Derived的拷贝构造函数,分别输出Base’s copy constructor与Derived’s copy constructor; (3)按引用捕获异常比按值捕获异常更加高效。分隔线以上按值捕获异常,导致对象d在传递时再次被拷贝一次,输出Base’s copy constructor,降低了系统效率,使用引用捕获异常可以避免额外的拷贝操作; (4)使用引用捕获异常,可以通过基类对象实现虚函数的虚调用,在运行时提现多态性。
基于效率和多态性的考虑,建议使用引用来捕获异常。
[1]陈刚.C++高级进阶教程[M].武汉:武汉大学出版社,2008.10.5用传引用的方式捕获异常