From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 10 May 2009 03:00:05 -0700 (PDT)

Message-ID:

<323454cd-16cc-43a2-9266-4a202d918d55@e24g2000vbe.googlegroups.com>

Bo Persson wrote:

Jeff Schwab wrote:

Discussing why interface and implementation is so much

better than header and implementation, sounds fun. :-)

SG wrote:

Discussing why interface and implementation is so much

better than header and implementation, sounds fun. :-)

The point is that "headers" are the C++ way of specifying an

interface. One of the worst ways ever invented, but still

better than nothing.

If you write lots of small classes and functions, then

maintaining separate declarations and definitions causes an

absurd amount of source code bloat (nearly 2x). There is also

the maintenance headache of keeping function signatures up to

date.

maintaining separate declarations and definitions causes an

absurd amount of source code bloat (nearly 2x). There is also

the maintenance headache of keeping function signatures up to

date.

There are tools that can help. But the point is that it should

require a significant amount of work to change the signature of

a function. Changing the signature breaks client code, and

shouldn't be done lightly.

For high-level interfaces, there's no problem, because the

interfaces are meant to be stable; arguably, changes to the

overall interface of an application ought to be difficult.

interfaces are meant to be stable; arguably, changes to the

overall interface of an application ought to be difficult.

Exactly.

If the guts of your code include tons of tiny functions,

though, then making changes ought to be easy, not hard.

though, then making changes ought to be easy, not hard.

There's no way you can avoid having to modify client code. If a

function is used in a hundred different places in client code,

you have to modify a hundred different places in your code base

anytime you change it. That's a lot of work at the coding

level, and in anything but the smallest projects, it also

involves significant manageerial work, in order to synchronize

the changes (in different groups).

Obviously, this only applies to classes and functions which are

"exported"---made available to client code. For classes which

are private, the only real argument for not putting the function

bodies in the class is readability, and that really doesn't

apply if the classes and the functions are simple enough. And

of course, for the most private classes, those with block scope,

you don't have a choice anyway.

For

example, I vastly prefer this:

template<class Coordinate>

class point

{

Coordinate m_x;

Coordinate m_y;

public:

typedef Coordinate coordinate;

point(coordinate x, coordinate y )

: m_x( x )

, m_y( y ) { }

coordinate x() const {

return m_x;

}

coordinate y() const {

return m_y;

}

};

To that:

template<class Coordinate>

class point

{

Coordinate m_x;

Coordinate m_y;

public:

typedef Coordinate coordinate;

point(coordinate x, coordinate y );

coordinate x() const;

coordinate y() const;

};

template<class Coordinate>

point<Coordinate>::point(coordinate x, coordinate y)

: m_x( x )

, m_y( y ) { }

template<class Coordinate>

typename point<Coordinate>::coordinate

point<Coordinate>::x() const {

return m_x;

}

template<class Coordinate>

typename point<Coordinate>::coordinate

point<Coordinate>::y() const {

return m_y;

}

example, I vastly prefer this:

template<class Coordinate>

class point

{

Coordinate m_x;

Coordinate m_y;

public:

typedef Coordinate coordinate;

point(coordinate x, coordinate y )

: m_x( x )

, m_y( y ) { }

coordinate x() const {

return m_x;

}

coordinate y() const {

return m_y;

}

};

To that:

template<class Coordinate>

class point

{

Coordinate m_x;

Coordinate m_y;

public:

typedef Coordinate coordinate;

point(coordinate x, coordinate y );

coordinate x() const;

coordinate y() const;

};

template<class Coordinate>

point<Coordinate>::point(coordinate x, coordinate y)

: m_x( x )

, m_y( y ) { }

template<class Coordinate>

typename point<Coordinate>::coordinate

point<Coordinate>::x() const {

return m_x;

}

template<class Coordinate>

typename point<Coordinate>::coordinate

point<Coordinate>::y() const {

return m_y;

}

Templates are a bit tricky, unless you have a compiler which

supports export. What I'd really prefer is the second, but with

the template exported, and the function definitions in another

file, which isn't visible to the client.

This is, of course, an extremely simple case. In practice,

the mess is far worse.

the mess is far worse.

Rather, this is an extremely simple case, where your solution

works (the class is really nothing more than a struct anyway).

But your solution doesn't scale.

In fact, of course, a lot depends on context. In some of the

more basic modules I write, where templates are involved, I'll

use a mixture. At the application level, however, anything that

is exported will not contain the implementation in a header

file.

--

James Kanze (GABI Software) email:james.kanze@gmail.com

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