C++：需要有关流的更多帮助

Question

我一直在阅读有关流的内容，但是当我尝试偏离书本来完成任务时，我似乎做错了一些事情。无论如何，这是我的代码的开始。如果此代码可以稍作修改即可工作，请告诉我，否则，如果您可以提供更好的“更像 C++”路线，我将不胜感激。

#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>

using namespace std;

std::multimap<std::string,float> _map;

istream& operator>>(istream& stream, pair<string,float> in ) { 
    return stream >> in.first >> in.second;
}

int main( int argc, char *argv[ ] ) { 
    istream is( );
    do {
        pair<string,float> input;
        is >> input;
        _map.insert(input);
    } while( is );
}

编译器错误为：

[mehoggan@desktop bjarne_stroustrup]$ g++ -o map -Wall ./key_value_stats.cpp
./key_value_stats.cpp: In function ‘int main(int, char**)’:
./key_value_stats.cpp:20:15: error: no match for ‘operator>>’ in ‘is >> input’
./key_value_stats.cpp:12:10: note: candidate is: std::istream& operator>>(std::istream&, std::pair<std::basic_string<char>, float>)
./key_value_stats.cpp:22:14: warning: the address of ‘std::istream is()’ will always evaluate as ‘true’

---

UPDATE[0]：

删除 ( ) 后 并更改：

istream& operator>>(istream& stream, pair<string,float> in ) {

我

istream& operator>>(istream& stream, pair<string,float> &in ) {

得到一组不同的编译器错误：

/usr/lib/gcc/i686-redhat-linux/4.5.1/../../../../include/c++/4.5.1/istream: In function ‘int main(int, char**)’:
/usr/lib/gcc/i686-redhat-linux/4.5.1/../../../../include/c++/4.5.1/istream:582:7: error: ‘std::basic_istream<_CharT, _Traits>::basic_istream() [with _CharT = char, _Traits = std::char_traits<char>]’ is protected
./key_value_stats.cpp:17:13: error: within this context

使用以下代码：

#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>

using namespace std;

std::multimap<std::string,float> _map;

istream& operator>>(istream& stream, pair<string,float> &in ) {
    return stream >> in.first >> in.second;
}

int main( int argc, char *argv[ ] ) { 
    istream is;
    do {
        pair<string,float> input;
        is >> input;
        _map.insert(input);
    } while( is );
}

---

UPDATE[1]

好吧，我解决了我认为的问题。回到 Stroustrup 的 desk_calculator 示例，我有以下至少可以编译的代码，我将在其中添加最终功能，然后为任何感兴趣的人重新发布最终产品。

#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>

using namespace std;

std::multimap<std::string,float> _map;

istream& operator>>(istream& stream, pair<string,float> &in ) {
    return stream >> in.first >> in.second;
}

//void print_pair( pair<string,float>

int main( int argc, char *argv[ ] ) { 
    istream *is = &cin;
    do {
        pair<string,float> input;
        (*is) >> input;
        _map.insert(input);
    } while( is );
}

---

更新[2]

我认为我没有满足问题的要求，但我已经掌握了技术知识。

#include <stdio.h>
#include <stdlib.h>
#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>

/********************************************************************
 * Read a sequence of possibly whitespace-separated (name,value)
 * pairs, where the name is a single whitespaace-separated word and
 * the value is an integer or floating-point value. Compute and print
 * the sum and mean for each name and the sum and mean for all names
 * ******************************************************************/

using namespace std;

std::multimap<string,float> _map;

istream& operator>>(istream& stream, pair<string,float> &in ) {
    return stream >> in.first >> in.second;
}

ostream& operator<<(ostream& stream, pair<string,float> &out ) {
    return stream << "(" << out.first 
                  << ", " << out.second << ")" << endl;
}

int main( int argc, char *argv[ ] ) { 
    istream *is = &cin;
    do {
        pair<string,float> input;
        (*is) >> input;
        _map.insert(input);
    } while( is->peek( ) != EOF );

    ostream *os = &cout;
    multimap<string,float>::iterator mit = _map.begin( );
    float sum = 0.0;
    while( mit != _map.end( ) ) {
        pair<string,float> p_pair = (*mit);
        (*os) << p_pair;
        sum+=p_pair.second;
        mit++;
    }
    float mean = static_cast<float>( sum/_map.size( ) );
    (*os) << "Sum: " << sum << " Mean: " << mean << endl;
}

Answer 1

使用

#include <map>
#include <string>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <iterator>

using namespace std;

std::multimap<std::string,float> _map;

istream& operator>>(istream& stream, pair<string,float>& in ) { 
    return stream >> in.first >> in.second;
}

int main( int argc, char *argv[ ] ) { 
    do {
        pair<string,float> input;
         cin >> input;
        _map.insert(input);
    } while( cin );
}

Answer 2

从行中删除 ()：

istream is();

Should be:

istream is;

Or:

istream is("text 1.2");

Or else is 是一个返回 istream 的函数。