用右值初始化左值引用

Question

I built this code with gcc/clang and got different results:

#include <iostream>
#include <sstream>

int main() {
    std::istream& is = 1 ? std::move(std::stringstream("")) : std::cin;
}

• Why gcc allows initializing lvalue reference with rvalue ( std::stringstream("") )?
• Why clang tries to call copy constructor?

gcc 4.9.1

No error

clang 3.4

prog.cc:5:63: error: call to implicitly-deleted copy constructor of 'istream' (aka 'basic_istream<char>')
    std::istream& is = 1 ? std::move(std::stringstream("")) : std::cin;
                                                             ^~~~~~~~
/usr/local/libcxx-3.4/include/c++/v1/istream:185:5: note: copy constructor is implicitly deleted because 'basic_istream<char, std::__1::char_traits<char> >' has a user-declared move constructor
   basic_istream(basic_istream&& __rhs);
   ^
prog.cc:5:28: error: calling a protected constructor of class 'std::__1::basic_istream<char, std::__1::char_traits<char> >'
   std::istream& is = 1 ? std::move(std::stringstream("")) : std::cin;
                          ^
/usr/local/libcxx-3.4/include/c++/v1/istream:185:5: note: declared protected here
   basic_istream(basic_istream&& __rhs);
   ^
prog.cc:5:28: error: calling a protected constructor of class 'std::__1::basic_istream<char, std::__1::char_traits<char> >'
   std::istream& is = 1 ? std::move(std::stringstream("")) : std::cin;
                          ^
/usr/local/libcxx-3.4/include/c++/v1/istream:185:5: note: declared protected here
   basic_istream(basic_istream&& __rhs);
   ^

Answer 1

GCC's behavior is a bug, and it's been fixed on trunk . Clang is correct. This is a messy case because you have mixed value categories for the second and third operands of the conditional operator:

• std::move(std::stringstream("")) is an xvalue ^* of type std::stringstream ;
• std::cin is an lvalue of type std::istream .

The relevant standard quote (§5.16 [expr.cond]/p3-6) can be found in this answer . It's long enough that I don't really want to copy it over. I'll just outline how it is applied to this code:

• Obviously std::istream cannot be converted to match std::stringstream in any way regardless of value category;
• An xvalue of type std::stringstream cannot be converted to type "lvalue reference to std::istream " given the constraint that the reference must bind directly to an lvalue - there's no lvalue here for the reference to bind to;
• std::istream is a base class of std::stringstream , so per the 3rd bullet of p3, the xvalue of type std::stringstream can and will be converted to a prvalue temporary of type std::istream by copy-initialization, which replaces the original operand for further analysis.
• Now the second operand is a prvalue of type std::istream , the third operand is an lvalue of type std::istream , they have different value categories so p4 doesn't apply.
• Hence the result is a prvalue per p5. Since they have the same type, overload resolution specified in p5 is not performed, and you proceed to p6.

• The applicable bullet in p6 is

  The second and third operands have the same type; the result is of that type. If the operands have class type, the result is a prvalue temporary of the result type, which is copy-initialized from either the second operand or the third operand depending on the value of the first operand.

  so it copy-initializes the result (which is a prvalue temporary) from either the converted first operand, or the second operand ( std::cin ).

Hence the errors:

• Copy-initializing the prvalue std::istream result from an lvalue ( std::cin ) would use the copy constructor, and streams cannot be copied.
• Copy-initializing the prvalue temporary std::istream for the second operand from a std::stringstream xvalue is a move, but std::istream 's move constructor is protected.

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_{^* For terminology (lvalue, xvalue, prvalue, etc.), see What are rvalues, lvalues, xvalues, glvalues, and prvalues?}