Cách tiếp cận tiêu chuẩn được đề xuất bởi Nate Kohl, chỉ cần sử dụng vector, sort + unique:
sort( vec.begin(), vec.end() );
vec.erase( unique( vec.begin(), vec.end() ), vec.end() );
không làm việc cho một vectơ con trỏ.
Hãy xem xét kỹ ví dụ này trên cplusplus.com .
Trong ví dụ của họ, "cái được gọi là trùng lặp" được di chuyển đến cuối thực sự được hiển thị là? (các giá trị không xác định), bởi vì các "phần tử được gọi là trùng lặp" là SOMETIMES "các phần tử bổ sung" và SOMETIMES có "các phần tử bị thiếu" nằm trong vectơ gốc.
Xảy ra sự cố khi sử dụng std::unique() trên vectơ con trỏ tới các đối tượng (rò rỉ bộ nhớ, đọc dữ liệu xấu từ HEAP, giải phóng trùng lặp, gây ra lỗi phân đoạn, v.v.).
Đây là giải pháp của tôi cho vấn đề: thay thế std::unique()bằngptgi::unique() .
Xem tập tin ptgi_unique.hpp bên dưới:
// ptgi::unique()
//
// Fix a problem in std::unique(), such that none of the original elts in the collection are lost or duplicate.
// ptgi::unique() has the same interface as std::unique()
//
// There is the 2 argument version which calls the default operator== to compare elements.
//
// There is the 3 argument version, which you can pass a user defined functor for specialized comparison.
//
// ptgi::unique() is an improved version of std::unique() which doesn't looose any of the original data
// in the collection, nor does it create duplicates.
//
// After ptgi::unique(), every old element in the original collection is still present in the re-ordered collection,
// except that duplicates have been moved to a contiguous range [dupPosition, last) at the end.
//
// Thus on output:
// [begin, dupPosition) range are unique elements.
// [dupPosition, last) range are duplicates which can be removed.
// where:
// [] means inclusive, and
// () means exclusive.
//
// In the original std::unique() non-duplicates at end are moved downward toward beginning.
// In the improved ptgi:unique(), non-duplicates at end are swapped with duplicates near beginning.
//
// In addition if you have a collection of ptrs to objects, the regular std::unique() will loose memory,
// and can possibly delete the same pointer multiple times (leading to SEGMENTATION VIOLATION on Linux machines)
// but ptgi::unique() won't. Use valgrind(1) to find such memory leak problems!!!
//
// NOTE: IF you have a vector of pointers, that is, std::vector<Object*>, then upon return from ptgi::unique()
// you would normally do the following to get rid of the duplicate objects in the HEAP:
//
// // delete objects from HEAP
// std::vector<Object*> objects;
// for (iter = dupPosition; iter != objects.end(); ++iter)
// {
// delete (*iter);
// }
//
// // shrink the vector. But Object * pointers are NOT followed for duplicate deletes, this shrinks the vector.size())
// objects.erase(dupPosition, objects.end));
//
// NOTE: But if you have a vector of objects, that is: std::vector<Object>, then upon return from ptgi::unique(), it
// suffices to just call vector:erase(, as erase will automatically call delete on each object in the
// [dupPosition, end) range for you:
//
// std::vector<Object> objects;
// objects.erase(dupPosition, last);
//
//==========================================================================================================
// Example of differences between std::unique() vs ptgi::unique().
//
// Given:
// int data[] = {10, 11, 21};
//
// Given this functor: ArrayOfIntegersEqualByTen:
// A functor which compares two integers a[i] and a[j] in an int a[] array, after division by 10:
//
// // given an int data[] array, remove consecutive duplicates from it.
// // functor used for std::unique (BUGGY) or ptgi::unique(IMPROVED)
//
// // Two numbers equal if, when divided by 10 (integer division), the quotients are the same.
// // Hence 50..59 are equal, 60..69 are equal, etc.
// struct ArrayOfIntegersEqualByTen: public std::equal_to<int>
// {
// bool operator() (const int& arg1, const int& arg2) const
// {
// return ((arg1/10) == (arg2/10));
// }
// };
//
// Now, if we call (problematic) std::unique( data, data+3, ArrayOfIntegersEqualByTen() );
//
// TEST1: BEFORE UNIQ: 10,11,21
// TEST1: AFTER UNIQ: 10,21,21
// DUP_INX=2
//
// PROBLEM: 11 is lost, and extra 21 has been added.
//
// More complicated example:
//
// TEST2: BEFORE UNIQ: 10,20,21,22,30,31,23,24,11
// TEST2: AFTER UNIQ: 10,20,30,23,11,31,23,24,11
// DUP_INX=5
//
// Problem: 21 and 22 are deleted.
// Problem: 11 and 23 are duplicated.
//
//
// NOW if ptgi::unique is called instead of std::unique, both problems go away:
//
// DEBUG: TEST1: NEW_WAY=1
// TEST1: BEFORE UNIQ: 10,11,21
// TEST1: AFTER UNIQ: 10,21,11
// DUP_INX=2
//
// DEBUG: TEST2: NEW_WAY=1
// TEST2: BEFORE UNIQ: 10,20,21,22,30,31,23,24,11
// TEST2: AFTER UNIQ: 10,20,30,23,11,31,22,24,21
// DUP_INX=5
//
// @SEE: look at the "case study" below to understand which the last "AFTER UNIQ" results with that order:
// TEST2: AFTER UNIQ: 10,20,30,23,11,31,22,24,21
//
//==========================================================================================================
// Case Study: how ptgi::unique() works:
// Remember we "remove adjacent duplicates".
// In this example, the input is NOT fully sorted when ptgi:unique() is called.
//
// I put | separatators, BEFORE UNIQ to illustrate this
// 10 | 20,21,22 | 30,31 | 23,24 | 11
//
// In example above, 20, 21, 22 are "same" since dividing by 10 gives 2 quotient.
// And 30,31 are "same", since /10 quotient is 3.
// And 23, 24 are same, since /10 quotient is 2.
// And 11 is "group of one" by itself.
// So there are 5 groups, but the 4th group (23, 24) happens to be equal to group 2 (20, 21, 22)
// So there are 5 groups, and the 5th group (11) is equal to group 1 (10)
//
// R = result
// F = first
//
// 10, 20, 21, 22, 30, 31, 23, 24, 11
// R F
//
// 10 is result, and first points to 20, and R != F (10 != 20) so bump R:
// R
// F
//
// Now we hits the "optimized out swap logic".
// (avoid swap because R == F)
//
// // now bump F until R != F (integer division by 10)
// 10, 20, 21, 22, 30, 31, 23, 24, 11
// R F // 20 == 21 in 10x
// R F // 20 == 22 in 10x
// R F // 20 != 30, so we do a swap of ++R and F
// (Now first hits 21, 22, then finally 30, which is different than R, so we swap bump R to 21 and swap with 30)
// 10, 20, 30, 22, 21, 31, 23, 24, 11 // after R & F swap (21 and 30)
// R F
//
// 10, 20, 30, 22, 21, 31, 23, 24, 11
// R F // bump F to 31, but R and F are same (30 vs 31)
// R F // bump F to 23, R != F, so swap ++R with F
// 10, 20, 30, 22, 21, 31, 23, 24, 11
// R F // bump R to 22
// 10, 20, 30, 23, 21, 31, 22, 24, 11 // after the R & F swap (22 & 23 swap)
// R F // will swap 22 and 23
// R F // bump F to 24, but R and F are same in 10x
// R F // bump F, R != F, so swap ++R with F
// R F // R and F are diff, so swap ++R with F (21 and 11)
// 10, 20, 30, 23, 11, 31, 22, 24, 21
// R F // aftter swap of old 21 and 11
// R F // F now at last(), so loop terminates
// R F // bump R by 1 to point to dupPostion (first duplicate in range)
//
// return R which now points to 31
//==========================================================================================================
// NOTES:
// 1) the #ifdef IMPROVED_STD_UNIQUE_ALGORITHM documents how we have modified the original std::unique().
// 2) I've heavily unit tested this code, including using valgrind(1), and it is *believed* to be 100% defect-free.
//
//==========================================================================================================
// History:
// 130201 dpb dbednar@ptgi.com created
//==========================================================================================================
#ifndef PTGI_UNIQUE_HPP
#define PTGI_UNIQUE_HPP
// Created to solve memory leak problems when calling std::unique() on a vector<Route*>.
// Memory leaks discovered with valgrind and unitTesting.
