what virtual functions?!

[alex goldman]

When compiling this code with
g++ -Wall -pedantic -ansi -c program.cpp # this is GCC-3.3.4

######################################
#include <vector>

class b {
public:
virtual void f() = 0;
};

class d : public b {
std::vector<int> v;
public:
void f() {}
};
#####################################

I get a warning:

program.cpp:8: warning: `class d' has virtual functions but non-virtual
destructor

Am I very confused about the presence of virtual functions in class d, or is
the compiler?

 

[Cy Edmunds]

When compiling this code with
g++ -Wall -pedantic -ansi -c program.cpp # this is GCC-3.3.4

######################################
#include <vector>

class b {
public:
virtual void f() = 0;
};

class d : public b {
std::vector<int> v;
public:
void f() {}

This line means the same thing as

virtual void f() {}

Once a function is virtual it stays virtual. Hence the keyword is optional.

 

[Peter Julian]

When compiling this code with
g++ -Wall -pedantic -ansi -c program.cpp # this is GCC-3.3.4

######################################
#include <vector>

class b {
public:
virtual void f() = 0;
};

class d : public b {
std::vector<int> v;
public:
void f() {}
};
#####################################

I get a warning:

program.cpp:8: warning: `class d' has virtual functions but non-virtual
destructor

The compiler is clearly telling you that the presence of a pure-virtual
member function in class b does *not* automatically generate a virtual
d~tor. And indeed, it doesn't - thats bad news.

in fact, you should have seen the following warnings with the
-ansi -pedantic -Wall options:

`class b' has virtual functions but non-virtual destructor
`class d' has virtual functions but non-virtual destructor

Am I very confused about the presence of virtual functions in class d, or is
the compiler?

This has nothing to do with the virtual function as per say. The basic rule
in inheritence is that a virtual d~tor should replace the default
non-virtual d~tor in a base class you plan to derive from.

So class B should look like...

class B
{
public:
B() { }
virtual ~B() { }
virtual void f() = 0;
};

class D : public B
{
std::vector<int> vn;
public:
D() : vn() { }
~D() { } // is also virtual since base d~tor is virtual
void f() {} // is also virtual...
};

int main()
{
D d;

system("PAUSE");
return EXIT_SUCCESS;
}

No warning anymore...

 

[alex goldman]

class D : public B
{
std::vector<int> vn;
public:
D() : vn() { }

Isn't the above line redundant?

 

[Rolf Magnus]

Isn't the above line redundant?

It is superfluous.

 

[Peter Julian]

Isn't the above line redundant?

You're right: ts not required, but quite handy. vn(40, 0) would have
initialize 40 int elements to 0.