c++ - 为什么我需要在分配时用()初始化动态分配的数组？

Question

我有以下代码:

#include <cassert>
#include <cstring>
#include <iostream>

class Line
{
public:
  Line(const char* c)
  {
    len = std::strlen(c);
    text = new char[len + 1]();
    std::strncpy(text, c, len);
  }

  Line& operator=(const Line& rhs)
  {
    if (&rhs == this) return *this;

    delete[] text;
    text = new char[rhs.getLen()]();
    std::strncpy(text, rhs.getText(), rhs.getLen());
    return *this;
  }

  Line(const Line& rhs)
  {
    len = rhs.getLen();
    text = new char[rhs.getLen() + 1]();
    std::strncpy(text, rhs.getText(), len);
  }

  ~Line()
  {
    delete[] text;
  }

  unsigned getLen() const
  {
    return len;
  }

  char* getText() const
  {
    return text;
  }

private:
  char* text;
  unsigned len;
};

// operator overloads

const Line operator*(const Line& lhs, const Line& rhs)
{
  unsigned newLen = lhs.getLen() + rhs.getLen();
  char* newC = new char[newLen + 1]();
  std::strncat(newC, lhs.getText(), lhs.getLen());
  std::strncat(newC, rhs.getText(), rhs.getLen());
  Line line(newC);
  delete[] newC;
  return line;
}

bool operator==(const Line& lhs, const Line& rhs)
{
  return strcmp(lhs.getText(), rhs.getText()) == 0;
}

std::ostream& operator<<(std::ostream& os, const Line& l)
{
  os << l.getText();
  return os;
}

int main(void)
{
  Line l("hello");
  Line l2("hello");
  Line r("world");
  Line x = l * r;

  assert(strcmp("hello", l.getText()) == 0);
  assert(strcmp("helloworld", x.getText()) == 0);
  assert(l == l2);
  assert(!(r == l2));

  std::cout << l << '\n';
  std::cout << r << '\n';
  std::cout << x << '\n';
}

行中没有括号:

    text = new char[len + 1];

我会在行中得到断言

assert(strcmp("helloworld", x.getText()) == 0);

a.out: main.cpp:83: int main(): Assertion `strcmp("hello", l.getText()) == 0' failed.

如果没有断言，我会得到一个 sanitizer 错误:

==29167==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200000eff6 at pc 0x7f0f48d679cb bp 0x7fffb3df3820 sp 0x7fffb3df2fd0
READ of size 7 at 0x60200000eff6 thread T0
    #0 0x7f0f48d679ca in __interceptor_strlen (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x6d9ca)
    #1 0x7f0f48a9f938 in std::basic_ostream<char, std::char_traits<char> >& std::operator<< <std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*) (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xb2938)
    #2 0x40114a in operator<<(std::ostream&, Line const&) /home/XXX/programming/c/XXX/main.cpp:72
    #3 0x401276 in main /home/XXX/programming/c/XXX/main.cpp:88
    #4 0x7f0f48432ec4 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21ec4)
    #5 0x400ed8  (/home/XXX/programming/c/XXX/a.out+0x400ed8)

0x60200000eff6 is located 0 bytes to the right of 6-byte region [0x60200000eff0,0x60200000eff6)
allocated by thread T0 here:
    #0 0x7f0f48d8f30a in operator new[](unsigned long) (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x9530a)
    #1 0x4014fd in Line::Line(char const*) /home/XXX/programming/c/XXX/main.cpp:11
    #2 0x40121d in main /home/XXX/programming/c/XXX/main.cpp:78
    #3 0x7f0f48432ec4 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21ec4)

SUMMARY: AddressSanitizer: heap-buffer-overflow ??:0 __interceptor_strlen
Shadow bytes around the buggy address:
  0x0c047fff9da0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9db0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9dc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa 00 03
=>0x0c047fff9df0: fa fa fd fd fa fa 06 fa fa fa 06 fa fa fa[06]fa
  0x0c047fff9e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
==29167==ABORTING

为什么我需要初始化这个内存？ (相当于使用 std::fill )

Answer 1

您需要这样做，因为您的 strncpy 太短，无法在原始字符串中包含终止零，从而使目标的最后一个元素保持不变。

这也意味着你的程序是未定义的。