Check if returned std::function is "valid" in C++11

Question

I want to implement a dynamic task queue like so:

typedef std::function<void(void)> Job;
typedef std::function<Job(void)> JobGenerator;

// ..

JobGenerator gen = ...;
auto job = gen(); 
while (IsValidFunction(job))
{
    job();
}

How can i implement IsValidFunction? Is there a sort of default value for std::function to check against?

I'm not clear what makes it valid. What do you mean by exists? — doctorlove, Nov 25 '13 at 12:42
http://en.cppreference.com/w/cpp/utility/functional/function/operator_bool — jrok, Nov 25 '13 at 12:42

score 10 · Accepted Answer · answered Nov 25 '13 at 12:42

10

You can simply check job as a bool:

while (auto job = gen())
{
    job();
}

That's a sort of shorthand which assigns job from gen() each time through the loop, stopping when job evaluates as false, relying on std::function<>::operator bool: http://en.cppreference.com/w/cpp/utility/functional/function/operator_bool

answered Nov 25 '13 at 12:42

John Zwinck

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Good stuff. I already knew about [conversion operators](http://stackoverflow.com/questions/1383606/conversion-operators-in-c), but I forgot about them entirely! I think, [this link](http://stackoverflow.com/questions/4600295/what-is-the-meaning-of-operator-bool-const-in-c) explains the concept better. – Domi Nov 25 '13 at 14:15
Careful @Domi, there are bugs on the `bool` conversion of `std::function` in the wild: http://stackoverflow.com/questions/19578237/strange-behavior-with-stdfunction – Yakk - Adam Nevraumont Nov 25 '13 at 14:50

score 1 · Answer 2 · answered Nov 25 '13 at 12:43

1

You could just check if the function has a valid target, by using its conversion to bool. Non-valid functions would then be empty ones which don't have a target, e.g. default-constructed ones or nullptr.

answered Nov 25 '13 at 12:43

Christian Rau

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Check if returned std::function is "valid" in C++11

2 Answers2