从Visual Studio 2010迁移到2012时出现C ++ 11问题

Question

I'm trying to port my project from Visual Studio 2010 to Visual Studio 2012. In my code I have some file handling that looks like this:

auto fileDeleter = [](FILE* f) { fclose(f); };

unique_ptr<FILE, decltype(fileDeleter)> fMinute(
    fopen(minuteLogName.c_str(), "w"), fileDeleter);


unique_ptr<FILE, decltype(fileDeleter)> fIndividual(
    fopen(individualLogName.c_str(), "w"), fileDeleter);

if (!fMinute || !fIndividual) {
    throw Exceptions::IOException("One of the log files failed to open",
                                  __FUNCTION__);
}

This built with no issues in 2010, but in 2012 it fails on the conditional with:

error C2678: binary '!' : no operator found which takes a left-hand operand of type > 'std::unique_ptr<_Ty,_Dx>' (or there is no acceptable conversion)
...
could be 'built-in C++ operator!(bool)'

The C++11 standard specifies that unique_ptr has a bool operator to allow you to do quick checks like I have above. Stranger yet, VS2012's unique_ptr definition has this very operator:

_OPERATOR_BOOL() const _NOEXCEPT
{   // test for non-null pointer
    return (this->_Myptr != pointer() ? _CONVERTIBLE_TO_TRUE : 0);
}

But I get that error upon compilation. Why?

Yes, I could just be using ofstream instead, but that's besides the point.

Answer 1

To build on what BigBoss said, C++11 requires that std::unique_ptr use explicit operator bool() noexcept , which solves the whole implicit conversion to bool problem. Except... VC2012 doesn't support explicit operators yet. Therefore, they have to use the safe-bool idiom .

While the safe-bool idiom is good, it can have flaws (which is why explicit operator bool() exists), depending on how you implement the idiom. And you apparently ran across one of them in VC2012. Reconfiguring your test, with !(fMinute && fIndividual) , should solve it.

But either way, it's a Visual Studio bug. Since the behavior changed, you should file a bug report even if you do manage to find a way around it.

Answer 2

You can use if (!(fMinute && fIndividual) ) instead of if (!fMinute || !fIndividual) . C++ say that they are convertible to bool, but operator bool usually create problems, for example you may have a function that accept int , if your class have an operator bool then it is convertible to int and you can pass it to this function, but for example in our case unique_ptr never intended to be used as an int , so many developers never use operator bool directly but instead write an operator that can be used as a bool in conditional expressions but it is not actually bool!

struct bool_convertible {
    typedef void (*convertible_to_bool)();
    static void convertible_to_true();
    operator convertible_to_bool () const {
        return test() ? &convertible_to_true : nullptr;
    }
};

Using this technique I may have bool_convertible c; if( c ) {...} bool_convertible c; if( c ) {...} but I can't have

void test_int( int );
bool_convertible c;
test_int( c );

Answer 3

In the recent C++11 standard, std::unique_ptr has no operator! defined, only the explicit conversion operator

explicit operator bool() const;

However, the built-in unary logical negation operator, ! , contextually converts its argument to bool according to 5.3.1/9:

The operand of the logical negation operator ! is contextually converted to bool (Clause 4); its value is true if the converted operand is false and false otherwise

Contextual conversion to bool will use an explicit conversion operator, if available. So, your code is actually expected to work under C++11 rules. You should probably file a bug report to Microsoft. Whether they support explicit conversion operators or not should not matter.

As a workaround try this:

if (!fMinute.get() || !fIndividual.get()) {
    ....