Forward declaring std::string in C++

One type you can't forward declare in C++ is std::string

class string; // Computer says no

This does not say a lot but what it does say is that string is a class and unfortunately it's not - it's a typedef of basic_string<...>

If I compile a file containing just the line

#include <string>

then g++'s -H flag tells me that pulls in over 100 other header files! Avoiding those #includes might make an appreciable difference to build times in some C++ codebases. You can fake an std::string forward declaration by using a type wrapper. For example:

#ifndef EG_HPP
#define EG_HPP

class fwd_string; // instead of #include <string>

void eg(const fwd_string &); 
// instead of
// void eg(const std::string &);

#endif

Where fwd_string.hpp looks like this:

#ifndef FWD_STRING_HPP
#define FWD_STRING_HPP

#include <string>

class fwd_string
{
public:
    fwd_string();
    fwd_string(const char *);
    fwd_string(const std::string &);
    std::string string;
    ...
};

#endif

You can also use this "type-tunneling" technique to forward declare enums in C. I've never seen this used in anger in an actual codebase. It's just an idea. Caveat emptor.

Accu conference charity book stall

Once again I'm going to give away a load of books at this years ACCU conference. As before I'll choose a charity and ask everyone who takes a book to make a voluntary donation. I've already collected two boxfulls. If you're coming to the conference please consider bringing along some books and contributing them to the stall.