您已经做了一个结构示例,看起来像这样:
#include <array>
enum class ItemType
{
Tree,
Bush,
Flower,
Grass,
Mushroom,
CountOfTypes
};
class Items;
class Item
{
public:
Item(Items* parentContainer, ItemType type) : parentContainer(parentContainer), type(type) { DoSomething(); }
ItemType type;
// Parent container needs to be acessed occasionally
Items* parentContainer;
// This cannot be constexpr
void DoSomething();
};
class Items
{
public:
using ItemArray = std::array<Item, ItemType::CountOfTypes>;
// Should contain one item per type, at index equal to that type
ItemArray items;
};
请注意对数组内容的要求,对于items
数组,必须始终为true:
ItemType someType = ... any of item type enum except of CountOfTypes ...;
/// must be true
items[(size_t)someType].type == someType;
此代码将无法编译,因为Item
没有默认构造函数,但我们尝试在Items::Items
中进行分配。即使可以编译,这些值也不会被初始化,我们不希望这样。
解决该问题的一种方法是按正确的顺序手动枚举类型:
class Items
{
public:
Item items[(size_t)ItemType::CountOfTypes] = { {this, ItemType::Tree}, {this, ItemType::Bush}, {this, ItemType::Grass}, {this, ItemType::Mushroom} };
};
另一个选择是添加默认的构造函数,然后调用一些初始化函数。我也不喜欢。
但是我过去已经使用过std::make_integer_sequence
了,我会尝试再做一次。如果上面的代码有效,则应按一定顺序生成它。因此,我尝试了这个:
class Items
{
public:
using ItemArray = std::array<Item, (size_t)ItemType::CountOfTypes>;
template <int... Indices>
static ItemArray GenerateItems(std::integer_sequence<int, Indices...>, Items* parent) { return { ({parent, Indices})... }; }
ItemArray items = GenerateItems(std::make_integer_sequence<int, static_cast<int>(ItemType::CountOfTypes)>{}, this);
};
这是基于我以前使用此功能的情况。我不确定这是否是正确的方法。
我在Visual Studio中遇到的错误是:
1>o:\projects\cpptest\cpptest\cpptest\arraywiththis.cpp(30): error C2760: syntax error: unexpected token '{', expected 'expression'
1>o:\projects\cpptest\cpptest\cpptest\arraywiththis.cpp(30): note: This diagnostic occurred in the compiler generated function 'Items::ItemArray Items::GenerateItems(std::integer_sequence<int,Indices...>,Items *)'
海湾合作委员会更加直言不讳(他们的错误信息在过去6年中得到了不可思议的改善:]
PS O:\projects\cpptest\CPPTest\CPPTest> g++ .\ArrayWithThis.cpp
.\ArrayWithThis.cpp: In static member function 'static Items::ItemArray Items::GenerateItems(std::integer_sequence<int, Indices ...>, Items*)':
.\ArrayWithThis.cpp:30:117: error: expected ';' before '}' token
static ItemArray GenerateItems(std::integer_sequence<int, Indices...>, Items* parent) { return { ({parent, Indices})... }; }
^
;
.\ArrayWithThis.cpp: In instantiation of 'static Items::ItemArray Items::GenerateItems(std::integer_sequence<int, Indices ...>, Items*) [with int ...Indices = {0, 1, 2, 3}; Items::ItemArray = std::array<Item, 4>]':
.\ArrayWithThis.cpp:32:116: required from here
.\ArrayWithThis.cpp:30:123: error: could not convert '{((void)0, 0), ((void)0, 1), ((void)0, 2), ((void)0, 3)}' from '<brace-enclosed initializer list>' to 'Items::ItemArray' {aka 'std::array<Item, 4>'}
static ItemArray GenerateItems(std::integer_sequence<int, Indices...>, Items* parent) { return { ({parent, Indices})... }; }
尤其是在困扰我:((void)0, 0)
为什么无效?
我正在尝试做的可能吗?如果是,该如何解决我的代码?
您有多余的括号;可能是:
class Items
{
public:
static constexpr std::size_t ItemCount = static_cast<std::size_t>(ItemType::CountOfTypes);
using ItemArray = std::array<Item, ItemCount>;
template <std::size_t ... Is>
static ItemArray GenerateItems(std::index_sequence<Is...>, Items* parent)
{ return {{ Item{parent, Indices}... }}; }
ItemArray items = GenerateItems(std::make_index_sequence<ItemCount>{}, this);
};