C++编写Config类读取配置文件

老外写的一段代码，在Server中编写这个类读取配置文件比较实用 

//Config.h
#pragma once

#include <string>
#include <map>
#include <iostream>
#include <fstream>
#include <sstream>


/*
* rief Generic configuration Class
*
*/
class Config {
    // Data
protected:
    std::string m_Delimiter;  //!< separator between key and value
    std::string m_Comment;    //!< separator between value and comments
    std::map<std::string,std::string> m_Contents;  //!< extracted keys and values

    typedef std::map<std::string,std::string>::iterator mapi;
    typedef std::map<std::string,std::string>::const_iterator mapci;
    // Methods
public:

    Config( std::string filename,std::string delimiter = "=",std::string comment = "#" );
    Config();
    template<class T> T Read( const std::string& in_key ) const;  //!<Search for key and read value or optional default value, call as read<T>
    template<class T> T Read( const std::string& in_key, const T& in_value ) const;
    template<class T> bool ReadInto( T& out_var, const std::string& in_key ) const;
    template<class T>
    bool ReadInto( T& out_var, const std::string& in_key, const T& in_value ) const;
    bool FileExist(std::string filename);
    void ReadFile(std::string filename,std::string delimiter = "=",std::string comment = "#" );

    // Check whether key exists in configuration
    bool KeyExists( const std::string& in_key ) const;

    // Modify keys and values
    template<class T> void Add( const std::string& in_key, const T& in_value );
    void Remove( const std::string& in_key );

    // Check or change configuration syntax
    std::string GetDelimiter() const { return m_Delimiter; }
    std::string GetComment() const { return m_Comment; }
    std::string SetDelimiter( const std::string& in_s )
    { std::string old = m_Delimiter;  m_Delimiter = in_s;  return old; }
    std::string SetComment( const std::string& in_s )
    { std::string old = m_Comment;  m_Comment =  in_s;  return old; }

    // Write or read configuration
    friend std::ostream& operator<<( std::ostream& os, const Config& cf );
    friend std::istream& operator>>( std::istream& is, Config& cf );

protected:
    template<class T> static std::string T_as_string( const T& t );
    template<class T> static T string_as_T( const std::string& s );
    static void Trim( std::string& inout_s );


    // Exception types
public:
    struct File_not_found {
        std::string filename;
        File_not_found( const std::string& filename_ = std::string() )
            : filename(filename_) {} };
        struct Key_not_found {  // thrown only by T read(key) variant of read()
            std::string key;
            Key_not_found( const std::string& key_ = std::string() )
                : key(key_) {} };
};


/* static */
template<class T>
std::string Config::T_as_string( const T& t )
{
    // Convert from a T to a string
    // Type T must support << operator
    std::ostringstream ost;
    ost << t;
    return ost.str();
}


/* static */
template<class T>
T Config::string_as_T( const std::string& s )
{
    // Convert from a string to a T
    // Type T must support >> operator
    T t;
    std::istringstream ist(s);
    ist >> t;
    return t;
}


/* static */
template<>
inline std::string Config::string_as_T<std::string>( const std::string& s )
{
    // Convert from a string to a string
    // In other words, do nothing
    return s;
}


/* static */
template<>
inline bool Config::string_as_T<bool>( const std::string& s )
{
    // Convert from a string to a bool
    // Interpret "false", "F", "no", "n", "0" as false
    // Interpret "true", "T", "yes", "y", "1", "-1", or anything else as true
    bool b = true;
    std::string sup = s;
    for( std::string::iterator p = sup.begin(); p != sup.end(); ++p )
        *p = toupper(*p);  // make string all caps
    if( sup==std::string("FALSE") || sup==std::string("F") ||
        sup==std::string("NO") || sup==std::string("N") ||
        sup==std::string("0") || sup==std::string("NONE") )
        b = false;
    return b;
}


template<class T>
T Config::Read( const std::string& key ) const
{
    // Read the value corresponding to key
    mapci p = m_Contents.find(key);
    if( p == m_Contents.end() ) throw Key_not_found(key);
    return string_as_T<T>( p->second );
}


template<class T>
T Config::Read( const std::string& key, const T& value ) const
{
    // Return the value corresponding to key or given default value
    // if key is not found
    mapci p = m_Contents.find(key);
    if( p == m_Contents.end() ) return value;
    return string_as_T<T>( p->second );
}


template<class T>
bool Config::ReadInto( T& var, const std::string& key ) const
{
    // Get the value corresponding to key and store in var
    // Return true if key is found
    // Otherwise leave var untouched
    mapci p = m_Contents.find(key);
    bool found = ( p != m_Contents.end() );
    if( found ) var = string_as_T<T>( p->second );
    return found;
}


template<class T>
bool Config::ReadInto( T& var, const std::string& key, const T& value ) const
{
    // Get the value corresponding to key and store in var
    // Return true if key is found
    // Otherwise set var to given default
    mapci p = m_Contents.find(key);
    bool found = ( p != m_Contents.end() );
    if( found )
        var = string_as_T<T>( p->second );
    else
        var = value;
    return found;
}


template<class T>
void Config::Add( const std::string& in_key, const T& value )
{
    // Add a key with given value
    std::string v = T_as_string( value );
    std::string key=in_key;
    trim(key);
    trim(v);
    m_Contents[key] = v;
    return;
}

