CODEWITHSUNDEEP
c++11copy-constructor
stewbasic
stewbasic

Reputation: 841

Copy constructor called on *this

I defined a copy constructor for a class A. Due to an unfortunate macro expansion, I ended up compiling the following:

A a = a;

I (eventually) realized this results in a call to A::A(const A& rhs) with this==&rhs.

  1. Why does the compiler allow this? Conceptually I would assume that since a is declared in this statement, it wouldn't yet be available for use on the RHS.

  2. Should I defensively check this==&rhs whenever I define a copy constructor?

I am using gcc version 5.4.0 with -std=c++11.

Upvotes: 0

Views: 32

Answers (1)

M.M
M.M

Reputation: 141598

  1. In a declaration, the identifier being declared is in scope as soon as it appears. There are some valid uses of this, e.g. void *p = &p;

  2. It's normal for the copy-constructor to assume no self-copy, leaving it up to the caller to not make this mistake. Preferably, follow the rule of zero.

It would be better to not write A a = a; in the first place. To avoid this you could get in the habit of using auto, e.g. 

#define F(x) auto a = A(x)

#define G(x) A a = x

Now if you write G(a); you silently get the bug, but F(a); fails to compile.

Upvotes: 1

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