C++ 内存泄漏，Valgrind 消息误导？

Question

我对 C++ 有点陌生，我能够无错误地运行我的代码，但是，当我通过 Valgrind 运行它时遇到内存泄漏，而且我一辈子都做不到似乎弄清楚我漏水的地方!这是我的错误消息:

==22902==     in use at exit: 72,728 bytes in 2 blocks
==22902==   total heap usage: 4 allocs, 2 frees, 73,816 bytes allocated
==22902==
==22902== 24 bytes in 1 blocks are definitely lost in loss record 1 of 2
==22902==    at 0x4C2E80F: operator new[](unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==22902==    by 0x401086: Bar::Bar(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, unsigned int) (Bar.cpp:10)
==22902==    by 0x400F76: main (Foo_main.cpp:20)

我有一个类 Foo，它用作 Bar 类的“附件”。 Bar继承自foo。它有两个私有(private)成员id_和nam​​e_，析构函数和use()在头文件中声明为virtual。

#include "Foo.h"

#include<iostream>

using std::cout;
using std::endl;
using std::string;

Foo::Foo(int id, const string& name) :
        id_(id),
        name_(name){}

Foo::~Foo() {}

int Foo::id() const { return id_; }

string Foo::name() const { return name_; }

void Foo::use() const {
    cout << "Using Foo #" << id_ << ", " << name_ << endl;
}

Bar 有两个私有(private)成员，num_attachments_ (unsigned int) 和 attachments_ (Foo**)

#include "Bar.h"
#include <iostream>

using std::cout;
using std::endl;

Bar::Bar(int id, const std::string& name, unsigned int num_attachments) :
        Foo(id, name),
        attachments_(new Foo*[num_attachments]),
        num_attachments_(num_attachments) {
    // explicity null out each entry in the new array
    for (unsigned int i=0; i<num_attachments; ++i) {
        attachments_[i] = NULL;
    }
}

void Bar::use() const {
    cout << "Using Bar #" << id() << endl;
    for (unsigned int i=0; i<num_attachments_; ++i) {
        if (attachments_[i] != NULL) {
            attachments_[i]->use();
        }
    }
}

(注意:一些我知道不会导致泄漏的代码被注释掉了)。我怀疑问题出在 Bar 的 use() 函数中，但我不太确定缺少什么!

最后，这是主要功能:

Foo* f = new Bar(1, "foobar", 3);
f->use();
delete f;

当然，我可以根据要求上传整个程序(尽管我觉得问题可能很明显，我只是完全遗漏了一些东西)。任何帮助都会很棒!

Answer 1

您需要为 Bar 声明并实现一个显式析构函数

内存泄漏可能一定会发生，因为你还没有释放你的属性attachments_的内存，你必须在你的 Bar 中分配构造函数。

它的析构函数应该像这样实现:

Bar::~Bar() {
    for (unsigned int i = 0; i < num_attachments_; i++) {
        if (attachments_[i] != NULL) {
            delete attachments_[i];
        }
    }
    if(attachments_) { // Just a safeguard good practice for defensive programming. You could omit this statement. This if statement is the same as if (attachments_ != NULL)
        delete [] attachments_;
    }
}

此外，由于Bar继承自 Foo ，在 Bar.h 中，您的析构函数 已经声明为虚拟 已经很好，这样派生类析构函数在您的基类之前被调用析构函数。由于派生类具有动态内存，因此您希望发生这种情况。所以在 Bar.h 中:

class Bar: public Foo {
   /* ...other class members... */
   public:
       /* ...other class operations... */
       virtual ~Bar(); // the virtual keyword here forces the program to visit Bar's (the derived class) destructor before Foo's (the base class)destructor. It is necessary, otherwise it only invokes the Foo's destructor (leaving leaked dynamic memory from Bar)
};