// Config.cpp

#include "Config.h"

using namespace std;


Config::Config( string filename, string delimiter,
               string comment )
               : m_Delimiter(delimiter), m_Comment(comment)
{
    // Construct a Config, getting keys and values from given file

    std::ifstream in( filename.c_str() );

    if( !in ) throw File_not_found( filename );

    in >> (*this);
}


Config::Config()
: m_Delimiter( string(1,'=') ), m_Comment( string(1,'#') )
{
    // Construct a Config without a file; empty
}



bool Config::KeyExists( const string& key ) const
{
    // Indicate whether key is found
    mapci p = m_Contents.find( key );
    return ( p != m_Contents.end() );
}


/* static */
void Config::Trim( string& inout_s )
{
    // Remove leading and trailing whitespace
    static const char whitespace[] = " 
	v
f";
    inout_s.erase( 0, inout_s.find_first_not_of(whitespace) );
    inout_s.erase( inout_s.find_last_not_of(whitespace) + 1U );
}


std::ostream& operator<<( std::ostream& os, const Config& cf )
{
    // Save a Config to os
    for( Config::mapci p = cf.m_Contents.begin();
        p != cf.m_Contents.end();
        ++p )
    {
        os << p->first << " " << cf.m_Delimiter << " ";
        os << p->second << std::endl;
    }
    return os;
}

void Config::Remove( const string& key )
{
    // Remove key and its value
    m_Contents.erase( m_Contents.find( key ) );
    return;
}

std::istream& operator>>( std::istream& is, Config& cf )
{
    // Load a Config from is
    // Read in keys and values, keeping internal whitespace
    typedef string::size_type pos;
    const string& delim  = cf.m_Delimiter;  // separator
    const string& comm   = cf.m_Comment;    // comment
    const pos skip = delim.length();        // length of separator

    string nextline = "";  // might need to read ahead to see where value ends

    while( is || nextline.length() > 0 )
    {
        // Read an entire line at a time
        string line;
        if( nextline.length() > 0 )
        {
            line = nextline;  // we read ahead; use it now
            nextline = "";
        }
        else
        {
            std::getline( is, line );
        }

        // Ignore comments
        line = line.substr( 0, line.find(comm) );

        // Parse the line if it contains a delimiter
        pos delimPos = line.find( delim );
        if( delimPos < string::npos )
        {
            // Extract the key
            string key = line.substr( 0, delimPos );
            line.replace( 0, delimPos+skip, "" );

            // See if value continues on the next line
            // Stop at blank line, next line with a key, end of stream,
            // or end of file sentry
            bool terminate = false;
            while( !terminate && is )
            {
                std::getline( is, nextline );
                terminate = true;

                string nlcopy = nextline;
                Config::Trim(nlcopy);
                if( nlcopy == "" ) continue;

                nextline = nextline.substr( 0, nextline.find(comm) );
                if( nextline.find(delim) != string::npos )
                    continue;

                nlcopy = nextline;
                Config::Trim(nlcopy);
                if( nlcopy != "" ) line += "
";
                line += nextline;
                terminate = false;
            }

            // Store key and value
            Config::Trim(key);
            Config::Trim(line);
            cf.m_Contents[key] = line;  // overwrites if key is repeated
        }
    }

    return is;
}
bool Config::FileExist(std::string filename)
{
    bool exist= false;
    std::ifstream in( filename.c_str() );
    if( in )
        exist = true;
    return exist;
}

void Config::ReadFile( string filename, string delimiter,
                      string comment )
{
    m_Delimiter = delimiter;
    m_Comment = comment;
    std::ifstream in( filename.c_str() );

    if( !in ) throw File_not_found( filename );

    in >> (*this);
}

//main.cpp
#include "Config.h"
int main()
{
    int port;
    std::string ipAddress;
    std::string username;
    std::string password;
    const char ConfigFile[]= "config.txt";
    Config configSettings(ConfigFile);

    port = configSettings.Read("port", 0);
    ipAddress = configSettings.Read("ipAddress", ipAddress);
    username = configSettings.Read("username", username);
    password = configSettings.Read("password", password);
    std::cout<<"port:"<<port<<std::endl;
    std::cout<<"ipAddress:"<<ipAddress<<std::endl;
    std::cout<<"username:"<<username<<std::endl;
    std::cout<<"password:"<<password<<std::endl;

    return 0;
}

config.txt的文件内容： 
ipAddress=10.10.90.125 
port=3001 
username=mark 
password=2d2df5a 


编译运行输出: 
port:3001 
ipAddress:10.10.90.125 
username:mark 
password:2d2df5a