Why cannot re-assign to a reference to istream / ostream?

Question

It sesms in C++ it's alright to re-assign a reference, bounding it to another object. However, the code below cannot compile:

ostream &os_ref = cerr;
os_ref = cout;

Can anybody tell me what's wrong? Error information is listed below.

temp.cpp: In function ‘int main()’:
temp.cpp:15:12: error: use of deleted function ‘std::basic_ostream<char>& std::basic_ostream<char>::operator=(const std::basic_ostream<char>&)’
     os_ref = cout;
            ^
In file included from /usr/include/c++/4.9/iostream:39:0,
                 from temp.cpp:1:
/usr/include/c++/4.9/ostream:58:11: note: ‘std::basic_ostream<char>& std::basic_ostream<char>::operator=(const std::basic_ostream<char>&)’ is implicitly deleted because the default definition would be ill-formed:
     class basic_ostream : virtual public basic_ios<_CharT, _Traits>
           ^
/usr/include/c++/4.9/ostream:58:11: error: use of deleted function ‘std::basic_ios<char>& std::basic_ios<char>::operator=(const std::basic_ios<char>&)’
In file included from /usr/include/c++/4.9/ios:44:0,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from temp.cpp:1:
/usr/include/c++/4.9/bits/basic_ios.h:66:11: note: ‘std::basic_ios<char>& std::basic_ios<char>::operator=(const std::basic_ios<char>&)’ is implicitly deleted because the default definition would be ill-formed:
     class basic_ios : public ios_base
           ^
In file included from /usr/include/c++/4.9/ios:42:0,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from temp.cpp:1:
/usr/include/c++/4.9/bits/ios_base.h:789:5: error: ‘std::ios_base& std::ios_base::operator=(const std::ios_base&)’ is private
     operator=(const ios_base&);
     ^
In file included from /usr/include/c++/4.9/ios:44:0,
                 from /usr/include/c++/4.9/ostream:38,
                 from /usr/include/c++/4.9/iostream:39,
                 from temp.cpp:1:
/usr/include/c++/4.9/bits/basic_ios.h:66:11: error: within this context
     class basic_ios : public ios_base
           ^

"In C++ it's alright to re-assign a reference, bounding it to another object. " **WRONG**. — juanchopanza, Jan 22 '15 at 14:49
The answer should be in here: http://stackoverflow.com/questions/8070537/how-should-i-correctly-assign-cout-to-a-static-ostream-reference-variable?rq=1 — stefan, Jan 22 '15 at 14:49
No it's in here: http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — Lightness Races in Orbit, Jan 22 '15 at 14:52

score 3 · Answer 1 · answered Jan 22 '15 at 14:50

3

References are bound to an object, you can never reassign it.

Reference remain as an alias to which it was initialized at time of creation.

answered Jan 22 '15 at 14:50

P0W

46,614
9
72
119

juanchopanza · Accepted Answer · 2015-01-22T14:56:58.703

3

In C++ it's alright to re-assign a reference, bounding it to another object.

No, it isn't

Can anybody tell me what's wrong?

In C++, you cannot re-assign a reference to bind it to another object. A reference is an alias to another object. If you "assign" a reference, you are assigning to the referred object, not changing the object the reference refers to.

For example:

flag

int a = 3, b = 4;
int &ref = a;
ref = b;                     // like saying a = b;
std::cout << a << std::endl; // prints 4

edited Jan 22 '15 at 14:56

answered Jan 22 '15 at 14:50

juanchopanza

223,364
34
402
480

Sorry I thought it was so. But why can the following code compile? – Warbean Jan 22 '15 at 14:54
int a = 3, b = 4; int &ref = a; ref = b; – Warbean Jan 22 '15 at 14:54
oops I understand now... – Warbean Jan 22 '15 at 14:57
@Warbean Why do you think that is binding the reference to another object? Did you check what happens? I added your example. – juanchopanza Jan 22 '15 at 14:57
Thanks! That 's a ridiculous misunderstand. – Warbean Jan 22 '15 at 15:02

Why cannot re-assign to a reference to istream / ostream?

2 Answers2