Question

我正在用C ++编写一个小程序。有一节课。你能在课堂上使用cout来输出数据。例如cout＆lt;＆lt; “美好时光”; 那么你不需要重载＆lt;＆lt;运营商？我试图这样做，但它不起作用只是一些错误。如何通过cout在课堂上输出数据 CPP文件：

#include "University.hpp"
#include "Course.hpp"
//#include "Lecture.hpp"
#include <string>
#include <sstream>
#include <iostream>

using namespace std;

University::University() {
}
University::University(int capacity, string uni) {
    university = uni;
    cap = capacity;
    length = 0;
    myCourse = new Course[cap];    
}

University::University(University& orig) {
    copy(orig);
}

University::~University() {
    delete [] myCourse;
}

University & University:: operator=(University & other){
    if(this != &other){
        delete [] myCourse;
        copy(other);
    }
    return *this;
}

void University:: copy(University & other){
    if(this != &other){
       length = other.length;
       cap = other.cap;
       myCourse = new Course[cap];

       for(int x = 0; x < length; x++){
           myCourse[x] = other.myCourse[x];
       }       
    }    
}
void University:: addCourse(Course syllabus){
    if(length == cap){
        cap *= 2;
        Course * temp = new Course[cap];

        for(int x = 0; x < length; x++){
            temp[x] = myCourse[x];
        }
        myCourse = temp;        
    }
    myCourse[length] = syllabus;
    length++;
}
void University::listAll(){
    for(int x = 0; x < length; x++){
        cout << toString();// toString();
    }
}

friend ostream& operator<<(ostream& os, const University& uni){
    os <<uni.university <<uni.myCourse;
    return os;
}

string University::toString(){
    ostringstream ans;

    for(int x = 0; x < length; x++){
    ans << "The University " << university << " teaches " << myCourse[x] << myCourse[x].toString() << endl;
    }
    return ans.str();
}

部首：

#include <string>
#include "Course.hpp"

using namespace std;

#ifndef UNIVERSITY_HPP
#define UNIVERSITY_HPP

class University {
     friend ostream& operator<< (ostream& os, const University& uni);
public:
    University();
    University(int capacity, string UoL);
    University(University& orig);
    ~University();
    void copy(University & other);
    University & operator=(University & other);
    void addCourse(Course syllabus);
    void listAll();
    string toString();

private:
    Course * myCourse;
    int length;
    int cap;
    string university;
};

#endif  /* UNIVERSITY_HPP */

错误：

g++    -c -g -MMD -MP -MF build/Debug/Cygwin-Windows/University.o.d -o build/Debug/Cygwin-Windows/University.o University.cpp
University.cpp:72: error: can't initialize friend function `operator<<'
University.cpp:72: error: friend declaration not in class definition
University.cpp: In member function `std::string University::toString()':
University.cpp:81: error: no match for 'operator<<' in 'std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)(+std::operator<< [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>](((std::basic_ostream<char, std::char_traits<char> >&)(+std::operator<< [with _Traits = std::char_traits<char>](((std::basic_ostream<char, std::char_traits<char> >&)((std::basic_ostream<char, std::char_traits<char> >*)(&ans))), ((const char*)"The University ")))), ((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >&)((const std::basic_string<char, std::char_traits<char>, std::allocator<char> >*)(((std::string*)((University*)this)) + 12u)))))), ((const char*)" teaches ")) << *(((University*)this)->University::myCourse + (+(((unsigned int)x) * 16u)))'
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:63: note: candidates are: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>&(*)(std::basic_ostream<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:74: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ios<_CharT, _Traits>&(*)(std::basic_ios<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:86: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base&(*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:121: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:155: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:98: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:178: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:189: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:193: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:204: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:179: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:214: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:238: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:219: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:261: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:284: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:307: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_streambuf<_CharT, _Traits>*) [with _CharT = char, _Traits = std::char_traits<char>]
University.cpp:72: note:                 std::ostream& operator<<(std::ostream&, const University&)
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:504: note:                 std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const unsigned char*) [with _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:499: note:                 std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const signed char*) [with _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:612: note:                 std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, const char*) [with _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:567: note:                 std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, const char*) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:465: note:                 std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, unsigned char) [with _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:460: note:                 std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, signed char) [with _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/ostream.tcc:505: note:                 std::basic_ostream<char, _Traits>& std::operator<<(std::basic_ostream<char, _Traits>&, char) [with _Traits = std::char_traits<char>]
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/ostream:449: note:                 std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&, char) [with _CharT = char, _Traits = std::char_traits<char>]
make[2]: *** [build/Debug/Cygwin-Windows/University.o] Error 1
make[1]: *** [.build-conf] Error 2
make: *** [.build-impl] Error 2
make[2]: Leaving directory `/cygdrive/g/Aristotelis/C++/Assessment_2'
make[1]: Leaving directory `/cygdrive/g/Aristotelis/C++/Assessment_2'

BUILD FAILED (exit value 2, total time: 3s)

Answer 1

您尝试使用ostringstream::operator<<参数调用Course。您还需要为operator<<定义Course。对不起我以前错过了。您还应该从friend的定义中删除operator<<(ostream&, const University&)，因为它只需要在类定义中。

Answer 2

试试这个例子。

#include <iostream>
using namespace std;

class A
{
  friend ostream& operator<<(ostream& os, const A& a); 
  int _member;
  public:
    A() : _member(0)
      { } 

    A(int member) : _member(member)
      { } 
};

ostream& operator<<(ostream&os, const A& a)
{
  os << a._member;
  return os; 
}

int main()
{
  A a(5);
  cout << a << endl;
  return 0;
}

使用它并满足您的需求。

Answer 3

University.cpp:72: error: can't initialize friend function `operator<<'
University.cpp:72: error: friend declaration not in class definition
University.cpp: In member function `std::string University::toString()':

所以，我们看一下在成员函数`std :: string University :: toString（）'中找到：

ans << ... << myCourse[x] << ...;

你确定你有超载吗？

friend ostream& operator<< (ostream& os, const Course& uni);

这不是你在这里展示的任何代码。

在c ++程序类中的cout

3 个答案: