Auto-constructor not working with <functional> objects

问题

Consider the following code:

class _c {
    public:
    _c(int);
    _c(function<void(void)>);
};

A class with two constructors defined for an int and a function<void(void)> respectively.

Which means that I can now instantiante objects of those class like this:

_c a = _c(0);
_c b = _c([]() {});

Now, I declare a function that takes a _c object as an argument:

void _f(_c __c) {};

And now I can call this function with my _c objects like this:

_f(_c(0));
_f(_c([]() {}));

Until here, everything looks good. Now, if I try to call my _f function without explicity invoking the _c constructor I see that:

_f(0) // works because C++ knows to construct a _c with an 0 by using _c(int)

but,

_f([]() {}) // fails with 'no matching function for call to _f'

I don't understand why is this happening, could someone please explain why doesn't it work when using a <functional> type?

Also, I'm compiling with: Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)

回答1:

When you call f(0), the argument is of type int which can convert into _c. It is usual one-step conversion.

However, when you call _f([]() {}), the argument is lambda-type (generated by the compiler), not of the type std::function<void(void)>. So in this case, it needs two conversions — one from lambda-type to std::function<void(void)>, then to _c (using the conversion-constructor).

The two-steps conversion is not allowed by the language,which is why your code doesn't work.

The solution is to add a templated constructor as:

template<typename Functor, typename =decltype(std::declval<Functor&>()())>
_c(Functor && func);

Then your code should work.

来源：https://stackoverflow.com/questions/31147081/auto-constructor-not-working-with-functional-objects