未调用类型转换运算符

Question

我正在尝试为对象之间的类型转换做一些示例代码，只是按照显示的内容 here。

这是父级 class

// ParentClass.h
class ParentClass {
public:
    ParentClass();
    virtual ~ParentClass();
};

这只是一些概念证明，它唯一做的就是打印一些东西

#include <iostream>
#include "ParentClass.h"
ParentClass::ParentClass()
{
    std::cout << "Parent\tDefault constructor\n";
}
ParentClass::~ParentClass()
{
    std::cout << "Parent\tDestructor\n";
}

然后，派生的class

// ChildClass.h
#include "ParentClass.h"

class ChildClass: public ParentClass {
public:
    ChildClass();
    // conversion constructor
    ChildClass( const ParentClass& p );
    // conversion from assignment
    ChildClass& operator= ( const ParentClass & p );
    // type-cast operator
    operator ParentClass();
    // destructor
    virtual ~ChildClass();
};

然后，代码，也比较笨

// ChildClass.cpp

#include <iostream>
#include "ChildClass.h"

ChildClass::ChildClass()
{
    std::cout << "Child\tDefault constructor\n";
}
ChildClass::~ChildClass()
{
    std::cout << "Child\tDestructor\n";
}
// conversion constructor
ChildClass::ChildClass( const ParentClass& p )
{
    std::cout << "Child\tConversion constructor\n";
}
// assignment operator    
ChildClass& ChildClass::operator= ( const ParentClass & p )
{
    std::cout << "Child\toperator= conversion\n";
    return *this;
}    
// type-cast operator
ChildClass::operator ParentClass()
{
    std::cout << "Child\toperatorParentClass()\n";
    return ParentClass();
}

然后，我在我的主函数中做一些转换

    std::cout << "calls Parent default constructor\n";
    ParentClass foo;
    std::cout << "calls Parent to child constructor\n";
    ChildClass bar = foo;
    std::cout << "calls assignment operator from child\n";
    bar = foo;
    std::cout << "calls Child type-cast operator\n";
    foo = bar;
    std::cout << "Exit\n";

但是当我运行这个代码没有从派生的 class 输入类型转换运算符时，它只是打印这个：

calls Parent default constructor
Parent  Default constructor
calls Parent to child constructor
Parent  Default constructor
Child   Conversion constructor
calls assignment operator from child
Child   operator= conversion
calls Child type-cast operator
Exit
Child   Destructor
Parent  Destructor
Parent  Destructor

我对此一头雾水，因为我尝试了上面 link 中的代码，它运行正常。

Answer 1

派生到基础的转换从不调用转换函数。 [class.conv.fct]/p1:

A conversion function is never used to convert a (possibly cv-qualified) object to the (possibly cv-qualified) same object type (or a reference to it), to a (possibly cv-qualified) base class of that type (or a reference to it), or to (possibly cv-qualified) void.

在您的 foo = bar; 中，bar 直接绑定到 ParentClass 的隐式声明的复制赋值运算符的 const ParentClass& 参数。

未调用类型转换运算符

Type casting operator is not being called

c++

c++11

typecast-operator

typecasting-operator