在C ++中遍历树期间更改指针

Question

So I am facing a weird issue. I have a tree of A's & am trying to find the min of a specific field within all of the A's. I do not have a vector of ptrs of ALL the nodes within the manager class. I merely have the root nodes stores there. Each node will however have a vector of ptrs of all it's children though.

Now if I call into this using the following:

std::vector<A*> treeRoots; //only keeps the roots of each tree
A* minVal = nullptr;
time_t minTime = std::numeric_limits<time_t>::max();

for(auto root : treeRoots){
    root->findMin(minVal, &minTime);
}

This calls into the following:

void A::findMin(A* minA, time_t* pMinVal){
  //find the smallest amongst the children
  for(auto child : children_){ //children_ is an std::vector<A*>
    child->findMin(minFactor, pMinVal);
  }

  time_t currentNodeData = getValue(); //the value we are comparing
  if(currentNodeData < *pMinVal){
    minA = this;
    *pMinVal  = currentNodeData;
  }
  log()->info("is null? %d",(minA == nullptr));
}

I seem to always get a true on the "is null?" comparison.

If I modify as follows, it seems to work fine:

for(auto root : treeRoots){
    root->findMin(&minVal, &minTime);
}

And call into the following function:

void A::findMin(A** minA, time_t* pMinVal){
  //find the smallest amongst the children
  for(auto child : children_){ //children_ is an std::vector<A*> within each node
    child->findMin(minFactor, pMinVal);
  }

  time_t currentNodeData = getValue(); //the value we are comparing
  if(currentNodeData < *pMinVal){
    *minA = this;
    *pMinVal  = currentNodeData;
  }
  log()->info("is null? %d",(*minA == nullptr));
}

Answer 1

Since you are passing the value of pointer minVal , which was initally zero, it will always return true because original value of minVal will not be modified. if you want to modify the value, use reference to a pointer like:

root->findMin(minval,&time);
then
void findMin(A* &minA,time_t *pminVal)
{
 .....
}

or a pointer to a pointer as you have used in second example.