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  Acquaintance C ++: the most simple C ++ program
  Add Date : 2017-08-31      
  Let's write a simple C ++ program

// Helloworld.cpp
#include < iostream>

int main () {
    std :: cout < < "! Hello, Wrold" < < std :: endl;
    return 0;

This code reflects the four questions:
1. Comment
2. preprocessor directive
3.main () function
4. Input / output streams

The following will mainly talk about preprocessor directive: to generate a C ++ program has three steps. First, the code preprocessor runs preprocessor identification code meta information; then, the code is compiled into a computer can recognize object files; Finally, each object files are linked together to become an application . Preprocessing directives begin with the # character, such as #include < iostream>, meaning that tell the preprocessor: extract < iostream> header file for all the contents of the current file to use. < Iostream> header file declarations provided by the C ++ input / output functions, only this program includes header files, in order to complete the program you want to output Hello, World! text.

Note: The students know the familiar C header files .h is ending, such as < stdio.h>. In C ++, the standard header file .h suffix is omitted, such as < iostream>, but still contains the standard C header files, just change the name, such as the original < stdio.h> becomes < cstdio>.

There preprocessing directive # define, # ifdef, # ifndef, # endif like. E.g:

#ifndef MYHEADER_H
#define MYHEADER_H
// ...

This is to avoid duplicate sample include the header file.

To be mentioned here is that if the compiler supports the #pragma once directive, you can use this command instead of the above command:

#pragma once
// ...

Then briefly about the std :: syntax: std :: endl ending represents the output stream will wrap, as \ n characters. :: Is called the scope resolution operator, std refers to a namespace, use them together in order to resolve name conflicts between different code segments. For example, I write a piece of code, which defines a fun () function, but one day, I use third-party libraries, which happens to also defines a fun () function, then the compiler can not determine the code calls fun () function in the end belongs to which version. So I need to set your code into a namespace, we first define a namespace:

namespace mycode {
    void fun ();

When I use the fun () function when you need to specify its namespace, there are two ways:

using namespace mycode;

int main () {
    fun (); // call is my own definition of fun () function
    return 0;


int main () {
    mycode :: fun (); // call is fun my custom () function
    return 0;

Now I understand, cout output stream belongs to namespace std, so write std :: cout. Incidentally, also belong to the input stream cin std namespace as for use, this is not to mention.
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