This question already has an answer here:
Consider the following code.
#include <functional>
int main(void)
{
std::function<void()> f1;
if (f1) { /* ok */
...
}
bool b = f1; /* compile-error */
bool B = !f1; /* ok */
...
}
std::function<>
converts implicitly to bool in some circumstances but not in all of them. Assigning it to a bool
-variable does not work, whereas the result of an operation or using it in an if()
-statement is OK.
Why is that so? It seems we have to do an boolean-operation on it, then the conversion works.
What I did to make work the b = f1
-line is the good ol' double bang: !!
. It looks like an antique in such modern C++-code.
EDIT: This compiles as well:
bool b = f1 || f1; /* OK */
Note that std::function::operator bool is
explicit
conversion function, implicit conversion is not allowed. Sobool b = f1;
won't work. (Explicit conversion will work well if you use static_cast likebool b = static_cast<bool>(f1);
.)When being used with
if
,operator!
oroperator||
, contextual conversions will take effect, and the explicit conversion function will be considered.(since C++11)