c++ - 条件自动变量

Question

void function(typeA* ptr_to_A) {
  if (!ptr_to_A) {
    typeB B; // typeB is a derived class of typeA
    ptr_to_A = &B;
  }
  do_stuff_to_ptr_to_A(ptr_to_A);
  // my hope is that B is still in scope here so that this function will operate on my B object (which is on the stack) which only gets created if ptr_to_A was initially NULL
}

这个函数会做我想做的事(我想让它做的事)吗？也就是说，如果参数是空指针，只在堆栈上分配 B 吗？

Answer 1

Will this function do what I think it does?

不，这是未定义的行为，因为 B 超出范围。由于这是未定义的行为，任何事情都可能发生，因此您无法预测结果。您希望将 B 至少保持在与​​函数调用相同的范围内，因此只需将其移至方法的顶部即可:

void function(typeA* ptr_to_A) {
   typeB B; // typeB is a derived class of typeA
   if (!ptr_to_A) {
      ptr_to_A = &B;
   }
   do_stuff_to_ptr_to_A(ptr_to_A);
}

但是如果你只想在 ptr_to_A 为 null 时分配一个 typeB 那么你可以这样做:

void function(typeA* ptr_to_A) {
  if (!ptr_to_A) {
    typeB B; // typeB is a derived class of typeA
    do_stuff_to_ptr_to_A(&B);
  } else {
    do_stuff_to_ptr_to_A(ptr_to_A);
  }
}