Passing non-const references to rvalues in C++

Question

In the following line of code:

bootrec_reset(File(path, size, off), blksize);

Calling a function with prototype:

static void bootrec_reset(File &file, ssize_t blksize);

I receive this error:

libcpfs/mkfs.cc:99:53: error: invalid initialization of non-const reference of type 'File&' from an rvalue of type 'File'

libcpfs/mkfs.cc:30:13: error: in passing argument 1 of 'void bootrec_reset(File&, ssize_t)'

I'm aware that you can not pass non-const references (const &) to rvalues according to the standard. MSVC however allows you to do this (see this question). This question attempts to explain why but the answer makes no sense as he is using references to literals, which are a corner case and should obviously be disallowed.

In the given example it's clear to see that following order of events will occur (as it does in MSVC):

1. File's constructor will be called.
2. A reference to the File, and blksize, are pushed on the stack.
3. bootrec_reset makes use of file.
4. After returning from bootrec_reset, the temporary File is destroyed.

It's necessary to point out that the File reference needs to be non-const, as it's a temporary handle to a file, on which non-const methods are invoked. Furthermore I don't want to pass the File's constructor arguments to bootrec_reset to be constructed there, nor do I see any reason to manually construct and destroy a File object in the caller.

So my questions are:

1. What justifies the C++ standard disallowing non-const references in this manner?
2. How can I force GCC to permit this code?
3. Does the upcoming C++0x standard change this in anyway, or is there something the new standard gives me that is more appropriate here, for example all that jibberish about rvalue references?

Answer 1

Yes, the fact that plain functions cannot bind non-const references to temporaries -- but methods can -- has always bugged me. TTBOMK the rationale goes something like this (sourced from this comp.lang.c++.moderated thread):

Suppose you have:

 void inc( long &x ) { ++x; }

 void test() {
     int y = 0;
     inc( y );
     std::cout << y;
 } 

If you allowed the long &x parameter of inc() to bind to a temporary long copy made from y, this code obviously wouldn't do what you expect -- the compiler would just silently produce code that leaves y unchanged. Apparently this was a common source of bugs in the early C++ days.

Had I designed C++, my preference would have been to allow non-const references to bind to temporaries, but to forbid automatic conversions from lvalues to temporaries when binding to references. But who knows, that might well have opened up a different can of worms...

Answer 2

• "What justifies the C++ standard disallowing non-const references in this manner?"

Practical experience with the opposite convention, which was how things worked originally. C++ is to a large degree an evolved language, not a designed one. Largely, the rules that are still there are those that turned out to work (although some BIG exceptions to that occurred with the 1998 standardization, e.g. the infamous export, where the committee invented rather than standardizing existing practice).

For the binding rule one had not only the experience in C++, but also similar experience with other languages such as Fortran.

As @j_random_hacker notes in his answer (which as I wrote this was scored 0, showing that the scoring in SO really doesn't work as a measure of quality), the most serious problems have to do with implicit conversions and overload resolution.

• "How can I force GCC to permit this code?"

You can't.

Instead of ...

bootrec_reset(File(path, size, off), blksize);

... write ...

File f(path, size, off);
bootrec_reset(f, blksize);

Or define an appropriate overload of bootrec_reset. Or, if "clever" code appeals, you can in principle write bootrec_reset(tempref(File(path, size, off)), blksize);, where you simply define tempref to return its argument reference appropriately const-casted. But even though that's a technical solution, don't.

• "Does the upcoming C++0x standard change this in anyway, or is there something the new standard gives me that is more appropriate here, for example all that jibberish about rvalue references?"

Nope, nothing that changes things for the given code.

If you're willing to rewrite, however, then you can use e.g. C++0x rvalue references, or the C++98 workarounds shown above.

Cheers & hth.,

Answer 3

Does the upcoming C++0x standard change this in anyway, or is there something the new standard gives me that is more appropriate here, for example all that jibberish about rvalue references?

Yes. Since every name is an lvalue, it is almost trivial to treat any expression as if it was an lvalue:

template <typename T>
T& as_lvalue(T&& x)
{
    return x;
}

// ...

bootrec_reset(as_lvalue(File(path, size, off)), blksize);

Answer 4

1. Is a fairly arbitrary decision - non-const references to temporaries are allowed when the temporary is the subject of a method call, for example (e.g. the "swap trick" to free the memory allocated by a vector, std::vector<type>().swap(some_vector);)
2. Short of giving the temporary a name, I don't think you can.
3. As far as I'm aware this rule exists in C++0x too (for regular references), but rvalue references specifically exist so you can bind references to temporaries - so changing bootrec_reset to take a File && should make the code legal.

Answer 5

Alternatively, simply overload.

static void bootrec_reset(File &&file, ssize_t blksize) {
    return bootrec_reset(file, blksize);
}

This is the easiest solution.

Answer 6

Please note that calling C++0x "jibberish" is not presenting a very favorable picture of your coding ability or desire to understand the language.

1) Is actually not so arbitrary. Allowing non-const references to bind to r-values leads to extremely confusing code. I recently filed a bug against MSVC which relates to this, where the non-standard behavior caused standard-compliant code to fail to compile and/or compile with a deviant behavior.

In your case, consider:

#include <iostream>

template<typename T>
void func(T& t)
{
    int& r = t;
    ++r;
}

int main(void)
{
    int i = 4;
    long n = 5;
    const int& r = n;

    const int ci = 6;
    const long cn = 7;

    //int& r1 = ci;
    //int& r2 = cn;

    func(i);
    //func(n);

    std::cout << r << std::endl;
}

Which of the commented lines to you want to compile? Do you want func(i) to change its argument and func(n) to NOT do so?

2) You can't make that code compile. You don't want to have that code. A future version of MSVC is probably going to remove the non-standard extension and fail to compile that code. Instead, use a local variable. You can always use a extra pair of braces to control the lifetime of that local variable and cause it to be destroyed prior to the next line of code, just like the temporary would be. Or r-value references.

{
  File ftemp(path, size, off);
  bootrec_reset(ftemp, blksize);
}

3) Yes, you can use C++0x r-value references in this scenario.

Answer 7

How can I force GCC to permit this code?

If you own the definition of File then you can try playing tricks such as this one:

class File /* ... */ {
public:
  File* operator&() { return this; }
/* ... */
};
/* ... */
bootrec_reset(*&File(path, size, off), blksize);

This compiles for me in c++98 mode.

Does the upcoming C++0x standard change this in anyway, or is there something the new standard gives me that is more appropriate here, for example all that jibberish about rvalue references?

Obviously this the way to go if at all possible.