在利用pass-by-value传参数时发生slicing,改用pass-by-reference-to-const
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今天晚上读EffectiveC++时,读到条款20时,书中提到“在利用pass-by-value传参数时发生slicing”,不确信是否是真的,编了个小程序,测试一下,果然如此,看来平时没注意,以前读过这个章节,当时大致翻过,总以为二者仅仅在于效率的问题,原来还有这个问题,看来以后使用要小心了,代码如下:
输出结果:
果然,pass-by-value会发生slicing,而pass-by-reference-to-const可以消除这个问题!
另外,还有一个收获,在使用pass-by-reference-to-const时,最初没有将display_window()后面加上const,编译时(编译器为g++)出现error: passing ‘const Base&’ as ‘this’ argument of ‘DisplayByRefer()’ discards qualifiers错误,后来发现是因为:编译器认为DisplayByRefer()内部的函数即display_window会改变obj,虽然不会改变,可编译器不那么认为,在display_window后面加上const,编译通过!
#include <iostream>#include <string>class Base{public:Base(const std::string& sname);virtual void display_window() const;virtual ~Base() {}protected:std::string m_sname;};Base::Base(const std::string& sname){m_sname = sname;}void Base::display_window() const{std::cout << "Display in Base class" << std::endl;}class Derived : public Base{public:Derived(const std::string& sname);virtual void display_window() const;};Derived::Derived(const std::string& sname): Base(sname){}void Derived::display_window() const{std::cout << "Display in Derived class" << std::endl;}void Display(Base obj){obj.display_window();}void DisplayByRefer(const Base& obj){obj.display_window();}int main(){Derived obj("io");Display(obj);DisplayByRefer(obj);return 0;}
输出结果:
Display in Base classDisplay in Derived class
果然,pass-by-value会发生slicing,而pass-by-reference-to-const可以消除这个问题!
另外,还有一个收获,在使用pass-by-reference-to-const时,最初没有将display_window()后面加上const,编译时(编译器为g++)出现error: passing ‘const Base&’ as ‘this’ argument of ‘DisplayByRefer()’ discards qualifiers错误,后来发现是因为:编译器认为DisplayByRefer()内部的函数即display_window会改变obj,虽然不会改变,可编译器不那么认为,在display_window后面加上const,编译通过!
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