CODEWITHSUNDEEP
c++c++11language-lawyerrvalue-reference
Belloc
Belloc

Reputation: 6390

Where in the C++ Standard is it stated that a const rvalue reference doesn't bind to an lvalue?

Where in the C++ Standard is it stated that a const rvalue reference doesn't bind to an lvalue?

For example the code below doesn't compile:

#include <iostream>

int i = 10;

int f(const int&& j) { return j; }
int main()
{
    std::cout << f(i) << '\n';
}

Upvotes: 1

Views: 127

Answers (2)

dyp
dyp

Reputation: 39151

In the last bullet of [dcl.init.ref]/5 (quoting n3485):

If T1 [the type of the initialized reference] is reference-related to T2 [the type of the initializer expression] and the reference is an rvalue reference, the initializer expression shall not be an lvalue.

The cv-qualification is irrelevant in this case.

The first (main) bullet of /5 doesn't apply since the reference is not an lvalue reference:

If the reference is an lvalue reference and the initializer expression [...]

The second (main) bullet point applies:

Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be const), or the reference shall be an rvalue reference

[emphasis mine]

The first sub bullet point of this doesn't apply since the initializer is is not an xvalue or function prvalue and doesn't have class type.

The second sub-bullet-point is an unconditional "otherwise", so that last sub bullet point applies.

If the initializer is not reference-related to the referred type (T1), the example compiles:

#include <iostream>

double i = 10;

int f(const int&& j) { return j; }
int main()
{
    std::cout << f(i) << '\n';
}

Live example

Upvotes: 6

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385405

[C++11: 8.5.3/3] lists how initialisers for references work; what you're looking for is there. It's too exhaustive to quote here verbatim, though.

Upvotes: 3

Related Questions