c++中的内存对齐是什么？

class A
{
    int a;
    char d;
};

// 创建给定类型对象大小满足对齐要求的未初始化内存块，在一个内存对齐的缓冲区上创建对象
// C++11后可以这样操作
void align_cpp11_after()
{
    static std::aligned_storage<sizeof(A),
                                alignof(A)>::type data;
    A *attr = new (&data) A;
}

// C++11之前
void align_cpp11_before()
{
    static char data[sizeof(void *) + sizeof(A)];
    const uintptr_t kAlign = sizeof(void *) - 1;
    char *align_ptr =
        reinterpret_cast<char *>(reinterpret_cast<uintptr_t>(data + kAlign) & ~kAlign);
    A *attr = new (align_ptr) A;
}
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 template<std::size_t _Len>
    struct __aligned_storage_msa
    {
      union __type
      {
    unsigned char __data[_Len];
    struct __attribute__((__aligned__)) { } __align;
      };
    };

  /**
   *  @brief Alignment type.
   *
   *  The value of _Align is a default-alignment which shall be the
   *  most stringent alignment requirement for any C++ object type
   *  whose size is no greater than _Len (3.9). The member typedef
   *  type shall be a POD type suitable for use as uninitialized
   *  storage for any object whose size is at most _Len and whose
   *  alignment is a divisor of _Align.
  */
  template<std::size_t _Len, std::size_t _Align =
       __alignof__(typename __aligned_storage_msa<_Len>::__type)>
    struct aligned_storage
    {
      union type
      {
    unsigned char __data[_Len];
    struct __attribute__((__aligned__((_Align)))) { } __align;
      };
    };
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#include <stdio.h>
// g++空结构体的内存大小为1，需要分配1字节用于占位，C++编译器不允许对象为0长度，无法获取地址等
// gcc中为0
struct A1
{
};

struct A2
{
    ;
};

struct A3
{
    char a;               // 1
    int b;                // 4
    unsigned short c;     // 2
    long d;               // 8
    unsigned long long e; // 8
    char f;               // 1
};
// A3大小为1+(3)+4+2+(6)+8+8+1+(7)=40，括号内是为了内存对齐加的padding大小

struct A4
{
    int b;
    char a;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
};
// A4大小为4+1+(1)+2+8+8+1+(7)=32

//pragma pack(n)
//alignment must be a power of two
#pragma pack(2) //指定按两字节对齐
struct B1
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
};
// B1大小为1+(1)+4+2+8+8+1+(1)=26
#pragma pack() //取消指定对齐

#pragma pack(4)
struct B2
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
};
// B2大小为1+(3)+4+2+(2)+8+8+1+(3)=32
#pragma pack()

#pragma pack(8)
struct B3
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
};
// B3大小为1+(3)+4+2+(6)+8+8+1+(7)=40
#pragma pack()

#pragma pack(16)
struct B4
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
};
// B4大小为1+(3)+4+2+(6)+8+8+1+(7)=40
#pragma pack()

//__attribute__
struct C1
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
} __attribute__((aligned));
// C1大小为1+(3)+4+2+(6)+8+8+1+(15)=48

struct C2
{
    char a;
    int b;
    unsigned short c;
    //long d;
    //unsigned long long e;
    char f;
} __attribute__((aligned));
// C2大小为1+(3)+4+2+(6)=16

struct C3
{
    char a;
    int b;
    unsigned short c;
    long d;
    //unsigned long long e;
    char f;
} __attribute__((aligned));
// C3大小为1+(3)+4+2+(6)+8+(8)=32

struct C4
{
    char a;
    //int b;
    unsigned short c;
    //long d;
    unsigned long long e;
    char f;
} __attribute__((aligned));
// C4大小为1+(1)+2+(4)+8+1+(15)=32

struct C5
{
    char a;
    //int b;
    //unsigned short c;
    //long d;
    //unsigned long long e;
    //char f;
} __attribute__((aligned));
// C5大小为1+(15)=16

struct D1
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
} __attribute__((aligned(1)));
// D1大小为1+(3)+4+2+(6)+8+8+1+(7)=40

struct D2
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
} __attribute__((aligned(4)));
// D2大小为1+(3)+4+2+(6)+8+8+1+(7)=40

struct D3
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
} __attribute__((aligned(8)));
// D3大小为1+(3)+4+2+(6)+8+8+1+(7)=40

struct D4
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
} __attribute__((aligned(16)));
// D4大小为1+(3)+4+2+(6)+8+8+1+(15)=48

struct E1
{
    char a;
    int b;
    unsigned short c;
    long d;
    unsigned long long e;
    char f;
} __attribute__((packed));
// E1大小为1+4+2+8+8+1=24

