Valgrind错误解引用std::vector返回的地址

为什么valgrind会为这段代码返回错误？

#include <iostream>
#include <vector>

int main()
{
    std::vector<int> vec;
    int *ptr;

    for (int i = 0; i < 1000; i++)
    {
        vec.push_back(i);
        if (i == 100)
        {
            ptr = &vec[i];
        }
    }
    std::cout << ptr << "n";  // Print address of -> Ok
    std::cout << *ptr << "n"; // Print content of -> Ok but with a valgrind error
}

编译： g++ -Wall -Wpedantic -O0 -o demo demo.cpp

valgrind 错误是：

==3982== Invalid read of size 4
==3982==    at 0x1093A7: main (in /home/david/demo)
==3982==  Address 0x4dae1e0 is 400 bytes inside a block of size 512 free'd
==3982==    at 0x483E1CF: operator delete(void*, unsigned long) (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==3982==    by 0x109EB4: __gnu_cxx::new_allocator<int>::deallocate(int*, unsigned long) (in /home/david/demo)
==3982==    by 0x109B5B: std::allocator_traits<std::allocator<int> >::deallocate(std::allocator<int>&, int*, unsigned long) (in /home/david/demo)
==3982==    by 0x10970F: std::_Vector_base<int, std::allocator<int> >::_M_deallocate(int*, unsigned long) (in /home/david/demo)
==3982==    by 0x109A3B: void std::vector<int, std::allocator<int> >::_M_realloc_insert<int const&>(__gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, int const&) (in /home/david/demo)
==3982==    by 0x10964F: std::vector<int, std::allocator<int> >::push_back(int const&) (in /home/david/demo)
==3982==    by 0x109354: main (in /home/david/demo)
==3982==  Block was alloc'd at
==3982==    at 0x483CE63: operator new(unsigned long) (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==3982==    by 0x10A0FF: __gnu_cxx::new_allocator<int>::allocate(unsigned long, void const*) (in /home/david/demo)
==3982==    by 0x109F73: std::allocator_traits<std::allocator<int> >::allocate(std::allocator<int>&, unsigned long) (in /home/david/demo)
==3982==    by 0x109DAD: std::_Vector_base<int, std::allocator<int> >::_M_allocate(unsigned long) (in /home/david/demo)
==3982==    by 0x1098BC: void std::vector<int, std::allocator<int> >::_M_realloc_insert<int const&>(__gnu_cxx::__normal_iterator<int*, std::vector<int, std::allocator<int> > >, int const&) (in /home/david/demo)
==3982==    by 0x10964F: std::vector<int, std::allocator<int> >::push_back(int const&) (in /home/david/demo)
==3982==    by 0x109354: main (in /home/david/demo)
==3982== 

这让我怀疑取消引用向量返回的地址是否具有未定义的行为，这是合法的代码吗？

回答