如何用std::array替代std::unordered_map并逐个填充指定元素？

问题背景 #

我原本用std::unordered_map存储枚举与纹理数据的映射，但这个map被数千次访问，性能开销占所在函数运行时间的20%。考虑到枚举值可以直接作为数组索引，想换成std::array来提升性能，但填充数组时遇到了"incomplete"错误。需要注意的是，并非所有枚举值都需要对应纹理对象，未设置的元素可以为nullptr，希望能逐个定义数组中的值。

原实现代码 #

Block.h #

enum class BlockType : uint8_t {
    BlockType_Air = 0,
    BlockType_Default = 1,
    BlockType_Grass = 2,
    BlockCount
};

struct BlockTextureData {
    std::string top;
    std::string bottom;
    std::string left;
    std::string right;
    std::string front;
    std::string back;
};

extern std::unordered_map<BlockType, BlockTextureData*> BlockTextures;

Block.cpp #

std::unordered_map<BlockType, BlockTextureData*> BlockTextures = {
    {BlockType::BlockType_Default, new BlockTextureData{"UV","UV","UV","UV","UV", "UV"}},
    {BlockType::BlockType_Grass, new BlockTextureData{"GrassTop","GrassBottom","GrassSide","GrassSide","GrassSide", "GrassSide"}}
};

错误的尝试代码 #

Block.h #

extern std::array<BlockTextureData*, static_cast<size_t>(BlockType::BlockCount)> BlockTextures;

Block.cpp #

std::array<BlockTextureData*, static_cast<size_t>(BlockType::BlockCount)> BlockTextures[static_cast<size_t>(BlockType::BlockType_Default)] = new BlockTextureData{"UV","UV","UV","UV","UV", "UV"};

std::array<BlockTextureData*, static_cast<size_t>(BlockType::BlockCount)> BlockTextures[static_cast<size_t>(BlockType::BlockType_Grass)] = new BlockTextureData{"GrassTop","GrassBottom","GrassSide","GrassSide","GrassSide", "GrassSide"};

正确的实现方式 #

你的错误在于重复定义了BlockTextures，并且错误地将数组当作二维数组来赋值。正确的做法是先创建一个大小为BlockCount的数组，默认初始化所有元素为nullptr，再通过枚举值转换的索引逐个赋值。

修正后的代码 #

Block.h

#include <array>
#include <string>

enum class BlockType : uint8_t {
    BlockType_Air = 0,
    BlockType_Default = 1,
    BlockType_Grass = 2,
    BlockCount
};

struct BlockTextureData {
    std::string top;
    std::string bottom;
    std::string left;
    std::string right;
    std::string front;
    std::string back;
};

// 声明全局数组，大小由枚举值BlockCount确定
extern std::array<BlockTextureData*, static_cast<size_t>(BlockType::BlockCount)> BlockTextures;

Block.cpp

#include "Block.h"
#include <array>

// 初始化数组，所有元素默认值为nullptr
std::array<BlockTextureData*, static_cast<size_t>(BlockType::BlockCount)> BlockTextures{};

// 初始化需要的元素，可放在初始化函数中
void initBlockTextures() {
    // 将枚举值转为size_t作为数组索引
    auto defaultIdx = static_cast<size_t>(BlockType::BlockType_Default);
    BlockTextures[defaultIdx] = new BlockTextureData{"UV","UV","UV","UV","UV", "UV"};

    auto grassIdx = static_cast<size_t>(BlockType::BlockType_Grass);
    BlockTextures[grassIdx] = new BlockTextureData{"GrassTop","GrassBottom","GrassSide","GrassSide","GrassSide", "GrassSide"};
}

// 确保初始化函数在程序启动时执行（比如用全局初始化器）
namespace {
    struct Initializer {
        Initializer() {
            initBlockTextures();
        }
    } init;
}

补充说明 #

1. 内存管理：如果使用new创建BlockTextureData对象，记得在程序退出时遍历数组，对非nullptr的元素执行delete，避免内存泄漏。
2. 访问方式：后续访问时直接用BlockTextures[static_cast<size_t>(BlockType::XXX)]即可，比unordered_map的查找快得多。

内容的提问来源于stack exchange，提问作者Marko Taht