虚函数默认参数 [英] Virtual functions default parameters

问题描述

有人可以解释c ++编译器保留虚函数参数的默认值吗？我知道这是一个坏主意，在儿童类中更改这些参数，但为什么？
谢谢。

Could anybody explain where c++ compilers keep default values for parameters for virtual functions? I know it is a bad idea to change these parameters in child classes but why? Thanks.

推荐答案

这是一个坏主意，因为它们不会保存在任何地方。

It's a bad idea because they aren't kept anywhere.

所使用的默认值将是在静态（编译时）类型中定义的值。因此，如果您要更改覆盖中的默认参数，但是您通过基类指针或引用调用该函数，则将使用基础中的默认值。

The default values that are used will be those defined in the static (compile-time) type. So if you were to change the default parameters in an override, but you called the function through a base class pointer or reference, the default values in the base would be used.

#include <iostream>

struct Base
{
    virtual ~Base(){ }
    virtual void foo(int a=0) { std::cout << "base: " << a << std::endl; }
};

struct Derived : public Base
{
    virtual ~Derived() { }
    virtual void foo(int a=1) { std::cout << "derived: " << a << std::endl; }
};

int main()
{
    Base* derived = new Derived();
    derived->foo();    // prints "derived: 0"
    delete derived;
}

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