C/C++内存布局

1. C 结构体的内存布局

以一个例子来看struct的内存结构


#define NP_FUNC_WRAPPER __attribute__((optimize(0)))struct StructBody
{int first_int_placeholder;int second_int_placeholder;double third_double_placeholder;
};class ClassBody
{public:int first_int_placeholder;int second_int_placeholder;double third_double_placeholder;
};class SecondClass
{public:virtual void function_call(){}private:int first_int_placeholder;int second_int_placeholder;double third_double_placeholder;};#include <cstdio>int main(int argc, char ** argv)
{int s_struct=  sizeof(StructBody);int s_class=  sizeof(ClassBody);int s_v_class=  sizeof(SecondClass);printf("sizeof(StructBody) = %d, sizeof(ClassBody) = %d, sizeof(SecondClass)=%d\n",s_struct, s_class, s_v_class);return 0;
}

使用g++编译器（-O3 -std=c++11 -fno-inline-small-functions）产生的汇编代码为：

.LC0:.string "sizeof(StructBody) = %d, sizeof(ClassBody) = %d, sizeof(SecondClass)=%d\n"
main:sub     rsp, 8mov     ecx, 24mov     edx, 16xor     eax, eaxmov     esi, 16mov     edi, OFFSET FLAT:.LC0call    printfxor     eax, eaxadd     rsp, 8ret

可以看出来，sizeof(StructBody) = 16，这是因为在64位平台，sizeof(int)=4, sizeof(double)=8

2. C++ 类对象的内存布局

同上，
sizeof(ClassBody) = 16，sizeof(SecondClass)=24，相ClassBody， SecondClass多了8bytes，这是因为SecondClass中有一个virtual function，会在class的开始位置创建一个vtable，本质是一个指针，指向一块内存区域，该区域存放虚函数的地址，因此多了8bytes.