Strange bool overloading

Question

Can you explain for me what the typedef here is doing and what the purpose is?

class C
{
    public:
        ...

        typedef bool (C::*implementation_defined_bool_type)(bool) const;

        operator implementation_defined_bool_type() const {
            return _spi ? &C::isPersistent : 0;
        }

};

It's an implementation of the obsolescent ["safe bool" idiom](http://stackoverflow.com/questions/6242768/is-the-safe-bool-idiom-obsolete-in-c11). — CB Bailey, Jun 19 '12 at 10:21

score 5 · Accepted Answer · edited May 23 '17 at 12:21

Can you explain for me what the typedef is doing here?

typedef bool (C::*implementation_defined_bool_type)(bool) const;

typedefs a pointer to a const member function of a type C, which takes a bool as input parameter and also returns a bool.

While,

operator implementation_defined_bool_type() const

Takes in an object of type C and returns a type implementation_defined_bool_type.
It is known as an Conversion Operator.

what is the purpose of it?

It implements the "Safe Bool Idiom", which aims to validate an object in a boolean context.
Note that the Safe Bool Idiom is obsolete with the C++11 Standard.

Good Read:
The Safe Bool Idiom

It doesn't explain what it actually does and why it is implemented that way, it's purpose and all. — Nawaz, Jun 19 '12 at 10:25

Strange bool overloading

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