Re: Function pointer problem
On May 9, 6:12 pm, Adrian <n...@bluedreamer.com> wrote:
Example A fails to compile with
exampleA.cpp: In member function 'void A::a() const':
exampleA:10: error: no matching function for call to 'A::b(const
std::string&) const'
exampleA:17: note: candidates are: virtual void (* A::b(const
std::string&))()const <near match>
Example B compiles fine.
Now I assume that the const is not binding to what I think it should
in the example A
Adding the typedef fixes this
Can anyone tell me what is going on
Adrian
+++++++++++++ Example A +++++++++++++++
#include <iostream>
#include <string>
class A
{
public:
void a() const
{
const std::string arg("test");
void(*func_ptr)(void)=b(arg);
if(func_ptr)
{
}
};
virtual ~A() throw();
virtual void (*b(const std::string &str))(void) const=0;
};
int main(int argc, char *argv[])
{
return 0;
}
+++++++++++++ Example B +++++++++++++++
#include <iostream>
#include <string>
class A
{
public:
typedef void(*func_ptr)(void);
void a() const
{
const std::string arg("test");
func_ptr func_ptr=b(arg);
if(func_ptr)
{
}
};
virtual ~A() throw();
virtual func_ptr b(const std::string &str) const=0;
};
int main(int argc, char *argv[])
{
return 0;
}
The reason the compiler freaks out in the first example is because of
the const. The function pointer is called in a constant member, and
when we make a call to a function from inside a constant member, to
another member function, the this pointer will get put on the stack as
expected, but with a constant member a constant this member gets put
(because its a constant function we are not allowed to modify our
classes data).
The const is in the wrong place and the signature of the virtual
function and the calling function do not match. I think as others
have said if you move the const to inside the brackets before the
(void), it will compile. It did for me anyway on gcc 3.4.
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