Question

I can’t seem to open a file without storing it in a variable. I can do this:

ifstream blob("somefile");
string line;
blob >> line;

But when I try this:

string line;
ifstream("somefile") >> line;

The compiler (clang) gives this error:

t.cpp:7:23: error: invalid operands to binary expression ('ifstream' (aka 'basic_ifstream<char>') and 'string' (aka 'basic_string<char>'))
    ifstream("thing") >> i;
    ~~~~~~~~~~~~~~~~~ ^  ~
In file included from t.cpp:1:
In file included from /usr/include/c++/4.6/iostream:39:
In file included from /usr/include/c++/4.6/ostream:39:
In file included from /usr/include/c++/4.6/ios:42:
In file included from /usr/include/c++/4.6/bits/ios_base.h:42:
In file included from /usr/include/c++/4.6/bits/locale_classes.h:41:
In file included from /usr/include/c++/4.6/string:53:
/usr/include/c++/4.6/bits/basic_string.h:2679:5: note: candidate function [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>] not
      viable: no known conversion from 'ifstream' (aka 'basic_ifstream<char>') to 'basic_istream<char> &' for 1st argument
    operator>>(basic_istream<char>& __is, basic_string<char>& __str);
    ^
In file included from t.cpp:1:
In file included from /usr/include/c++/4.6/iostream:40:
/    usr/include/c++/4.6/istream:121:7: note: candidate function not viable: no known conversion from 'string' (aka 'basic_string<char>') to
      '__istream_type &(*)(__istream_type &)' for 1st argument
      operator>>(__istream_type& (*__pf)(__istream_type&))
      ^
/usr/include/c++/4.6/istream:125:7: note: candidate function not viable: no known conversion 

... a few more hundred pages of crap ...

1 error generated.

So, what’s the difference between the two? With other classes, calling it directly works just fine. Is there some template magic involved that makes it ambiguous?

Answer 1

The code that you post is not valid in C++03 (i.e., what was the C++ language standard until September 2011). In C++03, there is only one overload of operator>> that might be considered here [I’ve removed all of the template code because it is not relevant]:

istream& operator>>(istream&, string&);

Note that the std::istream parameter is a non-const reference, so a temporary std::istream object cannot be used. In your code, you are trying to use a temporary object there.

In C++11 (i.e., the current C++ language standard), there is an additional overload which takes the std::istream object by rvalue reference. This allows either a temporary to be passed via that parameter.

Visual C++ already supports this feature of C++11, which is why, as Benjamin Lindley points out in a comment, your code will indeed compile if you use Visual C++ 2010 or later. Your build of Clang may support this if you use the -std=c++0x flag.