// __attribute__((packed))告诉编译器取消编译中的内存对齐优化，采用实际占用的字节数进行对齐
// __attribute__((aligned(N))) 告诉编译器在编译过程中按照N字节对齐，经过测试这个N只有大于结构体中最大的变量的大小才有用
// __attribute__((aligned)) 后面不接数字，告诉编译器根据目标机制采用最大最有益的方式对齐，基本上就是16字节对齐
// alignof(X) 返回某类型的对齐大小，与std::alignment_of类似，这两个功能完全相同，但是std::alignment_of可以用于模板元编程

int main()
{
    printf("sizeof(char):%2d...sizeof(int):%2d...sizeof(ushort):%2d...sizeof(long):%2d...sizeof(ulonglong):%2d\n\r",
           sizeof(char), sizeof(int), sizeof(unsigned short), sizeof(long), sizeof(unsigned long long));

    printf("sizeof(A1):%2d...sizeof(A2):%2d...sizeof(A3):%2d...sizeof(A4):%2d\n\r",
           sizeof(struct A1), sizeof(struct A2), sizeof(struct A3), sizeof(struct A4));

    printf("sizeof(B1):%2d...sizeof(B2):%2d...sizeof(B3):%2d...sizeof(B4):%2d\n\r",
           sizeof(struct B1), sizeof(struct B2), sizeof(struct B3), sizeof(struct B4));

    printf("sizeof(C1):%2d...sizeof(C2):%2d...sizeof(C3):%2d...sizeof(C4):%2d...sizeof(C5):%2d\n\r",
           sizeof(struct C1), sizeof(struct C2), sizeof(struct C3), sizeof(struct C4), sizeof(struct C5));

    printf("sizeof(D1):%2d...sizeof(D2):%2d...sizeof(D3):%2d...sizeof(D4):%2d\n\r",
           sizeof(struct D1), sizeof(struct D2), sizeof(struct D3), sizeof(struct D4));

    printf("sizeof(E1):%2d...\n\r", sizeof(struct E1));

    return 1;
}

g++编译输出结果如下：
sizeof(char): 1...sizeof(int): 4...sizeof(ushort): 2...sizeof(long): 8...sizeof(ulonglong): 8
sizeof(A1): 1...sizeof(A2): 1...sizeof(A3):40...sizeof(A4):32
sizeof(B1):26...sizeof(B2):32...sizeof(B3):40...sizeof(B4):40
sizeof(C1):48...sizeof(C2):16...sizeof(C3):32...sizeof(C4):32...sizeof(C5):16
sizeof(D1):40...sizeof(D2):40...sizeof(D3):40...sizeof(D4):48
sizeof(E1):24...
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#include <iostream>

using namespace std;

class A1
{
    int a;
    int b;
    int c;
    char d;
    void f()
    {
        cout << "f";
    }
};

class A2
{
    int a;
    int b;
    int c;
    char d;
};

class A3
{
    int a;
    int b;
    int c;
    char d;
    virtual void f()
    {
        cout << "f";
    }
};

int main()
{
    cout << "sizeof A1 " << sizeof(A1) << endl;
    cout << "alignof A1 " << alignof(A1) << endl;
    cout << "sizeof A2 " << sizeof(A2) << endl;
    cout << "alignof A2 " << alignof(A2) << endl;
    cout << "sizeof A3 " << sizeof(A3) << endl;
    cout << "alignof A4 " << alignof(A3) << endl;

    return 0;
}

编译运行结果如下：
sizeof A1 16
alignof A1 4
sizeof A2 16
alignof A2 4
sizeof A3 24
alignof A4 8
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