Valgrind：来自复制构造函数的大小为 8 的无效写入

Question

我正在使用 valgrind 尝试修复我正在处理的作业中的许多内存泄漏，它给了我一些不同的读/写/未初始化值错误。我认为我知道根据 valgrind 输出它来自哪里，但无法弄清楚如何在我的一生中修复它。我对 C++ 很陌生，所以我可能只是在为数组分配内存的方式（然后尝试访问错误分配的内存）做一些完全不正确的事情，但我不知道到底是什么会的。

这是各种 valgrind 输出：

Invalid write of size 8
==13371==    at 0x4013F5: family::setFriends(char**) (family.cpp:62)
==13371==    by 0x401231: family::family(family const&) (family.cpp:31)
==13371==    by 0x402358: hashtable::node::node(family const&) (hashtable.h:29)
==13371==    by 0x401E81: hashtable::insert(char const*, family const&) (hashtable.cpp:87)
==13371==    by 0x4018CD: familymgr::addFamily(family&) (familymgr.cpp:15)
==13371==    by 0x402779: main (housinghelper.cpp:86)
==13371==  Address 0x5ad1810 is 0 bytes inside a block of size 32 free'd
==13371==    at 0x4C2F650: operator delete[](void*) (vg_replace_malloc.c:621)
==13371==    by 0x4013B5: family::setFriends(char**) (family.cpp:60)
==13371==    by 0x401231: family::family(family const&) (family.cpp:31)
==13371==    by 0x402358: hashtable::node::node(family const&) (hashtable.h:29)
==13371==    by 0x401E81: hashtable::insert(char const*, family const&) (hashtable.cpp:87)
==13371==    by 0x4018CD: familymgr::addFamily(family&) (familymgr.cpp:15)
==13371==    by 0x402779: main (housinghelper.cpp:86)
==13371==  Block was alloc'd at
==13371==    at 0x4C2E8BB: operator new[](unsigned long) (vg_replace_malloc.c:423)
==13371==    by 0x40120F: family::family(family const&) (family.cpp:29)
==13371==    by 0x402358: hashtable::node::node(family const&) (hashtable.h:29)
==13371==    by 0x401E81: hashtable::insert(char const*, family const&) (hashtable.cpp:87)
==13371==    by 0x4018CD: familymgr::addFamily(family&) (familymgr.cpp:15)
==13371==    by 0x402779: main (housinghelper.cpp:86)

未初始化值消息：

Use of uninitialised value of size 8
==13371==    at 0x401E98: hashtable::insert(char const*, family const&) (hashtable.cpp:90)
==13371==    by 0x4018CD: familymgr::addFamily(family&) (familymgr.cpp:15)
==13371==    by 0x402779: main (housinghelper.cpp:86)
==13371==  Uninitialised value was created by a stack allocation
==13371==    at 0x401882: familymgr::addFamily(family&) (familymgr.cpp:11)
==13371== 
==13371== Use of uninitialised value of size 8
==13371==    at 0x401EB9: hashtable::insert(char const*, family const&) (hashtable.cpp:91)
==13371==    by 0x4018CD: familymgr::addFamily(family&) (familymgr.cpp:15)
==13371==    by 0x402779: main (housinghelper.cpp:86)
==13371==  Uninitialised value was created by a stack allocation
==13371==    at 0x401882: familymgr::addFamily(family&) (familymgr.cpp:11)

'friends' 是一个 char** 成员变量，在头文件中声明如下：

char **friends;

一切似乎都源于复制构造函数：

family::family(const family &fam) : ID(NULL), famName(NULL), 
friends(NULL){
    setID(fam.ID); 
    setFamName(fam.famName);
    setMembers(fam.members);
    setCapacity(fam.capacity);
    setNumFriends(fam.numFriends);

    setFriends(fam.friends);
}

这是主要的构造函数，以防我一开始就没有为friends正确分配内存：

    family::family(char *ID, char *famName, int members) :
    ID(NULL),
    famName(NULL),
    members(members),
    numFriends(-1),
    capacity(DEFAULT_CAPACITY)
{
    friends = new char*[capacity];
    for (int i = 0; i < numFriends; i++)
        friends[i] = NULL;

    setID(ID);
    setFamName(famName);
}

这里是被引用的setFriends 函数：

void family::setFriends(char** friendIn){
friends = new char*[sizeof(friendIn[0])/numFriends];
if(friends!=NULL)
    delete [] friends;
for (int i = 0; i < capacity;i++){
    this->friends[i] = friendIn[i];
    }
}

bool familymgr::addFamily(family &inputFam) {
    char fam[100];
    inputFam.getID(fam);

    table->insert(fam, inputFam);
}

获取ID：

void family::getID(char *id) const {
    strcpy(id, this->ID);
}

我在这里做错了什么来产生所有这些错误？

Answer 1

您的 setFriends 方法写入不属于您的内存：

void family::setFriends(char** friendIn){
    friends = new char*[sizeof(friendIn[0])/numFriends];
    if(friends!=NULL) // <-- This will always be true, since friends was
                      //     assigned a non-null value in the new[]
                      //     expression above
        delete [] friends;  // <-- Here you free the memory you just allocated
    for (int i = 0; i < capacity;i++){
        // At this point, friends is no longer pointing to a valid object
        // so you are trying to write to memory you don't own
        this->friends[i] = friendIn[i];
    }
}

您应该反转对NULL 和new[] 表达式的检查。此外，使用sizeof(friendIn[0])/numFriends 毫无意义。 sizeof(friendIn[0] 将始终为 4 或 8，具体取决于您平台的位数。我猜应该只是capacity:

void family::setFriends(char** friendIn){
    if(friends != nullptr) {
        delete [] friends;
    }
    friends = new char*[capacity];
    for (int i = 0; i < capacity;i++){
        this->friends[i] = friendIn[i];
    }
}

当从复制构造函数调用时，这将起作用，但请记住，您仍在制作friends 的浅表副本。两个family 对象将指向相同的字符串，如果您稍后delete[] 这些字符串，那么任何仍然指向它们的对象都将不起作用。你真的应该把friends 变成std::vector<std::string>>，而不是做所有这些手动内存管理。然后你可以让setFriends 更简单、更安全：

void family::setFriends(std::vector<std::string>> friendIn) {
    friends = std::move(friendIn);
}

Answer 2

Invalid Write 来自setFriends，您删除分配给friends 的内存然后写入它。您需要在setFriends 中进行新分配，然后才能复制到朋友中。

出现未初始化的值消息是因为您在 family 构造函数中有两个名为 ID 的变量：参数和类成员。当您调用setID(ID) 时，这是引用类成员（值为NULL），不是参数ID。将参数重命名为与类中的字段不同的名称。