create temporaries using a default constructor

Try to compile codes which demonstrate bolew

#include  < iostream >

class  Bar {
public :
    Bar() {}
};

class  Foo {
public :
    Foo( const  Bar &  b)  {}
     void  blah() {
        std::cout  <<   " Foo::blah() called. " ;
    }
};

int  main()
{
    Foo foo(Bar());
    foo.blah();
}

You got an error, aren't you? What's wrong with the codes?

let's changed the codes to:

#include  < iostream >

class  Bar {
public :
    Bar() {}
};

class  Foo {
public :
    Foo( const  Bar &  b)  {}
     void  blah() {
        std::cout  <<   " Foo::blah() called. " ;
    }
};

Bar bar()
{
     return  Bar();
}

int  main()
{
    Foo foo(Bar());   // this is just decclare an existence non-member function which param is another non-member functions and return value is Foo, and the Bar() part is the param of funtion fun and it returns Bar() and without any parameters

    Foo  x  =  foo(bar);
    x.blah();
}

Foo foo(Bar(*pFunc)())
{
    return Foo(pFunc());
}

So the codes demonstrate why create temporaries using a default constructor can't be compiled.

And it will be ok, when you just initialize Foo using:

Foo x  =  Bar();