如何在矢量中搜索结构项？

Question

I'm attempting to create an inventory system using a vector implementation, but I seem to be having some troubles. I'm running into issues using a struct I made. NOTE: This isn't actually in a game code, this is a separate Solution I am using to test my knowledge of vectors and structs!

struct aItem
{
    string  itemName;
    int     damage;
};

int main()
{
    aItem healingPotion;
    healingPotion.itemName = "Healing Potion";
    healingPotion.damage= 6;

    aItem fireballPotion;
    fireballPotion.itemName = "Potion of Fiery Balls";
    fireballPotion.damage = -2;

    vector<aItem> inventory;
    inventory.push_back(healingPotion);
    inventory.push_back(healingPotion);
    inventory.push_back(healingPotion);
    inventory.push_back(fireballPotion);

    if(find(inventory.begin(), inventory.end(), fireballPotion) != inventory.end())
                {
                        cout << "Found";
                }

    system("PAUSE");
    return 0;
}

The preceeding code gives me the following error:

1>c:\\program files (x86)\\microsoft visual studio 11.0\\vc\\include\\xutility(3186): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'aItem' (or there is no acceptable conversion)

There is more to the error, if you need it please let me know. I bet it's something small and silly, but I've been thumping at it for over two hours. Thanks in advance!

Answer 1

find looks for something that's equal to the item in the vector. You say you want to search using strings, but you haven't written code for that; it's trying to compare the entire struct. And you haven't written code to compare entire structs, so it's giving you an error.

The simplest solution is to use an explicit loop instead of find .

If you want to find things by string, use the find_if variant and write a predicate function that looks at the string. Or if you want to find things by the entire struct you can define an operator == on the struct that compares both itemName and damage .

Or you might also consider using the map or unordered_map data structures instead of vector . The map containers are designed for fast lookup using a key (such as the string).

Answer 2

The find method does not know how to compare two aItem objects for equality. You need to define the == operator in your struct definition, like this:

bool operator==(aItem other)
{
    if (itemName == other.itemName && damage == other.damage)
        return true;
    else
        return false;
}

This will allow find to determine if two aItem objects are equal, which is necessary for the algorithm to work.

Answer 3

try something like:

#include <iostream>
#include <vector>
using namespace std;
struct item {
    item(string const name,int const damage):name_(name),damage_(damage) {

    }
    string name_;
    int damage_;
};
int main(int argc, char** argv) {
    vector<item *> items;
    item healingPostion("cure light",-10);
    item fireballPostion("fireball",10);
    items.push_back(&healingPostion);
    items.push_back(&fireballPostion);
    if(find(items.begin(), items.end(), &fireballPostion) != items.end()) {
        cout << "Found";
    }
    return 0;
}