请考虑以下代码段：

#include <iostream>
struct S {
    virtual void pure1() = 0;
    virtual void pure2() = 0;
};

struct T : S {
    void pure1() { std::cout << "T::pure1" << '\n'; }
    void pure2() { std::cout << "T::pure2" << '\n'; }
};


void S::pure2() { std::cout << "S::pure2" << '\n';}

int main()
{
    T t;
    t.S::pure2();
}

它打印S :: pure2.

看看C 11标准,我不确切知道这是怎么发生的.我认为这与§3.4.5/ 4有关：

If the id-expression in a class member access is a qualified-id of
the form

class-name-or-namespace-name::…

the class-name-or-namespace-name following the . or -> operator is first looked up in the class of the object expression and the name, if
found, is used. Otherwise it is looked up in the context of the entire
postfix-expression.

但我不明白纯子函数pure2()是如何在基类S中找到的,表达式为t.S :: pure2();以上.

解决方法:

可以在基类中实现纯虚函数.该标准规定有效(强调我的)：

10.4 Abstract classes

2 An abstract class is a class that can be used only as a base class of some other class; no objects of an abstract class can be created except as subobjects of a class derived from it. A class is abstract if it has at least one pure virtual function. [ Note: Such a function might be inherited: see below. —end note ] A virtual function is specified pure by using a pure-specifier (9.2) in the function declaration in the class definition. A pure virtual function need be defined only if called with, or as if with (12.4), the qualified-id syntax (5.1).

如果你没有打电话

t.S::pure2();

那么,可以省略S :: pure2()的实现.如果你没有实现S :: pure2()但仍然被调用,那将是一个链接时间错误

t.S::pure2();