Usage of this sometimes necessary?

Question

just stumbled upon something i can't explain. The following code doesn't compile

template<int a>
class sub{
protected:
    int _attr;
};

template<int b>
class super : public sub<b>{
public:
    void foo(){
        _attr = 3;
    }
};

int main(){
    super<4> obj;
    obj.foo();
}

whereas when i change _attr = 3; to this->attr = 3; there seems to be no problem.

Why is that? Are there any cases you must to use this?

I used g++ test.cpp -Wall -pedantic to compile and i get the following error

test.cpp: in member function 'void super<b>::foo()':
test.cpp:11:3: error: '_attr' was not declared in this scope

Answer 1

Why is that? Are there any cases you must to use this?

Yes, there are some cases where you need to use this. In your example, when the compiler sees _attr, it tries to look for _attr inside the class and cannot find it. By adding this-> you delay lookup until instantiation time which allows the compiler to find it inside of sub.

Another very common reason to use this is to resolve ambiguity problems:

void foo (int i)
{
   this->i = i;
}