#include <algorithm> // std::swap
// instead of std::myUnique, call this instead, where arg3 is a function ptr
//
// like std::unique, it puts the dups at the end, but it uses swapping to preserve original
// vector contents, to avoid memory leaks and duplicate pointers in vector<Object*>.
#ifdef IMPROVED_STD_UNIQUE_ALGORITHM
#error the #ifdef for IMPROVED_STD_UNIQUE_ALGORITHM was defined previously.. Something is wrong.
#endif
#undef IMPROVED_STD_UNIQUE_ALGORITHM
#define IMPROVED_STD_UNIQUE_ALGORITHM
// similar to std::unique, except that this version swaps elements, to avoid
// memory leaks, when vector contains pointers.
//
// Normally the input is sorted.
// Normal std::unique:
// 10 20 20 20 30 30 20 20 10
// a b c d e f g h i
//
// 10 20 30 20 10 | 30 20 20 10
// a b e g i f g h i
//
// Now GONE: c, d.
// Now DUPS: g, i.
// This causes memory leaks and segmenation faults due to duplicate deletes of same pointer!
namespace ptgi {
// Return the position of the first in range of duplicates moved to end of vector.
//
// uses operator== of class for comparison
//
// @param [first, last) is a range to find duplicates within.
//
// @return the dupPosition position, such that [dupPosition, end) are contiguous
// duplicate elements.
// IF all items are unique, then it would return last.
//
template <class ForwardIterator>
ForwardIterator unique( ForwardIterator first, ForwardIterator last)
{
// compare iterators, not values
if (first == last)
return last;
// remember the current item that we are looking at for uniqueness
ForwardIterator result = first;
// result is slow ptr where to store next unique item
// first is fast ptr which is looking at all elts
// the first iterator moves over all elements [begin+1, end).
// while the current item (result) is the same as all elts
// to the right, (first) keeps going, until you find a different
// element pointed to by *first. At that time, we swap them.
while (++first != last)
{
if (!(*result == *first))
{
#ifdef IMPROVED_STD_UNIQUE_ALGORITHM
// inc result, then swap *result and *first
// THIS IS WHAT WE WANT TO DO.
// BUT THIS COULD SWAP AN ELEMENT WITH ITSELF, UNCECESSARILY!!!
// std::swap( *first, *(++result));
// BUT avoid swapping with itself when both iterators are the same
++result;
if (result != first)
std::swap( *first, *result);
#else
// original code found in std::unique()
// copies unique down
*(++result) = *first;
#endif
}
}
return ++result;
}
template <class ForwardIterator, class BinaryPredicate>
ForwardIterator unique( ForwardIterator first, ForwardIterator last, BinaryPredicate pred)
{
if (first == last)
return last;
// remember the current item that we are looking at for uniqueness
ForwardIterator result = first;
while (++first != last)
{
if (!pred(*result,*first))
{
#ifdef IMPROVED_STD_UNIQUE_ALGORITHM
// inc result, then swap *result and *first
// THIS COULD SWAP WITH ITSELF UNCECESSARILY
// std::swap( *first, *(++result));
//
// BUT avoid swapping with itself when both iterators are the same
++result;
if (result != first)
std::swap( *first, *result);
#else
// original code found in std::unique()
// copies unique down
// causes memory leaks, and duplicate ptrs
// and uncessarily moves in place!
*(++result) = *first;
#endif
}
}
return ++result;
}
// from now on, the #define is no longer needed, so get rid of it
#undef IMPROVED_STD_UNIQUE_ALGORITHM
} // end ptgi:: namespace
#endif
Và đây là chương trình UNIT Test mà tôi đã sử dụng để kiểm tra nó:
// QUESTION: in test2, I had trouble getting one line to compile,which was caused by the declaration of operator()
// in the equal_to Predicate. I'm not sure how to correctly resolve that issue.
// Look for //OUT lines
//
// Make sure that NOTES in ptgi_unique.hpp are correct, in how we should "cleanup" duplicates
// from both a vector<Integer> (test1()) and vector<Integer*> (test2).
// Run this with valgrind(1).
