C++ SFML Gamedev 书籍 - 来自 ResourceHolder 的未解析外部符号 class

Question

我有以下三个文件，我找不到其中产生错误的来源：

Main.cpp

#include <SFML/Graphics.hpp>
#include <iostream>

#include "ResourceHolder.h"

namespace Textures
{
    enum ID { Landscape, Airplane, Missile };
}

int main()
{
    //...

    try
    {
        ResourceHolder<sf::Texture, Textures::ID> textures;
        textures.load(Textures::Airplane, "Airplane.png");
    }
    catch (std::runtime_error& e)
    {
        std::cout << "Exception: " << e.what() << std::endl;
    }

    //...
}

ResourceHolder.h

#pragma once

#include <map>
#include <string>
#include <memory>
#include <stdexcept>
#include <cassert>

template <typename Resource, typename Identifier>
class ResourceHolder
{
public:
    void load(Identifier id, const std::string& fileName);

    Resource& get(Identifier id);
    const Resource& get(Identifier id) const;

private:
    void insertResource(Identifier id, std::unique_ptr<Resource> resource);

    std::map<Identifier, std::unique_ptr<Resource>> mResourceMap;
};

ResourceHolder.cpp

#include "ResourceHolder.h"

template <typename Resource, typename Identifier>
void ResourceHolder<Resource, Identifier>::load(Identifier id, const std::string& fileName)
{
    //Create and load resource
    std::unique_ptr<Resource> resource(new Resource());
    if (!resource->loadFromFile(fileName)) {
        throw std::runtime_error("ResourceHolder::load - Failed to load " + fileName);
    }

    //If loading was successful, insert resource to map
    insertResource(id, std::move(resource));
}

template <typename Resource, typename Identifier>
Resource& ResourceHolder<Resource, Identifier>::get(Identifier id)
{
    auto found = mResourcemap.find(id);
    assert(found != mResourceMap.end());

    return *found->second();
}

template <typename Resource, typename Identifier>
void ResourceHolder<Resource, Identifier>::insertResource(Identifier id, std::unique_ptr<Resource> resource)
{
    //Insert and check success
    auto inserted = mResourceMap.insert(std::make_pair(id, std::move(resource)));
    assert(inserted.second);
}

如果我删除 main.cpp 中的 try-catch 组合，代码可以正常编译；但是，如果我把它留在那里，它会给我一个 LNK2019（未解析的外部符号）错误。

此错误的来源是什么，我该如何解决？

Answer 1

您不能在 .cpp 文件中定义模板。它们必须在 header 中定义，以便编译器可以看到实现并生成特定的类.

这里有更好的 question/answer 解释为什么会这样 Why can templates only be implemented in the header file?。

编辑：get函数有什么问题

两件事。

首先是这个auto found = mResourcemap.find(id);。您的地图名称不正确，m 应该是大写 -> mResourceMap.

然后是 return *found->second(); 行。 map迭代器包含一对，第一个和第二个成员不是函数而是数据成员。你应该写 return *found->second;.

我建议您在使用模板之前了解您正在使用的结构。模板的编译错误非常混乱且难以阅读。您也可以制作一个单独的测试程序并制作一个没有模板的资源管理器以更容易地理解您的错误，然后在您的工作资源管理器之上构建模板。

Answer 2

所有其他答案都为您提供了足够的信息，说明为什么您的代码无法编译并且可能无效，这是我前段时间为 SFML 编写的资源管理器，可能对您有用：

HPP 文件：

#ifndef RESOURCEMANAGER_HPP
#define RESOURCEMANAGER_HPP

/************ INCLUDES ***********/
#include <iostream>
#include <map>
#include <vector>
#include <string>
#include <memory>
#include "SFML/Graphics.hpp"
#include "SFML/Audio.hpp"


class ResourceManager
{
private:
    std::map<std::string,std::unique_ptr<sf::Texture>> listImageContainer;
    std::map<std::string,std::pair<std::unique_ptr<sf::SoundBuffer>,std::unique_ptr<sf::Sound>>> listSoundContainer;
    std::map<std::string,std::unique_ptr<sf::Font>> listFontContainer;

public:
    ResourceManager();
    std::unique_ptr<sf::Sound>& LoadSound(const std::string);
    std::unique_ptr<sf::Font>& LoadFont(const std::string);
    std::unique_ptr<sf::Texture>& LoadImage(const std::string);
    ~ResourceManager();
};
#endif

CPP 文件：

#include "ResourceManager.hpp"

ResourceManager::ResourceManager()
{

}

std::unique_ptr<sf::Sound>& ResourceManager::LoadSound(const std::string _fileName)
{
    if (listSoundContainer.find(_fileName) == listSoundContainer.end())
    {
        std::unique_ptr<sf::SoundBuffer> soundBuffer(new sf::SoundBuffer());
        if (soundBuffer->loadFromFile("assets/sound/" + _fileName) != false)
        {
            std::unique_ptr<sf::Sound> sound(new sf::Sound(*soundBuffer));
            listSoundContainer[_fileName] = std::make_pair(std::move(soundBuffer), std::move(sound));
            return listSoundContainer[_fileName].second;
        }
        else
        {
            std::cerr << "Error loading sound..." << std::endl;
        }
    }
    else
    {
        return listSoundContainer[_fileName].second;
    }
}

std::unique_ptr<sf::Font>& ResourceManager::LoadFont(const std::string _fileName)
{
    if (listFontContainer.find(_fileName) == listFontContainer.end())
    {
        std::unique_ptr<sf::Font> font(new sf::Font());
        if (font->loadFromFile("assets/font/" + _fileName)!=false)
        {
            listFontContainer[_fileName] = std::move(font);
            return listFontContainer[_fileName];
        }
        else
        {
            std::cerr << "Error loading font..." << std::endl;
        }
    }
    else
    {
        return listFontContainer[_fileName];
    }
}

std::unique_ptr<sf::Texture>& ResourceManager::LoadImage(const std::string _fileName)
{
    if (listImageContainer.find(_fileName) == listImageContainer.end())
    {
        std::unique_ptr<sf::Texture> texture(new sf::Texture);
        if (texture->loadFromFile("assets/image/" + _fileName)!=false)
        {
            listImageContainer[_fileName] = std::move(texture);
            return listImageContainer[_fileName];
        }
        else
        {
            std::cerr << "Error loading image: " << _fileName << std::endl;
        }
    }
    else
    {
        return listImageContainer[_fileName];
    }
}

ResourceManager::~ResourceManager(){}

使用方法：

ResourceManager resourceManager;
auto& sound = resourceManager.LoadSound("nice.wav");
auto& image = resourceManager.LoadImage("head.png");
auto& sound2 = resourceManager.LoadSound("nice.wav"); //<--- already loaded
sound.play();
etc...

C++ SFML Gamedev 书籍 - 来自 ResourceHolder 的未解析外部符号 class

C++ SFML Gamedev Book - Unresolved External Symbol from ResourceHolder class

c++

compiler-errors

sfml

unresolved-external