When should I make explicit use of the `this` pointer?

Question

When should I explicitly write this->member in a method of a class?

Answer 1

The main (or I can say, the only) purpose of this pointer is that it points to the object used to invoke a member function.

Base on this purpose, we can have some cases that only using this pointer can solve the problem.

For example, we have to return the invoking object in a member function with argument is an same class object:

class human {

... 

human & human::compare(human & h){
    if (condition)
        return h;       // argument object
    else 
        return *this;   // invoking object
    }
};

Answer 2

Although I usually don't particular like it, I've seen others use this-> simply to get help from intellisense!

Answer 3

If you declare a local variable in a method with the same name as an existing member, you will have to use this->var to access the class member instead of the local variable.

#include <iostream>
using namespace std;
class A
{
    public:
        int a;

        void f() {
            a = 4;
            int a = 5;
            cout << a << endl;
            cout << this->a << endl;
        }
};

int main()
{
    A a;
    a.f();
}

prints:

5
4

Answer 4

Usually, you do not have to, this-> is implied.

Sometimes, there is a name ambiguity, where it can be used to disambiguate class members and local variables. However, here is a completely different case where this-> is explicitly required.

Consider the following code:

template<class T>
struct A {
   int i;
};

template<class T>
struct B : A<T> {

    int foo() {
        return this->i;
    }

};

int main() {
    B<int> b;
    b.foo();
}

If you omit this->, the compiler does not know how to treat i, since it may or may not exist in all instantiations of A. In order to tell it that i is indeed a member of A<T>, for any T, the this-> prefix is required.

Note: it is possible to still omit this-> prefix by using:

template<class T>
struct B : A<T> {

    using A<T>::i; // explicitly refer to a variable in the base class

    int foo() {
        return i; // i is now known to exist
    }

};

Answer 5

The other uses for this (as I thought when I read the summary and half the question... .), disregarding (bad) naming disambiguation in other answers, are if you want to cast the current object, bind it in a function object or use it with a pointer-to-member.

Casts

void Foo::bar() {
    misc_nonconst_stuff();
    const Foo* const_this = this;
    const_this->bar(); // calls const version

    dynamic_cast<Bar*>(this)->bar(); // calls specific virtual function in case of multi-inheritance
} 

void Foo::bar() const {}

Binding

void Foo::baz() {
     for_each(m_stuff.begin(), m_stuff.end(),  bind(&Foo:framboozle, this, _1));        
     for_each(m_stuff.begin(), m_stuff.end(), [this](StuffUnit& s) { framboozle(s); });         
} 

void Foo::framboozle(StuffUnit& su) {}

std::vector<StuffUnit> m_stuff;

ptr-to-member

void Foo::boz() {
    bez(&Foo::bar);
    bez(&Foo::baz);
} 

void Foo::bez(void (Foo::*func_ptr)()) {
    for (int i=0; i<3; ++i) {
        (this->*func_ptr)();
    }
}

Hope it helps to show other uses of this than just this->member.

Answer 6

There are few cases where using this must be used, and there are others where using the this pointer is one way to solve a problem.

1) Alternatives Available: To resolve ambiguity between local variables and class members, as illustrated by @ASk.

2) No Alternative: To return a pointer or reference to this from a member function. This is frequently done (and should be done) when overloading operator+, operator-, operator=, etc:

class Foo
{
  Foo& operator=(const Foo& rhs)
  {
    return * this;
  }
};

Doing this permits an idiom known as "method chaining", where you perform several operations on an object in one line of code. Such as:

Student st;
st.SetAge (21).SetGender (male).SetClass ("C++ 101");

Some consider this consise, others consider it an abomination. Count me in the latter group.

3) No Alternative: To resolve names in dependant types. This comes up when using templates, as in this example:

#include <iostream>


template <typename Val>
class ValHolder
{
private:
  Val mVal;
public:
  ValHolder (const Val& val)
  :
    mVal (val)
  {
  }
  Val& GetVal() { return mVal; }
};

template <typename Val>
class ValProcessor
:
  public ValHolder <Val>
{
public:
  ValProcessor (const Val& val)
  :
    ValHolder <Val> (val)
  {
  }

  Val ComputeValue()
  {
//    int ret = 2 * GetVal();  // ERROR:  No member 'GetVal'
    int ret = 4 * this->GetVal();  // OK -- this tells compiler to examine dependant type (ValHolder)
    return ret;
  }
};

int main()
{
  ValProcessor <int> proc (42);
  const int val = proc.ComputeValue();
  std::cout << val << "\n";
}

4) Alternatives Available: As a part of coding style, to document which variables are member variables as opposed to local variables. I prefer a different naming scheme where member varibales can never have the same name as locals. Currently I'm using mName for members and name for locals.