在std :: string上使用malloc / realloc / free時出現c ++內存錯誤

Question

我寫了這樣一小段代碼：

template <class T>
void
test()
{
    T* ptr = nullptr;

    ptr = (T*)malloc(1 * sizeof(T));

    new ((void*)ptr) T(T());

    ptr = (T*)realloc(ptr, 2 * sizeof(T));

    new ((void*)(ptr + 1)) T(T());

    (ptr)->~T();
    (ptr + 1)->~T();

    free(ptr);
}

struct foo
{
    foo() : ptr(malloc(10)) {}
    ~foo() { free(ptr); } 
    void* ptr;
};

int
main()
{ 
    test<int>(); // this is ok
    test<foo>(); // this is ok
    test<std::string>(); // memory error :(

    return 0;
}; 

當T為[int]或[foo]時，一切正常。 但是將[std :: string]用作T會使valgrind報告如下內存錯誤：

==18184== Memcheck, a memory error detector
==18184== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==18184== Using Valgrind-3.12.0 and LibVEX; rerun with -h for copyright info
==18184== Command: ./a.out
==18184== 
==18184== Invalid free() / delete / delete[] / realloc()
==18184==    at 0x4C2C20A: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==18184==    by 0x401074: void test<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >() (tmp.cpp:26)
==18184==    by 0x400CFC: main (tmp.cpp:44)
==18184==  Address 0x5a89e70 is 16 bytes inside a block of size 32 free'd
==18184==    at 0x4C2CC37: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==18184==    by 0x401042: void test<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >() (tmp.cpp:22)
==18184==    by 0x400CFC: main (tmp.cpp:44)
==18184==  Block was alloc'd at
==18184==    at 0x4C2AB8D: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==18184==    by 0x40100F: void test<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >() (tmp.cpp:18)
==18184==    by 0x400CFC: main (tmp.cpp:44)
==18184== 
==18184== 
==18184== HEAP SUMMARY:
==18184==     in use at exit: 0 bytes in 0 blocks
==18184==   total heap usage: 9 allocs, 10 frees, 72,856 bytes allocated
==18184== 
==18184== All heap blocks were freed -- no leaks are possible
==18184== 
==18184== For counts of detected and suppressed errors, rerun with: -v
==18184== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

為什么只有[std :: string]會導致內存問題，而[foo]在ctor和dtor中也都有malloc / free？

我正在使用g ++ 6.2.1和valgrind 3.12.0

Answer 1

malloc() ， free()和realloc()是C庫函數，它們對C ++類，其構造函數和析構函數絕對一無所知。

您正在使用malloc()和new放置，以使用malloc分配的內存構造std::string 。 這可以。

但是然后，您正在使用realloc()重新分配已分配的內存。

在內存中復制/移動C ++對象必須使用相應對象的復制/移動構造函數來完成。 無法使用realloc()復制/移動內存中的C ++對象。

做到這一點的唯一方法是malloc()一個新的內存塊，使用placement new調用對象的復制/移動構造函數，以便將它們復制/移動到新的內存塊中，最后調用對象的析構函數。舊的內存塊，之后可以對其進行free() ed。

Answer 2

realloc與非POD類型不兼容。

因為它可以在不移動的對象意識到的情況下移動內存中的內容。