抽象类：成员函数“虚拟...”的抽象返回类型无效 [英] Abstract class : invalid abstract return type for member function ‘virtual...’
本文介绍了抽象类：成员函数“虚拟...”的抽象返回类型无效的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
在我的程序中，我有这样的类层次结构：
In my program I have such class hierarchy:
#include <iostream>
#include <cmath>
#include <sstream>
using namespace std;
class aa;
class bb;
class root
{
public:
virtual ~root() {}
virtual root add(const aa& a) const=0;
virtual root add(const bb& a) const=0;
};
class aa: public root
{
public:
aa() { }
aa(const aa& a) { }
virtual root add(const aa& a) const
{ return root(new aa()); }
virtual root add(const bb& a) const
{ return root(new bb()); }
};
class bb: public root
{
public:
bb() { }
bb(const bb& b) {}
virtual root add(const aa& a) const
{ return root(new bb()); }
virtual root add(const bb& a) const
{ return root(new bb()); }
};
int main(int argc, char **argv)
{
}
，但是我在编译过程中仍然遇到错误。我无法更改班级层次结构，但是可以在这里做些我想做的事吗？
but Im still getting errors in compilation process. I cant change my class hierarchy but it is possible to make something I want here?
编辑的班级：
#include <iostream>
#include <cmath>
#include <sstream>
using namespace std;
class root
{
public:
virtual ~root() {}
virtual root add(const root& a) const=0;
virtual root add(const root& b) const=0;
};
class aa: public root
{
public:
aa() { }
aa(const aa& a) { }
virtual root add(const root& a) const
{ return root(new aa()); }
virtual root add(const root& b) const
{ return root(new bb()); }
};
class bb: public root
{
public:
bb() { }
bb(const bb& b) {}
virtual root add(const root& a) const
{ return root(new bb()); }
virtual root add(const root& b) const
{ return root(new bb()); }
};
int main(int argc, char **argv)
{
}
编辑类的错误：
/home/brian/Desktop/Temp/Untitled2.cpp|11|error: ‘virtual root root::add(const root&) const’ cannot be overloaded|
/home/brian/Desktop/Temp/Untitled2.cpp|10|error: with ‘virtual root root::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|10|error: invalid abstract return type for member function ‘virtual root root::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: because the following virtual functions are pure within ‘root’:|
/home/brian/Desktop/Temp/Untitled2.cpp|10|note: virtual root root::add(const root&) const|
/home/brian/Desktop/Temp/Untitled2.cpp|11|error: invalid abstract return type for member function ‘virtual root root::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|20|error: invalid abstract return type for member function ‘virtual root aa::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|22|error: invalid abstract return type for member function ‘virtual root aa::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|22|error: ‘virtual root aa::add(const root&) const’ cannot be overloaded|
/home/brian/Desktop/Temp/Untitled2.cpp|20|error: with ‘virtual root aa::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp||In member function ‘virtual root aa::add(const root&) const’:|
/home/brian/Desktop/Temp/Untitled2.cpp|20|error: invalid abstract return type for member function ‘virtual root aa::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|21|error: cannot allocate an object of abstract type ‘root’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp||In member function ‘virtual root aa::add(const root&) const’:|
/home/brian/Desktop/Temp/Untitled2.cpp|22|error: invalid abstract return type for member function ‘virtual root aa::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|23|error: cannot allocate an object of abstract type ‘root’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|23|error: expected primary-expression before ‘(’ token|
/home/brian/Desktop/Temp/Untitled2.cpp|23|error: expected type-specifier before ‘bb’|
/home/brian/Desktop/Temp/Untitled2.cpp|23|error: expected ‘)’ before ‘bb’|
/home/brian/Desktop/Temp/Untitled2.cpp|32|error: invalid abstract return type for member function ‘virtual root bb::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|34|error: invalid abstract return type for member function ‘virtual root bb::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|34|error: ‘virtual root bb::add(const root&) const’ cannot be overloaded|
/home/brian/Desktop/Temp/Untitled2.cpp|32|error: with ‘virtual root bb::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp||In member function ‘virtual root bb::add(const root&) const’:|
/home/brian/Desktop/Temp/Untitled2.cpp|32|error: invalid abstract return type for member function ‘virtual root bb::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|33|error: cannot allocate an object of abstract type ‘root’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp||In member function ‘virtual root bb::add(const root&) const’:|
/home/brian/Desktop/Temp/Untitled2.cpp|34|error: invalid abstract return type for member function ‘virtual root bb::add(const root&) const’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
/home/brian/Desktop/Temp/Untitled2.cpp|35|error: cannot allocate an object of abstract type ‘root’|
/home/brian/Desktop/Temp/Untitled2.cpp|6|note: since type ‘root’ has pure virtual functions|
||=== Build finished: 38 errors, 0 warnings ===|
推荐答案
您不能返回 root 按值，因为 root 是抽象的，因此永远不会存在 root 。
You cannot return a root by value, since root is abstract and thus there can never exist any values of type root.
您可能想返回一个指针：
You might want to return a pointer:
#include <memory>
std::unique_ptr<root> do_you_feel_lucky(aa const & x, bb const & y)
{
if (rand() % 2 == 0)
return { new aa(x) };
else
return { new bb(y) };
}
您的感觉很像克隆或虚拟副本函数，但是：
What you have feels a lot like a "clone" or "virtual copy" function, though:
struct Base
{
virtual std::unique_ptr<Base> clone() const = 0;
};
struct Derived : Base
{
virtual std::unique_ptr<Base> clone() const
{
return { new Derived(*this); }
}
};
自从您询问引用以来，这是您可以做的另一件事，尽管似乎没有意义：
Since you asked about references, here's another thing you could do, though it seems a bit pointless: Pick a reference to one among several derived objects and return a base reference.
root & pick_one_from_two(aa & x, bb & y)
{
return rand() % 2 == 0 ? x : y;
}
这篇关于抽象类：成员函数“虚拟...”的抽象返回类型无效的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
查看全文
