c++ - 带有模板函数的未解析外部符号

Question

我在网上搜索过，但还没有找到关于为什么会出现此错误的答案:

Error 1 error LNK2019: unresolved external symbol "public: class Mesh * __thiscall AssetManager::GetAsset(class std::basic_string,class std::allocator >)" (??$GetAsset@PAVMesh@@@AssetManager@@QAEPAVMesh@@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function "public: void __thiscall SceneManager::AddMesh(class std::basic_string,class std::allocator >)" (?AddMesh@SceneManager@@QAEXV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) C:\Users\Dirk\documents\visual studio 2010\Projects\OpenGameEngine\OpenGameEngine\SceneManager.obj

这是我的代码:

AssetManager.h

#pragma once
#include <string>
#include <map>
#include "Asset.h"
#include "Mesh.h"

using namespace std;

class AssetManager
{
    public:
        AssetManager(string rootFolder);
        bool LoadAsset(string assetName, int assetType, string assetFile, bool subDirectory);
        void UnloadAsset(string assetName);
        template <class T> T GetAsset(string assetName);
        bool AddAssetSubDirectory(int assetType, string subDirectory);  

    private:
        string m_rootFolder;
        map<int, string> m_assetSubs;
        map<string, Asset*> m_assets;
};

AssetManager.cpp

#include "AssetManager.h"

AssetManager::AssetManager(string rootFolder)
{
    m_rootFolder = rootFolder;
}

bool AssetManager::AddAssetSubDirectory(int assetType, string subDirectory)
{
    if (m_assetSubs.find(assetType) == m_assetSubs.end())
    {
        m_assetSubs[assetType] = subDirectory;
        return true;
    }
    else
    {
        return false;
    }
}

bool AssetManager::LoadAsset(string assetName, int type, string assetFile, bool subDirectory)
{
    string filePos;
    if (subDirectory)
    {
        filePos = m_rootFolder.append(m_assetSubs[type]).append(assetFile);
    }
    else
    {
        filePos = m_rootFolder.append(assetFile);
    }
    return true;
}

void AssetManager::UnloadAsset(string assetName)
{
    if (m_assets.find(assetName) != m_assets.end())
    {
        m_assets.erase(assetName);
    }
}

template <class T> T AssetManager::GetAsset(string assetName)
{
    if (m_assets.find(assetName) != m_assets.end())
    {
        return m_assets[assetName];
    }
    else
    {
        return null;
    }
}

SceneManager.h

#pragma once
#include <string>
#include <map>
#include "AssetManager.h"

using namespace std;

class SceneManager
{
    public:
    static SceneManager* Instance();
    void AddMesh(string assetName);
    void RemoveMesh(string assetName);
    void Draw();
    void Run();
    void SetAssetManager(AssetManager*);
    void Destroy();

    private:
    SceneManager();
    SceneManager(SceneManager const&);
    ~SceneManager();
    SceneManager& operator=(SceneManager const&){};
    static SceneManager* m_Instance;
    AssetManager *m_assetMgr;

    private:
    map<string, Mesh*> m_staticMeshes;
};

SceneManager.cpp

#include "SceneManager.h"
#include "AssetManager.h"

SceneManager* SceneManager::m_Instance = NULL;

SceneManager::SceneManager()
{
    m_assetMgr = 0;
}

SceneManager::SceneManager(SceneManager const&)
{

}

SceneManager::~SceneManager()
{
    delete m_assetMgr;
    m_assetMgr = 0;
}

void SceneManager::Destroy()
{
    delete m_Instance;
    m_Instance = 0;
}

SceneManager* SceneManager::Instance()
{
    if (!m_Instance)
        m_Instance = new SceneManager();

    return m_Instance;
}

void SceneManager::SetAssetManager(AssetManager *am)
{
    m_assetMgr = am; 
}

void SceneManager::AddMesh(string assetName)
{
    m_assetMgr->GetAsset<Mesh*>(assetName);
}

void SceneManager::RemoveMesh(string assetName)
{
    if (m_staticMeshes.find(assetName) != m_staticMeshes.end())
    {
        m_staticMeshes.erase(assetName);
    }
}

void SceneManager::Draw()
{
    for (map<string, Mesh*>::Iterator it = m_staticMeshes.begin(); it != m_staticMeshes.end(); ++it)
    {
        it->second->Draw();
    }
}

void SceneManager::Run()
{

}

提前感谢您的回复!

Answer 1

C++ 不允许您在头文件中声明模板并在.cpp 中定义它。文件。原因是模板只有在已知模板参数的情况下才能创建，无法提前编译。

要解决您的问题，您需要声明和定义 template <class T> T GetAsset(string assetName)在AssetManager.h文件