与内存对齐有关的sizeof_内存对齐对sizeof影响-优快云博客

本文通过多个实例演示了不同编译器指令下结构体内存对齐的原理与效果，包括默认对齐、#pragmapack(1)、#pragmapack(2)和#pragmapack(3)的情况。

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内存对齐，大家应该都听说过，我们直接来看程序：

#include <iostream>
using namespace std;

struct 
{
	int a;
	short b;
}A;

struct 
{
	short b;
	int a;
}B;

struct 
{
	short b;
	short c;
	int a;
}C;

struct 
{
	bool b;
	bool c;
	int a;
}D;

struct 
{
	bool c;
	int a;
	bool b;
}E;

struct 
{
	double a;
	int b;
	float c;
	char d;
	char f;
}F;

int main()
{
	cout << sizeof(A) << endl; // 8
	cout << sizeof(B) << endl; // 8
	cout << sizeof(C) << endl; // 8
	cout << sizeof(D) << endl; // 8
	cout << sizeof(E) << endl; // 12
	cout << sizeof(F) << endl; // 24

	return 0;
}

继续看：

#include <iostream>
using namespace std;

#pragma pack(1)

struct 
{
	int a;
	short b;
}A;

struct 
{
	short b;
	int a;
}B;

struct 
{
	short b;
	short c;
	int a;
}C;

struct 
{
	bool b;
	bool c;
	int a;
}D;

struct 
{
	bool c;
	int a;
	bool b;
}E;

struct 
{
	double a;
	int b;
	float c;
	char d;
	char f;
}F;

int main()
{
	cout << sizeof(A) << endl; // 6
	cout << sizeof(B) << endl; // 6
	cout << sizeof(C) << endl; // 8
	cout << sizeof(D) << endl; // 6
	cout << sizeof(E) << endl; // 6
	cout << sizeof(F) << endl; // 18

	return 0;
}

继续看：

#include <iostream>
using namespace std;

#pragma pack(2)

struct 
{
	int a;
	short b;
}A;

struct 
{
	short b;
	int a;
}B;

struct 
{
	short b;
	short c;
	int a;
}C;

struct 
{
	bool b;
	bool c;
	int a;
}D;

struct 
{
	bool c;
	int a;
	bool b;
}E;

struct 
{
	double a;
	int b;
	float c;
	char d;
	char f;
}F;

int main()
{
	cout << sizeof(A) << endl; // 6
	cout << sizeof(B) << endl; // 6
	cout << sizeof(C) << endl; // 8
	cout << sizeof(D) << endl; // 6
	cout << sizeof(E) << endl; // 8
	cout << sizeof(F) << endl; // 18

	return 0;
}

继续看：

#include <iostream>
using namespace std;

#pragma pack(3)

struct 
{
	int a;
	short b;
}A;

struct 
{
	short b;
	int a;
}B;

struct 
{
	short b;
	short c;
	int a;
}C;

struct 
{
	bool b;
	bool c;
	int a;
}D;

struct 
{
	bool c;
	int a;
	bool b;
}E;

struct 
{
	double a;
	int b;
	float c;
	char d;
	char f;
}F;

int main()
{
	cout << sizeof(A) << endl; // 8
	cout << sizeof(B) << endl; // 8
	cout << sizeof(C) << endl; // 8
	cout << sizeof(D) << endl; // 8
	cout << sizeof(E) << endl; // 12
	cout << sizeof(F) << endl; // 24

	return 0;
}

此时，我们得到 warning C4086: expected pragma parameter to be '1', '2', '4', '8', or '16'

好吧，消化了上面的程序，内存对其基本算是入门了。