Question

I want to know the meaning of the code “\”. I have seen some explanation before, but I forget, who can tell me?

#include "stdafx.h"
#include <fstream>
using namespace std;

ofstream out("order.out");

#define CLASS(ID) class ID {\
public: \
   ID(int) { out<<#ID " constructor\n;} \
   ~ID() { out<< #ID "destructor\n";}\
};

Answer 1

It makes the compiler see the current line and the line after \ as the same line.

For defines this is important, as the preprocessor only takes into account what’s on the current line.

For example:

#define CLASS(ID) class ID {
public: 
   ID(int) { out<<#ID " constructor\n;} 
   ~ID() { out<< #ID "destructor\n";}
};

CLASS(foo)

would expand to

class foo {

whereas

#define CLASS(ID) class ID {\
public: \
   ID(int) { out<<#ID " constructor\n;} \
   ~ID() { out<< #ID "destructor\n";}\
};

CLASS(foo)

would expand to

class foo {
public: 
   foo(int) { out<<"foo" " constructor\n;} 
   ~foo() { out<<"foo" "destructor\n";}
};