//
// In test2(), IF we use the call to std::unique(), we get this problem:
//
// [dbednar@ipeng8 TestSortRoutes]$ ./Main7
// TEST2: ORIG nums before UNIQUE: 10, 20, 21, 22, 30, 31, 23, 24, 11
// TEST2: modified nums AFTER UNIQUE: 10, 20, 30, 23, 11, 31, 23, 24, 11
// INFO: dupInx=5
// TEST2: uniq = 10
// TEST2: uniq = 20
// TEST2: uniq = 30
// TEST2: uniq = 33427744
// TEST2: uniq = 33427808
// Segmentation fault (core dumped)
//
// And if we run valgrind we seen various error about "read errors", "mismatched free", "definitely lost", etc.
//
// valgrind --leak-check=full ./Main7
// ==359== Memcheck, a memory error detector
// ==359== Command: ./Main7
// ==359== Invalid read of size 4
// ==359== Invalid free() / delete / delete[]
// ==359== HEAP SUMMARY:
// ==359== in use at exit: 8 bytes in 2 blocks
// ==359== LEAK SUMMARY:
// ==359== definitely lost: 8 bytes in 2 blocks
// But once we replace the call in test2() to use ptgi::unique(), all valgrind() error messages disappear.
//
// 130212 dpb dbednar@ptgi.com created
// =========================================================================================================
#include <iostream> // std::cout, std::cerr
#include <string>
#include <vector> // std::vector
#include <sstream> // std::ostringstream
#include <algorithm> // std::unique()
#include <functional> // std::equal_to(), std::binary_function()
#include <cassert> // assert() MACRO
#include "ptgi_unique.hpp" // ptgi::unique()
// Integer is small "wrapper class" around a primitive int.
// There is no SETTER, so Integer's are IMMUTABLE, just like in JAVA.
class Integer
{
private:
int num;
public:
// default CTOR: "Integer zero;"
// COMPRENSIVE CTOR: "Integer five(5);"
Integer( int num = 0 ) :
num(num)
{
}
// COPY CTOR
Integer( const Integer& rhs) :
num(rhs.num)
{
}
// assignment, operator=, needs nothing special... since all data members are primitives
// GETTER for 'num' data member
// GETTER' are *always* const
int getNum() const
{
return num;
}
// NO SETTER, because IMMUTABLE (similar to Java's Integer class)
// @return "num"
// NB: toString() should *always* be a const method
//
// NOTE: it is probably more efficient to call getNum() intead
// of toString() when printing a number:
//
// BETTER to do this:
// Integer five(5);
// std::cout << five.getNum() << "\n"
// than this:
// std::cout << five.toString() << "\n"
std::string toString() const
{
std::ostringstream oss;
oss << num;
return oss.str();
}
};
// convenience typedef's for iterating over std::vector<Integer>
typedef std::vector<Integer>::iterator IntegerVectorIterator;
typedef std::vector<Integer>::const_iterator ConstIntegerVectorIterator;
// convenience typedef's for iterating over std::vector<Integer*>
typedef std::vector<Integer*>::iterator IntegerStarVectorIterator;
typedef std::vector<Integer*>::const_iterator ConstIntegerStarVectorIterator;
// functor used for std::unique or ptgi::unique() on a std::vector<Integer>
// Two numbers equal if, when divided by 10 (integer division), the quotients are the same.
// Hence 50..59 are equal, 60..69 are equal, etc.
struct IntegerEqualByTen: public std::equal_to<Integer>
{
bool operator() (const Integer& arg1, const Integer& arg2) const
{
return ((arg1.getNum()/10) == (arg2.getNum()/10));
}
};
// functor used for std::unique or ptgi::unique on a std::vector<Integer*>
// Two numbers equal if, when divided by 10 (integer division), the quotients are the same.
// Hence 50..59 are equal, 60..69 are equal, etc.
struct IntegerEqualByTenPointer: public std::equal_to<Integer*>
{
// NB: the Integer*& looks funny to me!
