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# 开始

## 编写简单的C++程序

一个函数的定义包含四部分：返回类型、函数名、一个括号包围的形参列表（允许为空）以及函数体。

## 熟悉编译器

**g++**：

- 编译：`g++  1.1.cpp -o main`
- 运行：`./main`
- 查看运行状态：`echo $?`
- 编译多个文件:`g++ ch2.cpp Sales_item.cc -o main`

输入 `g++ --help`，查看编译器选项：

```
Usage: g++ [options] file...
Options:
  -pass-exit-codes         Exit with highest error code from a phase
  --help                   Display this information
  --target-help            Display target specific command line options
  --help={common|optimizers|params|target|warnings|[^]{joined|separate|undocumented}}[,...]
                           Display specific types of command line options
  (Use '-v --help' to display command line options of sub-processes)
  --version                Display compiler version information
  -dumpspecs               Display all of the built in spec strings
  -dumpversion             Display the version of the compiler
  -dumpmachine             Display the compiler's target processor
  -print-search-dirs       Display the directories in the compiler's search path
  -print-libgcc-file-name  Display the name of the compiler's companion library
  -print-file-name=<lib>   Display the full path to library <lib>
  -print-prog-name=<prog>  Display the full path to compiler component <prog>
  -print-multiarch         Display the target's normalized GNU triplet, used as
                           a component in the library path
  -print-multi-directory   Display the root directory for versions of libgcc
  -print-multi-lib         Display the mapping between command line options and
                           multiple library search directories
  -print-multi-os-directory Display the relative path to OS libraries
  -print-sysroot           Display the target libraries directory
  -print-sysroot-headers-suffix Display the sysroot suffix used to find headers
  -Wa,<options>            Pass comma-separated <options> on to the assembler
  -Wp,<options>            Pass comma-separated <options> on to the preprocessor
  -Wl,<options>            Pass comma-separated <options> on to the linker
  -Xassembler <arg>        Pass <arg> on to the assembler
  -Xpreprocessor <arg>     Pass <arg> on to the preprocessor
  -Xlinker <arg>           Pass <arg> on to the linker
  -save-temps              Do not delete intermediate files
  -save-temps=<arg>        Do not delete intermediate files
  -no-canonical-prefixes   Do not canonicalize paths when building relative
                           prefixes to other gcc components
  -pipe                    Use pipes rather than intermediate files
  -time                    Time the execution of each subprocess
  -specs=<file>            Override built-in specs with the contents of <file>
  -std=<standard>          Assume that the input sources are for <standard>
  --sysroot=<directory>    Use <directory> as the root directory for headers
                           and libraries
  -B <directory>           Add <directory> to the compiler's search paths
  -v                       Display the programs invoked by the compiler
  -###                     Like -v but options quoted and commands not executed
  -E                       Preprocess only; do not compile, assemble or link
  -S                       Compile only; do not assemble or link
  -c                       Compile and assemble, but do not link
  -o <file>                Place the output into <file>
  -pie                     Create a position independent executable
  -shared                  Create a shared library
  -x <language>            Specify the language of the following input files
                           Permissible languages include: c c++ assembler none
                           'none' means revert to the default behavior of
                           guessing the language based on the file's extension

```

输入 `g++ -v --help`可以看到更完整的指令。
例如还有些常用的：
```
-h FILENAME, -soname FILENAME: Set internal name of shared library
-I PROGRAM, --dynamic-linker PROGRAM: Set PROGRAM as the dynamic linker to use
-l LIBNAME, --library LIBNAME: Search for library LIBNAME
-L DIRECTORY, --library-path DIRECTORY: Add DIRECTORY to library search path
```

**获得程序状态**:

- windows: ``echo %ERRORLEVEL%``
- UNIX: ``echo $?``

## IO

- ```#include <iostream>```
- ```std::cout << "hello"```
- ```std::cin >> v1```

记住`>>`和`<<`返回的结果都是左操作数，也就是输入流和输出流本身。

**endl**：这是一个被称为**操纵符**（manipulator）的特殊值，效果是结束当前行，并将设备关联的缓冲区（buffer）中的内容刷到设备中。

UNIX和Mac下键盘输入文件结束符：`ctrl+d`，Windows下：`ctrl+z`

**头文件**：类的类型一般存储在头文件中，标准库的头文件使用`<>`，非标准库的头文件使用`""`。申明写在`.h`文件，定义实现写在`.cpp`文件。

**避免多次包含同一头文件**：

```cpp
#ifndef SALESITEM_H
#define SALESITEM_H
// Definition of Sales_itemclass and related functions goes here
#endif
```

**成员函数（类方法）**：使用`.`调用。

**命名空间（namespace）**：使用作用域运算符`::`调用。

## 注释

- 单行注释： `//`
- 多行注释： `/**/`。编译器将`/*`和`*/`之间的内容都作为注释内容忽略。注意不能嵌套。
```cpp
#define SALESITEM_H
/*
 * 多行注释格式
 * 每一行加一个*
 */
```

## while语句

循环执行，（直到条件（condition）为假。

## for语句

循环头由三部分组成：

- 一个初始化语句（init-statement）
- 一个循环条件（condition）
- 一个表达式（expression）

## 使用文件重定向

``./main <infile >outfile``