当我们用C++开发一些C++控制台小工具时，会需要一些用户输入的参数来决定程序如何工作和执行，而用户输入参数的方式大部分都是采用命令行参数的方式。
比如上一篇文章 玩转Windows服务系列——命令行管理Windows服务 中介绍的sc和net工具。
既然命令行参数这么普遍也这么有用，那么就有必要学习一下如何解析命令行参数。
如何解析命令行参数
那么命令行参数要如何解析呢。
最最最笨的办法就是每次程序中需要解析命令行参数时，就写一堆解析的代码，但是这样的效率其实很低的，不如使用开源库中的帮助类来解析。
我所了解的开源库的帮助类有
- ACE库的ACE_Get_Opt
- boost的Program_options
ACE_Get_Opt类的主要使用方法介绍
那么我们主要看一下ACE_Get_Opt类
先看它的构造方法
ACE_Get_Opt (int argc,
ACE_TCHAR **argv,
const ACE_TCHAR *optstring = ACE_TEXT (""),
int skip_args = 1,
int report_errors = 0,
int ordering = PERMUTE_ARGS,
int long_only = 0);
下面是代码中的注释
/**
* Constructor initializes the command line to be parsed. All information
* for parsing must be supplied to this constructor.
*
* @param argc The number of @a argv elements to parse.
* @param argv Command line tokens, such as would be passed
* to @c main().
* @param optstring Nul-terminated string containing the legitimate
* short option characters. A single colon ":"
* following an option character means the option
* requires an argument. A double colon "::" following
* an option character means the argument is optional.
* The argument is taken from the rest of the current
* @a argv element, or from the following @a argv
* element (only valid for required arguments;
* optional arguments must always reside in the same
* @a argv element). The argument value, if any is
* returned by the @c opt_arg() method.
* @a optstring can be extended by adding long options
* with corresponding short options via the
* @c long_option() method. If the short option
* already appears in @a optstring, the argument
* characteristics must match, otherwise it is added.
* See @c long_option() for more information.
* If 'W', followed by a semi-colon ';' appears in
* @a optstring, then any time a 'W' appears on the
* command line, the following argument is treated as
* a long option. For example, if the command line
* contains "program -W foo", "foo" is treated as a
* long option, that is, as if "program --foo" had
* been passed.
* The following characters can appear in @a optstring
* before any option characters, with the described
* effect:
* - '+' changes the @a ordering to @a REQUIRE_ORDER.
* - '-' changes the @a ordering to @a RETURN_IN_ORDER.
* - ':' changes the return value from @c operator()
* and get_opt() from '?' to ':' when an option
* requires an argument but none is specified.
*
* @param skip_args Optional (default 1). The specified number of
* initial elements in @a argv are skipped before
* parsing begins. Thus, the default prevents
* @a argv[0] (usually the command name) from being
* parsed. @a argc includes all @a argv elements,
* including any skipped elements.
* @param report_errors Optional, if non-zero then parsing errors cause
* an error message to be displayed from the
* @c operator() method before it returns. The
* error message is suppressed if this argument is 0.
* This setting also controls whether or not an error
* message is displayed in @c long_option() encounters
* an error.
* @param ordering Optional (default is @c PERMUTE_ARGS); determines
* how the @a argv elements are processed. This argument
* is overridden by two factors:
* -# The @c POSIXLY_CORRECT environment variable. If
* this environment variable is set, the ordering
* is changed to @c REQUIRE_ORDER.
* -# Leading characters in @a optstring (see above).
* Any leading ordering characters override both
* the @a ordering argument and any effect of the
* @c POSIXLY_CORRECT environment variable.
* @param long_only Optional. If non-zero, then long options can be
* specified using a single '-' on the command line.
* If the token is not a long option, it is processed
* as usual, that is, as a short option or set of
* short options.
*
* Multiple short options can be combined as long as only the last
* one can takes an argument. For example, if @a optstring is defined as
* @c "abc:" or @c "abc::" then the command line @e "program -abcxxx" short
* options @e a, @e b, and @e c are found with @e "xxx" as the argument for
* @e c.
* However, if the command line is specified as @e "program -acb" only
* options @e a and @e c are found with @e "b" as the argument for @e c.
