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Here:
http://en.cppreference.com/w/cpp/utility/functional/function
operator bool is described: "Checks whether the stored callable object is valid".
Presumably a default constructed std::function is not valid but is this the only case?
Also, how does it check whether it is valid?
Is the case where operator() raises std::bad_function_call exactly the case where the object is not valid?
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You can check if a std::function is empty with std::function::operator bool . true: if the object is callable. foo is not empty. bar is empty.
Instances of std::function can store, copy, and invoke any Callable target -- functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to data members.
std::function is a type erasure object. That means it erases the details of how some operations happen, and provides a uniform run time interface to them. For std::function , the primary1 operations are copy/move, destruction, and 'invocation' with operator() -- the 'function like call operator'.
std::function can hold more than function pointers, namely functors. Live example on Ideone. As the example shows, you also don't need the exact same signature, as long as they are compatible (i.e., the parameter type of std::function can be passed to the contained function / functor).
It's poorly written as is, your confusion is justified. By "valid" they mean "has a target".
A std::function "has a target" when it's been assigned a function:
std::function<void()> x; // no target
std::function<void()> y = some_void_function; // has target
x = some_other_void_function; // has target
y = nullptr; // no target
x = y; // no target
They should have either defined "valid" before they used it, or simply stuck with the official wording.
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The language standard says
explicit operator bool() const noexcept;Returns: true if *this has a target, otherwise false.
Meaning that the function has anything to call. The default constructed function obviously does not.
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