C++ rand不適用於屬性初始值設定程序

Question

我是C++的新手，在C語言中有一些經驗，並且學習它我在做作業的時候測試了一些東西。現在，我有

#define MAX_OBJS 4 

using namespace std; 

class Object { 
    public: 
     int x, rand; 

     Object(int y) { 
      x = y; 
      rand = rand() % 5; 
     }; 
}; 

class Many { 
    public: 
     vector<Object> obj_list; 

    Many(int n): obj_list (MAX_OBJS, n) {} 
}; 

int main() { 
    srand(time(NULL)); 

    Many many(42); 

    cout << "Example: looking for " << many.obj_list.back().rand "\n"; 

    vector<Object>::iterator j; 
    Object t = many.obj_list.back(); 
    for (j = many.obj_list.begin(); j != many.obj_list.end(); j++) { 
/*A*/ cout << j->rand << "\n"; 
/*B*/ if (&(*j) == &t) 
/*C*/  cout << "Found!" << "\n"; 
    } 

    return EXIT_SUCCESS; 
} 

從這段代碼，我可以說

1. 我讀了時間（0）可能改變了一些成績，但並沒有改變什麼。
2. 是的，我想獲得實例化時，許多對象來創建它的對象載體，以初始化所有MAX_OBJS元素X屬性與價值ñ的載體。
3. 我試圖讓一些代碼找到一個給定的元素，在這種情況下，最後一個，在一些向量（在這種情況下，在相同的元素是）。我試了幾件事情，如找到和find_if，沒有成功。

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我想一些建議/我的問題，這是

1. A線）打印幫助相同數量（隨機一個）MAX_OBJS次
2. 我不知道比我更好的方法來比較對象B）
3. C）從不打印出「找到！」

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發現不起作用，即使@Nawaz建議。編譯器說：

/usr/include/c++/4.5/bits/stl_algo.h: In function ‘RandomAccessIterator std::_find(_RandomAccessIterator, _RandomAccessIterator, const _Tp&, std::random_access_iterator_tag) [with _RandomAccessIterator = __gnu_cxx::__normal_iterator >, _Tp = Object]’:

/usr/include/c++/4.5/bits/stl_algo.h:4209:45: instantiated from ‘_IIter std::find(_IIter, _IIter, const _Tp&) [with _IIter = __gnu_cxx::__normal_iterator >, _Tp = Object]’

../src/Many.cpp:48:74: instantiated from here

/usr/include/c++/4.5/bits/stl_algo.h:158:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

/usr/include/c++/4.5/bits/stl_algo.h:4209:45: instantiated from ‘_IIter std::find(_IIter, _IIter, const _Tp&) [with _IIter = __gnu_cxx::__normal_iterator >, _Tp = Object]’

../src/Many.cpp:48:74: instantiated from here

/usr/include/c++/4.5/bits/stl_algo.h:162:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

/usr/include/c++/4.5/bits/stl_algo.h:166:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

/usr/include/c++/4.5/bits/stl_algo.h:170:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

/usr/include/c++/4.5/bits/stl_algo.h:178:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

/usr/include/c++/4.5/bits/stl_algo.h:182:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

/usr/include/c++/4.5/bits/stl_algo.h:186:4: error: no match for ‘operator==’ in ‘_first._gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = Object*, _Container = std::vector, __gnu_cxx::__normal_iterator<_Iterator, _Container>::reference = Object& == __val’

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謝謝！

Answer 1

if (&(*j) == &t) 

您正在比較上面if條件中的對象的地址。但以下代碼行的原始對象的副本：

Object t = many.obj_list.back(); 

副本意味着t不是從列表中同一個對象。因此你的程序永遠不會打印Found!。

我想你需要寫爲：

Object & t = many.obj_list.back(); 
    //^note this! 

它存儲的對象的引用。它不會複製obj_list中的原始對象。

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順便說一句，你爲什麼不使用std::find從<algorithm>頭文件？

#include <algorithm> 

std::vector<Object>::iterator it = std::find(many.obj_list.begin(), many.obj_list.end(), t) 
if (it != many.obj_list.end()) 
    std::cout << "Found!"<< std::endl;