* Also, for options with optional arguments, that is, those followed by
* "::", the argument must be in the same @a argv element, so "program -abc
* xxx" will only find "xxx" as the argument for @e c if @a optstring is
* specified as @c "abc:" not @c "abc::".
*/
构造函数共有七个参数，
argc、argv为程序入口函数main方法的参数，也就是程序的命令行。
optstring 为指定的参数选项，并且是一个字符作为一个选项，主要包含三种形式
- 单纯的一个字符选项，比如 s，表示此选项后面不能添加选项的参数
- 一个字符选项后跟一个冒号，比如 s:，表示此选项后面会有一个参数
- 一个字符后面跟两个冒号，比如 s::，表示此选项后面即可以有参数也可以无参数
skip_args 表示从argv的第几个元素开始，默认为1，一般情况下argv[0]为程序的path
report_errors遇到不识别的参数时是否提示错误
long_only表示是否只包含字符串的选项参数。
下面解释一下 字符选项和字符串选项，也就是 short option 和 long option.
- short option 以 字符’-’开始，比如 -s
- long option 以两个字符’-’开始，日入 –server
这是在默认的情况下，也就是long_only = 0的情况下。而当long_only不等于0的时候，就可以通过-server来表示long option了。
那么来看一个定义的例子
ACE_Get_Opt opt(argc, argv, "s:u:p:", 1, 0);
三个重载方法long_option的介绍
由于构造方法中的optstring只能制定short option，所以必须通过long_option方法来指定long option。
ACE_Get_Opt类中有三个重载的long_option方法，下面来一一介绍
long_option方法一：
/// Adds a long option with no corresponding short option.
/**
* If the @a name option is seen, @c operator() returns 0.
*
* @param name The long option to add.
* @param has_arg Defines the argument requirements for
* the new option.
*
* @retval 0 Success
* @retval -1 The long option can not be added.
*/
int long_option (const ACE_TCHAR *name,
OPTION_ARG_MODE has_arg = NO_ARG);
这个方法可以增加long_option，第一个参数name表示要添加的option的名字，第二个参数表示这个option是否包含参数，下面是OPTION_ARG_MODE的定义：
/// Mutually exclusive option argument mode used by long options.
enum OPTION_ARG_MODE
{
/// Doesn't take an argument.
NO_ARG = 0, /// Requires an argument, same as passing ":" after a short option
/// character in @a optstring.
ARG_REQUIRED = 1, /// Argument is optional, same as passing "::" after a short
/// option character in @a optstring.
ARG_OPTIONAL = 2
};
- NO_ARG表示没有参数，对应short option中的没有冒号的情况
- ARG_REQUIRED表示必须有参数，对应short option中有一个冒号的情况
- ARG_OPTIONAL表示参数可有可无，对应short option中有两个冒号的情况
此方法可以这样使用
if(opt.long_option("server", ACE_Get_Opt::ARG_REQUIRED) == -1)
{
cout << "server option can not be added" << endl;
}
long_option方法二：
/// Adds a long option with a corresponding short option.
/**
* @param name The long option to add.
* @param short_option A character, the short option that corresponds
* to @a name.
* @param has_arg Defines the argument requirements for
* the new option. If the short option has already
* been supplied in the @a optstring, @a has_arg
* must match or an error is returned; otherwise, the
* new short option is added to the @a optstring.
*
* @retval 0 Success
* @retval -1 The long option can not be added.
*/
int long_option (const ACE_TCHAR *name,
int short_option,
OPTION_ARG_MODE has_arg = NO_ARG);
这个方法中增加了一个short_option参数，表示这个long_option与对应的short_option所表达的意义是一致的，当解析命令行时，遇到这个long_option时，就按此处的short option进行处理。
此方法可以这样使用
if(opt.long_option("server", 's', ACE_Get_Opt::ARG_REQUIRED) == -1)
{
cout << "server option can not be added" << endl;
}
long_option方法三：
/// Returns the name of the long option found on the last call to
/// @c operator() or 0 if none was found.
const ACE_TCHAR *long_option (void) const;
这个方法表示解析命令行时，如果遇到long_option，则可以通过这个方法来获取是哪一个long_option。
命令行的解析
命令行的解析就要依赖 operator()()方法，和opt_arg方法，前者解析遇到的是哪个short option 或者 long option，而 opt_arg方法则获取option的参数。
下面看一下方法的声明
/**
* Scan elements of @a argv (whose length is @a argc) for short option
* characters given in @a optstring or long options (with no short
* option equivalents).
