c++ - cout 覆盖了我的一些指针

Question

我有一个奇怪的问题，一个简单的 cout 弄乱了我之前定义的指针。这是代码:

union Value {
    bool a;
    long long val;
    int next;
};

struct Clos { //not a "closure" in its actual sense.
    vector<Value**> args;
    function<void()> method;
};

Clos* prepare() {
  Clos* closure = new Clos();

  Value* a = nullptr; //these values do not exist yet, but I need these pointers to rename them at RT
  Value* b = nullptr;
  Value* out = new Value; //this exists
  closure->args.push_back(&a); //save adresses to rename them later
  closure->args.push_back(&b);
  closure->method = [&a, &b, &out](){out->val = a->val + b->val;}; //just some operation on arguments
  return closure;
}

在这里，我创建了一个带有绑定(bind)函数(一种“方法”)的对象“闭包”，它使用尚未定义的指针作为参数，稍后将在运行时绑定(bind)。

后来:

 int main(void) {
  Clos* clos = prepare();
  Value a; //now we get input values at RT
  a.val = 7;
  Value b;
  b.val = 8;
  *clos->args[0] = &a; //we bind them to previously "dangling" pointers
  *clos->args[1] = &b;
  cout << "WOLOLOLOLO"; //<<<<---- COMMENT OUT THIS LINE AND BOOM!
  clos->method(); //this works, as long cout is not called 
}

我最初定义 a 和 b(作为空指针？)的方式有问题吗？它们会被释放还是什么？我试过使它们“静态”，但它也不起作用。总的来说有些问题:(

Answer 1

主要问题在这里:

closure->method = [&a, &b, &out](){out->val = a->val + b->val;};

这个 lambda 正在创建一个引用局部变量的闭包。一旦变量超出范围，这个闭包就不再可调用。我想你想要更像这样的东西:

#include <vector>
#include <iostream>
#include <functional>

using std::vector;
using std::function;
using std::cout;


union Value {
  bool a;
  long long val;
  int next;
};

struct Clos { //not a "closure" in its actual sense.
  vector<Value*> args;
  function<void()> method;

  ~Clos()
  {
    for (auto &arg : args) {
      delete arg;
    }
  }
};

Clos* prepare() {
  Clos* closure = new Clos();

  Value* a = new Value;
  Value* b = new Value;
  Value* out = new Value; //this exists
  closure->args.push_back(a);
  closure->args.push_back(b);
  closure->args.push_back(out);

  // Make copies of the pointers, not references.
  closure->method = [a, b, out](){out->val = a->val + b->val;};

  return closure;
}

int main(void) {
  Clos* clos = prepare();
  Value a; a.val = 7;
  Value b; b.val = 8;
  *clos->args[0] = a;
  *clos->args[1] = b;
  clos->method();
  cout << clos->args[2]->val << "\n";

  delete clos;
}