C ++：尝试理解对矢量的引用

Question

I am learning C++ and I don't really understand how to pass objects through functions. I read that there were three ways to do it. Passing by value,reference and pointer. I think I am trying to do by reference?

I am pretty sure I am having issues with passing by reference because I tried to test it in the main without calling functions by doing

std::string nameInput = "Penguin";
std::string colorInput = "Black";
Animal temp(nameInput,colorInput);
zoo.addAnimal(temp);
zoo.printDatabase();

And it works out fine. But when I try to call it through functions I am not even able to add it to the vector.Before posting this I tried addAnimal(Animal &toAdd) but I am not sure what is really going on. Because aren't I just passing in the address to the function? And not the object animal itself.

Or maybe I am totally looking at the wrong direction. Which is why I like posting here because having another set of eyes is always nice!

//Zoo.h

#include<vector>
#include<animal.h>

#ifndef ZOO_H_
#define ZOO_H_

class Zoo{
    private:
        std::vector<Animal> database;
    public:
        void addAnimal(animal toAdd);
        void printDatabase();
};

#endif ZOO_H_

//Animal.h

#include<string>

#ifndef ANIMAL_H_
#define ANIMAL_H_

class Animal{
    private:
        std::string name;
        std::string color;

    public:
        Animal(std::string name, std::string color);
        void printInfo();
}

#endif ANIMAL_H_


//Zoo methods
void Zoo::printDatabase(){
    for(std::vector<Animal>::iterator list = database.begin(); list != list.end(); list++){
        (*list).printInfo();
    }
}

void Zoo::addAnimal(Animal toAdd){
    database.push_back(toAdd);
}

//Animal Methods
Animal::Animal(std::string inputName, std::string inputColor){
    name = inputName;
    color = inputColor;
}

void Animal::printInfo(){
    std::cout << "Name: " << name << "\n";
    std::cout << "Color: " << color >> "\n";
}


//main.cpp
int main(){

    Zoo zoo;
    std::string input;
    do{
        printMenu();
        std::getline(std::cin, input);
        if(!input.empty()){
            decide(input, zoo);
        }
    }while(input != "3";
}

void printMenu(){
    std::cout <<"Zoo database\n";
    std::cout << "1.Add  Animal \n";
    std::cout << "2.Print \n";
    std::cout << "3.Exit \n";
}

void decide(std::string input, Zoo zooInput){

    std::string name;
    std::string color;

    if(input == "1"){
        std::cout << "Please enter the name of the animal to add \n";
        std::getline(std::cin,name);
        std::cout << "Please enter the color of the animal \n";
        std::getline(std::cin,color);

        Animal temp(name,color);
        zooInput.addAnimal(temp);
    }

    if(input == "2"){
        zooInput.printDatabase();
    }
}

Answer 1

Not sure if I understand what you're asking but in your decide function, you aren't passing by reference or pointer, but by value.

You want to pass the zoo by reference since you expect the function to modify the zoo passed to it, not make a copy of it and modify that.

Also, addAnimal is fine, because you do want to pass by value here. Not by reference or pointer, because the animal in the calling function is going to get destroyed later.

Answer 2

As already stated in the comment, you want to change your function's signature to:

void decide(std::string input, Zoo& zooInput){

In these lines here:

Animal temp(name,color);
zooInput.addAnimal(temp);

A copy of temp is passed to addAnimal and subsequently copied/moved into zooInput 's vector. Passing it by reference will make no difference because it is copied into the vector anyway.

However, because zooInput is passed by value, changes to it will not be reflected outside the function.

Answer 3

First of all, this should not even compile...

1. You forgot to put ; at the end of Animal class declaration.
2. In main() function, there is no closing ) in while loop, here: while(input != "3";
3. Then you have methods like void addAnimal(animal toAdd); and animal is not declared anywhere, should have been Animal .
4. In your for loop, you have for(std::vector<Animal>::iterator list = database.begin(); list != list.end(); list++){ . Obviously, list is an iterator and it does not have end() method, so list != list.end() is plain wrong.
5. In std::cout << "Color: " << color >> "\\n"; , you use >> "\\n" , which is just wrong.
6. Using std::getline(std::cin, input); and then if(!input.empty()) does not work. It could just get stuck in infinitive loop on EOF .

And finally, back to references. You pass Zoo by value (copy) into your decide() function. Therefore, it adds an animal into its own private copy of Zoo which is destroyed upon leaving the function scope. Therefore, Zoo object defined in main() is never modified. To fix it, change void decide(std::string input, Zoo zooInput) to void decide(std::string input, Zoo& zooInput) .

And here is your program, somewhat fixed:

#include <string>
#include <vector>
#include <iostream>

class Animal {
    std::string name;
    std::string color;

public:
    Animal(std::string name, std::string color);
    void printInfo();
};

class Zoo {
    std::vector<Animal> database;
public:
    void addAnimal(Animal toAdd);
    void printDatabase();
};

void Zoo::printDatabase(){
    for(std::vector<Animal>::iterator list = database.begin(); list != database.end(); list++){
        (*list).printInfo();
    }
}

void Zoo::addAnimal(Animal toAdd){
    database.push_back(toAdd);
}

Animal::Animal(std::string inputName, std::string inputColor){
    name = inputName;
    color = inputColor;
}

void Animal::printInfo(){
    std::cout << "Name: " << name << "\n";
    std::cout << "Color: " << color << "\n";
}

void printMenu(){
    std::cout <<"Zoo database\n";
    std::cout << "1.Add  Animal \n";
    std::cout << "2.Print \n";
    std::cout << "3.Exit \n";
}

void decide(std::string input, Zoo& zooInput) {
    std::string name;
    std::string color;

    if(input == "1"){
        std::cout << "Please enter the name of the animal to add \n";
        std::getline(std::cin,name);
        std::cout << "Please enter the color of the animal \n";
        std::getline(std::cin,color);

        Animal temp(name,color);
        zooInput.addAnimal(temp);
    }

    if(input == "2"){
        zooInput.printDatabase();
    }
}

int main() {
    Zoo zoo;
    std::string input;
    printMenu();
    while (std::getline(std::cin, input)) {
        decide(input, zoo);
        if (input == "3")
            break;
        printMenu();
    }
}

Example run:

$ g++ -Wall -pedantic -std=c++11 -o test ./test.cc  && ./test 
Zoo database
1.Add  Animal 
2.Print 
3.Exit 
1
Please enter the name of the animal to add 
cow
Please enter the color of the animal 
blue
Zoo database
1.Add  Animal 
2.Print 
3.Exit 
2
Name: cow
Color: blue
Zoo database
1.Add  Animal 
2.Print 
3.Exit 
1
Please enter the name of the animal to add 
lobster
Please enter the color of the animal 
green
Zoo database
1.Add  Animal 
2.Print 
3.Exit 
2
Name: cow
Color: blue
Name: lobster
Color: green
Zoo database
1.Add  Animal 
2.Print 
3.Exit 
3
$