invalid initialization of non-const reference of type ‘int&' from a temporary of type 'MyClass<int>::iterator*'

Question

I'm getting the following error from g++ while trying to add iterator support for my linked list class.

 LinkedList.hpp: In member function ‘Type& exscape::LinkedList<Type>::iterator::operator*() [with Type = int]’: tests.cpp:51: instantiated from here LinkedList.hpp:412: error: invalid initialization of non-const reference of type ‘int&’ from a temporary of type ‘exscape::LinkedList<int>::iterator*’

Likely relevant code snippets:

LinkedList.hpp:

template <typename Type> class LinkedList { private: struct node { struct node *prev; struct node *next; Type data; }; public: class iterator : public std::iterator<...> { node *p; public: Type &operator*(); }; ... }; template <typename Type> LinkedList<Type>::iterator::iterator(struct node *in_node) : p(in_node) {} template <typename Type> inline Type &LinkedList<Type>::iterator::operator*() { return this-p->data; ///// Line 412 }

tests.cpp:

... LinkedList<int> l1; ... LinkedList<int>::iterator it; for (it = l1.begin(); it != l1.end(); ++it) { std::cout << "Element: " << *it << std::endl; ///// Line 51 }

I've googled (and searched SO, of course), and checked my code to no avail - either I'm missing something basic (aka. doing something stupid), or I'm missing the knowledge required. Any advice?

Looks like you might have a typo on line 412

Kyle Walsh
– Kyle Walsh

2009-11-20 19:55:43 +00:00
Commented Nov 20, 2009 at 19:55 — Kyle Walsh
– Kyle Walsh, Commented Nov 20, 2009 at 19:55

KeatsPeeks · Accepted Answer · 2009-11-20 19:59:21Z

you're returning a reference on a temporary object : this - p->data (I emphasized the typo) computes a pointer interval, and the result of the operation is a temporary rvalue: you can't take a reference from it.

Just remove the typo:

this->p->data;

Ugh! That was it, thanks. I guess typos fall into the former category of errors... ;)

Johannes Schaub - litb · Accepted Answer · 2009-11-20 19:58:10Z

2

The issue can be demonstrated by the following snippet

struct A { int a; A *get() { return this - a; } }; int main() { A a = { 0 }; assert(&a == a.get()); }

Replace line 412 by the following

return this->p->data; // "this->" is optional

answered Nov 20, 2009 at 19:58

Johannes Schaub - litb

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2 Comments

Marius Over a year ago

Actually, this-> is not always optional. When dealing with template classes that inherit from other template classes one may need it: comeaucomputing.com/techtalk/templates/#whythisarrow

Johannes Schaub - litb Over a year ago

@Marius, yes but here it is optional. I don't dare to try to enclose all my statements with all the exceptions found in C++ that apply to them. That wouldn't help people and it steals my time :)

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