C++ 方法中的抽象 class 指针

Question

So I have a c++ project that I'm working on and I'm trying to understand an other guys code but all I get are errors and confusion about c++ abstract classes.

So What I have is a header file a cpp file and a main program.

I want to create an abstract class pointer and then pass it to a method that initializes it to a subclass and then back in the main program I can use it as the subclass:

main.cpp:

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

using namespace std;

void parse(A* a){
  a = new B();
  a->foo();
}

int main() {
  A* a;
  parse(a);
  a->foo();
}

ah:

class A{
  public:
    virtual void foo() const = 0;
    virtual ~A(){ }
};

class B : public A{
  public:
    void foo() const override;
};

class C : public A{
  public:
    void foo() const override;
};

a.cpp

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

void B::foo() const{
  std::cout << "B" << std::endl;
}

void C::foo() const{
  std::cout << "C" << std::endl;
}

Basically here I think I should cee a B but I get a segmentation error or the program exits without printing anything.

Thank you for your help!

Answer 1

In main you have uninitialized pointer a

int main() {
  A* a;
  parse(a);
  a->foo();
}

So this statement

  a->foo();

results in undefined behavior.

As for the function parse

void parse(A* a){
  a = new B();
  a->foo();
}

then it deals with its local variable a . Changing the local variable does not affect the original pointer declared in main.

You need to declare the parameter as having a referenced type

void parse(A* &a){
  a = new B();
  a->foo();
}

In this case the pointer a declared in main will be passed to the function by reference and the function will change the original pointer a.

Answer 2

Besides the issue of undefined behviour described in Vlad's answer , your example also leaks memory, even if the pointer in main was correctly initialised.

I recommend an alternative solution. Instead of modifying the pointer through a reference, I recommend to not use owning bare pointers. Instead, return the pointer, and use a smart pointer for ownership:

std::unique_ptr<A>
parse(){
    auto a = std::make_unique<B>;
    a->foo();
    return a;
}

int
main() {
    auto a = parse();
    a->foo();
}