// TECHNICAL PROBLEM ELSEWHERE so had to remove the & from *&
//OUT bool operator() (const Integer*& arg1, const Integer*& arg2) const
//
bool operator() (const Integer* arg1, const Integer* arg2) const
{
return ((arg1->getNum()/10) == (arg2->getNum()/10));
}
};
void test1();
void test2();
void printIntegerStarVector( const std::string& msg, const std::vector<Integer*>& nums );
int main()
{
test1();
test2();
return 0;
}
// test1() uses a vector<Object> (namely vector<Integer>), so there is no problem with memory loss
void test1()
{
int data[] = { 10, 20, 21, 22, 30, 31, 23, 24, 11};
// turn C array into C++ vector
std::vector<Integer> nums(data, data+9);
// arg3 is a functor
IntegerVectorIterator dupPosition = ptgi::unique( nums.begin(), nums.end(), IntegerEqualByTen() );
nums.erase(dupPosition, nums.end());
nums.erase(nums.begin(), dupPosition);
}
//==================================================================================
// test2() uses a vector<Integer*>, so after ptgi:unique(), we have to be careful in
// how we eliminate the duplicate Integer objects stored in the heap.
//==================================================================================
void test2()
{
int data[] = { 10, 20, 21, 22, 30, 31, 23, 24, 11};
// turn C array into C++ vector of Integer* pointers
std::vector<Integer*> nums;
// put data[] integers into equivalent Integer* objects in HEAP
for (int inx = 0; inx < 9; ++inx)
{
nums.push_back( new Integer(data[inx]) );
}
// print the vector<Integer*> to stdout
printIntegerStarVector( "TEST2: ORIG nums before UNIQUE", nums );
// arg3 is a functor
#if 1
// corrected version which fixes SEGMENTATION FAULT and all memory leaks reported by valgrind(1)
// I THINK we want to use new C++11 cbegin() and cend(),since the equal_to predicate is passed "Integer *&"
// DID NOT COMPILE
//OUT IntegerStarVectorIterator dupPosition = ptgi::unique( const_cast<ConstIntegerStarVectorIterator>(nums.begin()), const_cast<ConstIntegerStarVectorIterator>(nums.end()), IntegerEqualByTenPointer() );
// DID NOT COMPILE when equal_to predicate declared "Integer*& arg1, Integer*& arg2"
//OUT IntegerStarVectorIterator dupPosition = ptgi::unique( const_cast<nums::const_iterator>(nums.begin()), const_cast<nums::const_iterator>(nums.end()), IntegerEqualByTenPointer() );
// okay when equal_to predicate declared "Integer* arg1, Integer* arg2"
IntegerStarVectorIterator dupPosition = ptgi::unique(nums.begin(), nums.end(), IntegerEqualByTenPointer() );
#else
// BUGGY version that causes SEGMENTATION FAULT and valgrind(1) errors
IntegerStarVectorIterator dupPosition = std::unique( nums.begin(), nums.end(), IntegerEqualByTenPointer() );
#endif
printIntegerStarVector( "TEST2: modified nums AFTER UNIQUE", nums );
int dupInx = dupPosition - nums.begin();
std::cout << "INFO: dupInx=" << dupInx <<"\n";
// delete the dup Integer* objects in the [dupPosition, end] range
for (IntegerStarVectorIterator iter = dupPosition; iter != nums.end(); ++iter)
{
delete (*iter);
}
// shrink the vector
// NB: the Integer* ptrs are NOT followed by vector::erase()
nums.erase(dupPosition, nums.end());
// print the uniques, by following the iter to the Integer* pointer
for (IntegerStarVectorIterator iter = nums.begin(); iter != nums.end(); ++iter)
{
std::cout << "TEST2: uniq = " << (*iter)->getNum() << "\n";
}
// remove the unique objects from heap
for (IntegerStarVectorIterator iter = nums.begin(); iter != nums.end(); ++iter)
{
delete (*iter);
}
// shrink the vector
nums.erase(nums.begin(), nums.end());
// the vector should now be completely empty
assert( nums.size() == 0);
}
//@ print to stdout the string: "info_msg: num1, num2, .... numN\n"
void printIntegerStarVector( const std::string& msg, const std::vector<Integer*>& nums )
{
std::cout << msg << ": ";
int inx = 0;
ConstIntegerStarVectorIterator iter;
// use const iterator and const range!
// NB: cbegin() and cend() not supported until LATER (c++11)
for (iter = nums.begin(), inx = 0; iter != nums.end(); ++iter, ++inx)
{
// output a comma seperator *AFTER* first
if (inx > 0)
std::cout << ", ";
// call Integer::toString()
std::cout << (*iter)->getNum(); // send int to stdout
// std::cout << (*iter)->toString(); // also works, but is probably slower
}
// in conclusion, add newline
std::cout << "\n";
}
