Question

我写了一小段这样的代码：

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;
};

当T为[int]或[foo]时，一切正常。但是使用[std :: string]作为T会导致valgrind报告这样的内存错误：

==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)

为什么只有[std :: string]导致内存问题，而[foo]在ctor和amp;中都有malloc / free。 dtor？

我正在使用g ++ 6.2.1和valgrind 3.12.0

Answer 1

malloc()，free()和realloc()是C库函数，它们对C ++类及其构造函数和析构函数一无所知。

您正在使用展示位置malloc()的{{1}}来使用new内存来构建std::string。这很好。

但是，您正在使用malloc来重新分配已分配的内存。

必须使用相应对象的复制/移动构造函数在内存中复制/移动C ++对象。使用realloc()无法在内存中复制/移动C ++对象。

执行此操作的唯一方法是realloc()新的内存块，使用placement malloc()来调用对象的复制/移动构造函数，以便将它们复制/移动到新的内存块中，最后调用旧内存块中对象的析构函数，之后可以new - 编辑。

Answer 2

null与非POD类型不兼容。

因为它可以在内存中移动内容而移动的对象无法识别它。

在std :: string上使用malloc / realloc / free时出现c ++内存错误

2 个答案: