[英]Why is this function causing memory leak?
我編寫了自己的數據結構(鏈表)並在下面的代碼中使用了它。 當我用valgrind分析程序時,鏈表的push和push_back方法都會導致內存泄漏。 你能幫我找出為什么會這樣嗎?
鏈接列表:
template <typename T>
struct Node {
T data;
Node *next;
};
/**
* @brief Simple Linked List implementation
*
* @tparam T
*/
template <typename T> class List{
private:
public:
Node<T> *head;
/**
* @brief Amount of nodes in the list
*
*/
int length;
/**
* @brief Construct a new List object
*
*/
List(){
head = NULL;
length = 0;
}
/**
* @brief Add new node to the list and increase size
*
* @param val
*/
void push(T val){
Node<T> *n = new Node<T>();
n->data = val;
n->next = head;
head = n;
length++;
}
/**
* @brief Add new node to the end of the list and increase size
*
* @param val
*/
void push_back(T val) {
Node<T> *n = new Node<T>();
Node<T> * temp = head;
n->data = val;
n->next = nullptr;
if (head) {
while (temp->next != NULL) {
temp = temp->next;
}
temp->next = n;
} else {
head = n;
}
length++;
}
/**
* @brief Remove the node from the list and decrease size
*
* @return T
*/
T pop(){
if(head) {
T p = head->data;
head = head->next;
length--;
return p;
}
}
/**
* @brief Get n-th item on the list
*
* @param index Index of the item
* @return T
*/
T get(int index) {
T value_to_return;
Node<T> * temp = head;
if (index == 0) {
return head->data;
}
for (int i = 0; i < index; i++) {
temp = temp->next;
value_to_return = temp->data;
}
return value_to_return;
}
};
導致錯誤的代碼:
/**
* @file file_reader.h
* @author Dawid Cyron (software@dawidcyron.me)
* @brief File with functions used for processing required text files
* @version 0.1
* @date 2020-01-26
*
* @copyright Copyright (c) 2020
*
*/
#include <vector>
#include "bibliography.h"
#include <fstream>
#include <regex>
#include "list.h"
#include "map.h"
/**
* @brief Function used for sorting list of bibliography
*
* @param bibliography_list List to sort
*/
void sort_bibliography_list(List<bibliography> bibliography_list) {
Node <bibliography> * current = bibliography_list.head, * index = NULL;
bibliography temp;
if (bibliography_list.head == NULL) {
return;
} else {
while (current != NULL) {
index = current->next;
while (index != NULL) {
if (current->data.author.substr(current->data.author.find(" "), current->data.author.length() - 1) > index->data.author.substr(index->data.author.find(" "), index->data.author.length() - 1)) {
temp = current->data;
current->data = index->data;
index->data = temp;
}
index = index->next;
}
current = current->next;
}
}
}
/**
* @brief Funciton used for reading the contents of bibliography file
*
* @param filename Name of the file containing bibliography
* @return std::vector < bibliography > Vector containing bibliography objects (tag, author, book title), alphabetically sorted by surname
*/
List < bibliography > readBibliographyFile(char * filename) {
std::ifstream bibliography_file(filename);
std::string line;
int line_counter = 0;
bibliography bib;
List<bibliography> storage_test;
if (bibliography_file.is_open()) {
while (getline(bibliography_file, line)) {
if (line_counter == 0) {
if (line == "") {
std::cout << "Incorrect data format. Exiting" << std::endl;
exit(1);
}
bib.label = line;
} else if (line_counter == 1) {
if (line == "") {
std::cout << "Incorrect data format. Exiting" << std::endl;
exit(1);
}
bib.author = line;
} else if (line_counter == 2) {
if (line == "") {
std::cout << "Incorrect data format. Exiting" << std::endl;
exit(1);
}
bib.book = line;
storage_test.push_back(bib);
line_counter = 0;
// Skip the empty line
getline(bibliography_file, line);
continue;
}
line_counter++;
}
}
sort_bibliography_list(storage_test);
return storage_test;
}
/**
* @brief Function used to load references footer
*
* @param references List of references
* @param output Reference to the output file
*/
void loadReferenceFooter(List<std::string> references, std::ofstream & output) {
output << "\nReferences\n \n";;
for (int i = 0; i < references.length; i++) {
output << references.get(i);
}
}
/**
* @brief Function used to replace cite tags with referenes
*
* @param filename Name of the file containing the text
* @param data Vector of Bibliography objects (tag, author, book title), has to be sorted by surname
* @param output_filename Name of the file where the content should be saved
*/
void replaceCites(char * filename, List < bibliography > data, char * output_filename) {
std::ifstream text_file(filename);
std::string content;
content.assign((std::istreambuf_iterator < char > (text_file)), (std::istreambuf_iterator < char > ()));
//std::map < std::string, int > map;
std::ofstream output(output_filename);
int cite_counter = 1;
List<std::string> references;
Hashtable<std::string, int> hash_table;
HashtableItem<std::string, int> * item;
for (int i=0; i < data.length; i++) {
std::smatch matches;
std::regex regex("\\\\cite\\{" + data.get(i).label + "\\}");
std::regex_search(content, matches, regex);
if (!matches.empty()) {
item = hash_table[data.get(i).label];
if (item != nullptr) {
content = std::regex_replace(content, regex, "[" + std::to_string(item->Value()) + "]");
} else {
content = std::regex_replace(content, regex, "[" + std::to_string(cite_counter) + "]");
references.push_back("[" + std::to_string(cite_counter) + "] " + data.get(i).author + ", " + data.get(i).book + "\n");
hash_table.Add(data.get(i).label, cite_counter);
cite_counter++;
}
}
}
output << content << std::endl;
text_file.close();
loadReferenceFooter(references, output);
output.close();
}
據我所知,數據結構應該可以正常工作。 我嘗試創建一個析構函數,它遍歷鏈表的所有節點並一個一個刪除它們,但這也不起作用(實際上,它導致應用程序甚至無法啟動)。
“為什么這個函數會導致內存泄漏?” - 簡單:您使用new
分配內存,而您永遠不會使用delete
釋放。
在現代 C++ 中,您通常應該更喜歡使用智能指針(在本例中為std::unique_ptr
)和/或容器類,而不是使用new
/ delete
進行手動內存管理。
內存泄漏是因為沒有析構函數來釋放分配的Node
s。 提問者指出,他們刪除了析構函數,因為當他們擁有析構函數時程序崩潰了。 這是應該解決的錯誤,因為使用析構函數是正確的做法。
把析構函數放回去,解決析構函數導致程序崩潰的原因。
List
違反了三定律。 這意味着當List
被復制並且您有兩個對象都指向同一個 head Node
。 這個Node
只能被delete
一次,兩個對象都會嘗試在List
析構函數中delete
它。 該程序遲早會痛苦地死去。
通常你可以用引用傳遞替換值傳遞,然后通過delete
特殊成員函數來禁止復制。 例如。 添加
List(const List & src) = delete;
List& operator=(List src) = delete;
到List
並等待編譯器開始在刪除復制特殊函數時發出關於List
被復制的尖叫聲。 用按引用傳遞替換所有按值傳遞。
很遺憾
List<bibliography> readBibliographyFile(char * filename)
按值返回。 一個局部變量的引用返回是注定的。 局部變量超出范圍並被銷毀,留下對無效對象的引用。 這意味着你必須以艱難的方式做到這一點:
實現所有三個特殊成員函數:
// destructor
~List()
{
while (head)
{
Node<T> * temp = head->next;
delete head;
head = temp;
}
}
// copy constructor
List(const List & src): head(NULL), length(src.length)
{
Node<T> ** destpp = &head; // point at the head.
// using a pointer to a pointer because it hides
// the single difference between head and a Node's
// next member: their name. This means we don't need
// any special cases for handling the head. It's just
// another pointer to a Node.
Node<T> * srcnodep = src.head;
while (srcnodep) // while there is a next node in the source list
{
*destpp = new Node<T>{srcnodep->data, NULL}; // copy the node and store it at
// destination
destpp = &((*destpp)->next); // advance destination to new node
srcnodep = srcnodep->next; // advance to next node in source list
}
}
List& operator=(List src) // pass list by value. It will be copied
{
length = src.length; // Technically we should swap this, but the copy
// is gonna DIE real soon.
// swap the node lists. use std::swap if you can.
Node<T> * temp = head;
head = src.head;
src.head = temp;
// now this list has the copy's Node list and the copy can go out of scope
// and destroy the list that was in this List.
return *this;
}
operator=
正在利用Copy and Swap Idiom 。 它通常不是最快的解決方案,但它很容易編寫並且幾乎不可能出錯。 我從復制和交換開始,只有在分析代碼的性能表明我必須這樣做時才遷移到更快的東西,而且這幾乎從未發生過。
復制構造函數中使用的指針到指針技巧在插入和刪除列表項時也非常方便。
了解並理解三定律及其朋友。 沒有它,您就無法編寫復雜而高效的 C++ 程序。 可能至少部分是為了迫使你學習它。
如果你可以選擇你實現的列表類型,這里有一個類似版本的向量(與你的鏈表相比)
#include <cstdlib>
#include <iostream>
template <typename T>
struct list final {
T* values;
std::size_t capacity, size;
list() : values{nullptr}, capacity{0u}, size{0u} {}
~list() { std::free(values); }
void push_back(T value) {
if (size == capacity) {
capacity = (capacity + 1) * 2;
if (void* mem = std::realloc(values, capacity * sizeof(T))) {
values = reinterpret_cast<T*>(mem);
} else {
throw std::bad_alloc();
}
}
values[size++] = value;
}
void pop_back() { --size; }
};
int main() {
list<int> integers;
for (int i = 0; i < 10; ++i) {
integers.push_back(i);
}
for (int i = 0; i < integers.size; ++i) {
std::cout << integers.values[i] << std::endl;
}
}
好的,我找到了我的問題所在,我做了一個非常骯臟的修復,但它有效。 對於那些告訴我更關心代碼的人,我會在考試結束后立即改進它,但由於截止日期如此嚴格,我只需要最快的解決方案。 你可以在下面找到它。
我將此功能添加到列表中:
void clear() {
while (head) {
Node<T> * temp = head->next;
delete head;
head = temp;
length--;
}
}
然后在 replaceCites 函數的末尾我調用:
data.clear();
references.clear();
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