C++ 抽象类：成员函数的抽象返回类型无效‘；虚拟…’；

在我的程序中，我有这样的类层次结构：

#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)
{
}

---

不能按值返回

根

，因为

根

是抽象的，因此不可能存在任何类型的

根

值

您可能希望返回一个指针：

#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) };
}

---

你的设计有点乱。您的基类需要了解派生类型，这并不好。您应该考虑使用单个成员函数<代码>虚拟根添加（const root和a）const＝0；<代码>@juanchopanza:Ok更改了它，但仍然有错误。看看我编辑过的问题，这个问题在KerrekBS的回答中得到了很好的处理。另一种选择是返回一个引用，但是你必须确保总是有东西可以引用。@juanchopanza你提到的检查也适用于指针对吗？@Koushik否，因为你可以返回一个空指针，可以在调用方进行检查。+1。但这是因为它不能构造抽象类的对象吗？我们可以通过引用返回吗？@Koushik:引用什么？是的，在我的示例中，您可能会尝试只返回

x

或

y

作为

根常量&

，但您会遇到生存期问题。使用非常量引用会更好，但通常不是很有用（毕竟，您希望创建新对象）。我会做一些编辑。@Koushik:

root

是一个类。我的意思是“参考哪个对象？”。

/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 ===|

#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) };
}

struct Base
{
    virtual std::unique_ptr<Base> clone() const = 0;
};

struct Derived : Base
{
    virtual std::unique_ptr<Base> clone() const
    {
        return { new Derived(*this); }
    }
};

root & pick_one_from_two(aa & x, bb & y)
{
    return rand() % 2 == 0 ? x : y;
}