*
* If an element of @a argv starts with '-', and is not exactly "-"
* or "--", then it is a short option element. The characters of this
* element (aside from the initial '-') are option characters. If
* it starts with "--" followed by other characters it is treated as
* a long option. If @c operator() is called repeatedly, it returns
* each of the option characters from each of the option elements.
*
* @return The parsed option character. The following characters have
* special significance.
* @retval 0 A long option was found
* @retval '\?' Either an unknown option character was found, or the
* option is known but requires an argument, none was
* specified, and @a optstring did not contain a leading
* colon.
* @retval ':' A known option character was found but it requires an
* argument and none was supplied, and the first character
* of @a optstring was a colon. @c opt_opt() indicates
* which option was specified.
* @retval '1' @c RETURN_IN_ORDER was specified and a non-option argument
* was found.
* @retval EOF No more option characters were found. @c opt_ind() will
* return the index in @a argv of the first @a argv element
* that is not an option. If @c PERMUTE_ARGS was
* specified, the @a argv elements have been permuted so that
* those that are not options now come last.
*
* @note The standards are unclear with respect to the conditions under
* which '?' and ':' are returned, so we scan the initial characters of
* @a optstring up unto the first short option character for '+', '-',
* and ':' in order to determine ordering and missing argument behavior.
*/
int operator () (void);
/**
* For communication from @c operator() to the caller. When
* @c operator() finds an option that takes an argument, the argument
* value is returned from this method, otherwise it returns 0.
*/
ACE_TCHAR *opt_arg (void) const;
operator()方法有几种返回值：
- 具体的option 字符，表示命令行中遇到了此 short option，或者关联了short option的 long option
- 0，表示遇到了一个long option，可以通过上面介绍的long_option的方法三来获取一下具体是哪一个long option
- EOF，也就是-1，表示解析结束。
当operator()方法返回一个short option或者 long option时，如果这个option可以有参数，则通过opt_arg方法来获取具体的参数。
完整的命令行解析代码
下面展示一个完整但是比较简单的命令行解析的示例程序代码
#include "ace/Get_Opt.h"
#include <iostream>
using namespace std;int _tmain(int argc, _TCHAR* argv[])
{
//定义了三个short option, 类型为 ARG_REQUIRED
ACE_Get_Opt opt(argc, argv, "s:u:p:", 1, 0);
//定义三个long option, 并与相应的short option 进行关联
if(opt.long_option("server", 's', ACE_Get_Opt::ARG_REQUIRED) == -1)
{
cout << "server option can not be added" << endl;
}
if(opt.long_option("user", 'u', ACE_Get_Opt::ARG_REQUIRED) == -1)
{
cout << "user option can not be added" << endl;
}
if(opt.long_option("pwd", 'p', ACE_Get_Opt::ARG_REQUIRED) == -1)
{
cout << "pwd option can not be added" << endl;
} //开始解析
for(int arg = 0; (arg = opt()) != -1;)
{
switch (arg)
{
case 's':
cout << "server is " << opt.opt_arg() << endl;
break;
case 'u':
cout << "user is " << opt.opt_arg() << endl;
break;
case 'p':
cout << "pwd is " << opt.opt_arg() << endl;
break;
//解析 long option
case 0:
if(_stricmp(opt.long_option(), "server") == 0)
{
cout << "long option server is" << opt.opt_arg() << endl;
}
break;
default:
break;
}
}
//如果直接运行，或者使用方式不对，则显示帮助
if(opt.opt_ind() + 2 < argc)
{
cout << "the usage is : GetOptTest.exe [option]" << endl;
cout << "\t [-s] server address" << endl;
cout << "\t [-u] user name" << endl;
cout << "\t [-p] password of user" << endl
<< endl;
}
return 0;
}
命令行解析实战
至此命令行解析的基本功能已经都介绍了，而且相应的代码已经完成，那么就让我们来欣赏一下解析的效果吧。
具体的命令行
-s 127.0.0.1 --server 192.168.0.1 -u hbccdf -p pwdfortest
运行效果图
其实功能很简单，ACE_Get_Opt的使用也很简单，但是一点点学习，并进行总结，是我一直需要提高的地方。以后会坚持把学到的东西进行总结，并分享到博客上。
坚持！！！
