C++ using 'this' keyword in member initialization list

Question

I was writing an Integer struct

struct Integer
{
    int value;
    Integer(int value_) : value(_value){ }
};

This works fine but instead I wanted to use value name for the parameter too and this causes a name clash. That's why I tried to use this keyword like below

struct Integer
{
    int value;
    Integer(int value) : this->value(value){ }
};

But I got an error and I don't seem to understand the problem here. I would be very happy to learn. Thanks for your answers.

It *shouldn't* clash. The names in the initializer list are always members or constructors, while the name inside the parentheses will always be the argument. — Some programmer dude, Oct 22 '20 at 10:06
"The names in the initializer list are always members or " **classes**. Constructors don't have names, classes do. Just noting so you can find the correct name lookup rules. E.g https://stackoverflow.com/questions/25549652/why-is-there-an-injected-class-name — MSalters, Oct 22 '20 at 10:19

score 6 · Accepted Answer · answered Oct 22 '20 at 10:30

This works fine but instead I wanted to use value name for the parameter too and this causes a name clash.

No it doesn't.

Integer(int value) : value(value){ }

This is just fine (if a little confusing to some).

It is simply not permitted in the language grammar to write this-> there, probably because it would make no sense to specify any other object: the this-> is implicitly done for you.

score 5 · Answer 2 · answered Oct 22 '20 at 10:10

The standard way is to write

struct Integer
{
    int value;
    Integer(int value) : value(value){ }
};

In the initializer list : value(value), the first value refers to the member (because that's what a member initializer initializes), while the second value refers to the constructor argument (because that's an expression, and the name in an expression is looked up by the ordinary rules).

Bernd · Answer 3 · 2020-10-22T11:19:13.990

As other answers already mentioned, it is not defined in the C++grammer / syntax. See it here.

And there is no need to specify this. To disambiguate class vs. member you don't use this, you use the scope resoultion operator.

See this (strange) example:


#include <iostream>


class A{
public:
    A(int a){
        std::cout << "A("<< a << ")\n";
    }
    A(){
        std::cout << "A()\n";
    }
};

class B : public A
{
public:
    int A;
     B(int a) : A::A(42), A(a)
    {}
};

int main()
{  
    B b = 12;
    std::cout << b.A;
    return 0;
}

And of course this already exists:

#include <iostream>

class B;

class A{
public:
    A(int a){
        std::cout << "A("<< a << ")\n";
    }
    A(){
        std::cout << "A()\n";
    }
    A(B& b);
};

class B : public A
{
public:
    int A;
     B(int a) : A::A(*this), A(a)
    {}
};

inline A::A(B& b){
    std::cout << "A(B&)\n";
}

int main()
{  
    B b = 12;
    std::cout << b.A;
    return 0;
}

C++ using 'this' keyword in member initialization list

3 Answers3