将结构复制(使用赋值)到并集内部的结构中，从而导致段错误 [英] Copying (using assignment) a structure to a structure inside a union causing seg fault

问题描述

我编写了以下代码:

#include <iostream>
#include <string>
#include <cstring>

struct bar
{
  std::string s3;
  std::string s4;
}Bar;

union foo
{
  char * s1;
  char * s2;
  bar    b1;

  foo(){};
  ~foo(){};
}Foo;


int main ()
{
  foo f1;
  bar b2;

  std::string temp("s3");
  b2.s3 = temp;
  b2.s4 = temp;

  //f1.b1 = b2;                           //-- This Fails (Seg faults)

  /*
    #0  0x00002b9fede74d25 in std::string::_Rep::_M_dispose(std::allocator<char> const&) [clone .part.12] ()
        from /usr/local/lib64/libstdc++.so.6
    #1  0x00002b9fede75f09 in std::string::assign(std::string const&) () from /usr/local/lib64/libstdc++.so.6
    #2  0x0000000000400ed1 in bar::operator= (this=0x7fff3f20ece0) at un.cpp:5
    #3  0x0000000000400cdb in main () at un.cpp:31
  */

  memcpy( &f1.b1, &b2, sizeof(b2) );  //-- This Works 

  std::cout << f1.b1.s3 << " " << f1.b1.s4 << std::endl;
  return 0;
} 

能否请您解释细分错误的原因?我无法解读回溯跟踪中的数据所暗示的含义.

Can you please explain why the segmentation fault ? I am unable to decipher what that the data in the back trace suggests.

推荐答案

union foo无法初始化bar对象(它如何知道要调用哪个成员的初始化程序?)，因此无法初始化std::string s .如果要使用foo内的bar，则需要手动对其进行初始化，就像这样...

union foo can't initialize the bar object (how does it know which member's initializer to call?) and consequently can't initialize the std::strings. If you want to use the bar inside of foo, then you need to manually initialize it, like so...

new (&f1.b1) bar; // Placement new
f1.b1 = b2;
// And later in code you'll have to manually destruct the bar, because
//   foo doesn't know to destruct the bar either...
f1.b1.~bar();

或者，您可以尝试将此功能本身引入联合的构造函数和析构函数中.

Alternatively, you can try to roll this functionality into the union's constructors and destructors yourself.

foo() : b1() {}
// Or you construct like this, which you might need to for a non-trivial union...
// foo() { new (&b1) bar; }  // Placement new.
~foo() { b1.~bar(); }

请注意，复制也需要特殊处理.

Note that copying also needs special handling.

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