Problems encountered in writing abstract classes and derived classes

Question

When I was implementing a data structure, I encountered the following problem of use of undeclared identifier 'a'. Isn't an inherited from the base class? Why can't I access it?

#pragma once
#include <iostream>
template <typename T> class A {
public:
  T a;
  virtual void show() = 0;
};

template <typename T> class B : public A<T> {
public:
  B() { a = 0; }
  void show() { std::cout << "Hello, world!" << std::endl; }
};

But when I added the this pointer, there was no error message：

#pragma once
#include <iostream>
template <typename T> class A {
public:
  T a;
  virtual void show() = 0;
};

template <typename T> class B : public A<T> {
public:
  B() { this->a = 0; }
  void show() { std::cout << "Hello, world!" << std::endl; }
};

Why???

Answer 1

When you write this->a, you are explicitly telling the compiler to look for the member a in the scope of the current instance. This helps the compiler resolve the name correctly, even in the context of templates with potential scope ambiguities.

In your first example;

B() { a = 0; }

The compiler encounters a in the constructor of class B, and it's trying to find a within the scope of class B. However, since a is inherited from class A<T>, it's considered a dependent name and requires using this-> to indicate the correct scope.

In your second example;

B() { this->a = 0; }

You are explicitly using this->a to indicate that a is a member of the current instance. This resolves the scope ambiguity and lets the compiler know that a is inherited from the base class.