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雞同鴨講3，sizeof範例補充

Yang | 2024-08-31 20:31:57 | 巴幣 0 | 人氣 79

接續前一篇，C語言列舉(enum)成員的走訪與避免被誤改，補充紀錄sizeof範例

參考資料：

https://stackoverflow.com/questions/9439240/sizeof-class-with-int-function-virtual-function-in-c

https://stackoverflow.com/questions/34179352/why-the-sizeofd-increased-by-8-bytes-in-vs2015-when-i-derived-d-from-a-virtual

https://stackoverflow.com/questions/22113519/size-of-class-with-virtual-function

class UTestSizeOfRecordWithoutVirtual

{

protected:

template<int N>

struct StaticRec

{

static constexpr int Value = N;

const int GetValue1() const { return Value; };

int GetValue2() { return GetValue1(); };

};

struct Static5: StaticRec<5>

{

const int GetValue3() const { return Value; };

};

struct Static5B: Static5

{

const int GetValue4() const { return GetValue3(); };

};

struct Static5C: Static5B

{

int Value2 = Value;

};

static constexpr size_t SizeOfStaticRec0 = sizeof(StaticRec<0>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRecM1 = sizeof(StaticRec<-1>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRecM5 = sizeof(StaticRec<-5>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRec1 = sizeof(StaticRec<1>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRec5 = sizeof(StaticRec<5>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5 = sizeof(Static5) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5B = sizeof(Static5B) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5BVaule = sizeof(Static5B::Value) == 4 ? 4 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5C = sizeof(Static5C) == 4 ? 4 : throw std::logic_error("");

};

class UTestSizeOfRecordWithVirtual

{

protected:

template<int N>

struct StaticRec

{

static constexpr int Value = N;

const int GetValue1() const { return Value; };

int GetValue2() { return GetValue1(); };

};

struct Static5: StaticRec<5>

{

virtual const int GetValue3() const { return Value; };

};

struct Static5B: Static5

{

virtual const int GetValue4() const { return GetValue3(); };

};

struct Static5C: Static5B

{

int Value2 = Value;

};

static constexpr size_t SizeOfStaticRec0 = sizeof(StaticRec<0>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRecM1 = sizeof(StaticRec<-1>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRecM5 = sizeof(StaticRec<-5>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRec1 = sizeof(StaticRec<1>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStaticRec5 = sizeof(StaticRec<5>) == 1 ? 1 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5 = sizeof(Static5) == 8 ? 8 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5B = sizeof(Static5B) == 8 ? 8 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5BVaule = sizeof(Static5B::Value) == 4 ? 4 : throw std::logic_error("");

static constexpr size_t SizeOfStatic5C = sizeof(Static5C) == 16 ? 16 : throw std::logic_error("");

};

參考資料其實有看沒有懂，但是範例程式碼有無virtual，對於sizeof(Static5C)的影響會很大，難以理解

而且範例程式碼在不同平台(Windows/Linux/AIX7/...？)上結果一樣！很神奇

總之，團隊開發或是有甲乙方不同腳色的專案開發，別認為在改版時，

只是為struct或class內方法加上virtual，或是移除virtual，

不會對其他開發成員造成影響，其實有可能是埋地雷引發災難！

20240908補充

加上pragma pack(1)，sizeof(Static5C)會變成12

pragma pack(8)，sizeof(Static5C)會變成16

#pragma pack(1)

class UTestSizeOfRecordWithVirtualAndPack1