Passing a vector pointer and looping it to change its value-CodePudding

I'm trying to pass a vector to a function, loop over it, and modify the values before sending it back, but I'm having a very hard time with the pointer and reference to make it work:

I understand that itr is a pointer. I'm confused about resource on the for loop. I believe it to be a reference, but I keep getting the error:

error: no viable overloaded '='resource = resource value

ACTION test::addto(name player){  
    
  auto itr = player.require_find(wallet.value, USER_NOT_FOUND(wallet));
    
  asset value1 = asset(100, ACTION_SYMBOL);
  asset value2 = asset(100, FOOD_SYMBOL);
    
  changeResourceValue(itr->resources, value1);
  changeResourceValue(itr->resources, value2);
    
  player.modify(itr, _self, [&](auto& p) { 
      p.resources = resources;
  });
}

void changeResourceValue(const vector<asset>* resources, asset value){
   for (auto &resource : *resources){
      if(resource.symbol == value.symbol){
         resource = resource   value;
      }
   }
}

CodePudding user response：

For starters the first parameter of the function changeResourceValue is declared with the qualifier const

void changeResourceValue(const vector<asset>* resources, asset value){

It means that you may not change elements of the vector pointed to by the pointer resources.

So you need at least to remove the qualifier const.

The second problem is that the operator = is not defined for objects of the type asset. Maybe the error message is a consequence that the variable resource is a constant reference.

Also you need to check whether the member function require_find returns a pointer or an iterator in this record

auto itr = player.require_find(wallet.value, USER_NOT_FOUND(wallet));

Old C versions of std::vector defined iterators as pointers. But in new versions of std::vector iterators are not pointers.

So maybe the function should be declared like

void changeResourceValue( std::vector<asset>::iterator resources, asset value);

CodePudding user response：

IIUC, resource is indeed a reference, but to const asset and not asset because you have a (pointer to) const vector. If you want to change a vector, don’t pretend you don’t, i.e. pass a regular, non-const pointer.