标签：des style blog http color io 使用 ar for
数据结构基础课的教材使用C来实现数据结构，上学期看别的书用C++实现时还没什么感觉，一对比就发现C++的类和模板用起来比C方便多了。
在写四则运算计算器的时候，int写一遍，char还得写一遍感觉十分痛苦，百度了一下有没有别的解决办法，发现了下面这篇文章。
如何利用堆栈及逆波兰表达式进行数学四则运算：http://www.xuebuyuan.com/1602553.html
关键是在于理解 void 指针与一般指针的不同之处是它只保存数据的首地址，而不知道也不关心数据究竟有多少个字节。
因此利用 memcpy 可以无视数据类型进行赋值，这就让我们能够创建多种类型的Stack。
由于 memcpy 的参数是 void 指针，所以Push和Pop的都只接受地址作为参数，并且直接将改变应用于相应的内存单元，并不返回值。
此外，此处实现的Top指针也和一般的Stack定义不同，此处的Top指针总是指向下一个可用的内存单元。
仿照着写了一个Stack的实现如下：
1 // stack.h 2 // This stack is implemented by void pointer, so it is GENERIC. 3 // 4 // How the void pointer allows assignments of different data types: 5 // first, a normal pointer is binded to a data type, so it knows the number of bytes of the 6 // data it points to. second, it has an address, which is the start-address of the data. 7 // so they can tell the compiler where to start and where to stop accessing the data. 8 // 9 // but void pointer is binded to no data type. it only knows the starting-address. 10 // so it can be forcely assigned to any pointers, thus allowing assignments of all data types. 11 // so what we do is: when we create the stack, we MALLOC a block of free space, and we take a 12 // integer, the typesize, which specifies the length of the data. 13 // We use the function MEMCPY, which directly access the addresses,to push the data in the stack 14 // and pop data out of the stack. 15 // 16 // The difference from the definiton: 17 // the stack pointer here is different from the definition. 18 // when the stack is empty, Top == base. 19 // when an element is pushed in, Top points to the address after the last byte of the element. 20 // so the Top always points to the next available space. 21 22 #include<stdio.h> 23 #include<stdlib.h> 24 25 struct Record 26 { 27 void *Top; 28 void *base; 29 int stacksize; 30 size_t typesize; 31 }; 32 33 typedef struct Record *Stack; 34 35 Stack CreateStack(int stacksize, size_t typesize); 36 void Push(Stack S, void *data); 37 void Pop(Stack S, void *data); 38 int IsEmpty(Stack S); 39 void Clear(Stack S); 40 void Destroy(Stack S);
// stack.c #include"stack.h" #include<string.h> Stack CreateStack(int stacksize, size_t typesize) { Stack S = (Stack)malloc(sizeof(struct Record)); S->base = malloc(stacksize*typesize); if(!S->base) { printf("Out of space.\n"); exit(1); } S->Top = S->base; S->stacksize = stacksize; S->typesize = typesize; return S; } void Push( Stack S, void *data) { if( (int)(S->Top - S->base) + S->typesize > S->stacksize ) { printf("Out of space to Push.\n"); exit(1); } memcpy(S->Top, data, S->typesize); S->Top = (void*)( (int)S->Top + S->typesize ); } // GNU defines the arithmetic of void* equals to char(which is defined as byte). // ANSI says we cannot do arithmetic on void* because we don‘t know the type. // so when we update the Top pointer, first regard it as an int, and convert it // back when we are finished. void Pop( Stack S, void *data) { if( S->Top == S->base ) { printf("Error:Popping an empty stack.\n"); exit(1); } S->Top = (void*)( (int)S->Top - S->typesize); memcpy(data, S->Top, S->typesize); } void Clear(Stack S) { S->Top = S->base; } int IsEmpty(Stack S) { return S->Top == S->base; } void Destroy(Stack S) { free(S->base); }
标签：des style blog http color io 使用 ar for
原文地址：http://www.cnblogs.com/jeffo/p/stack-using-void-pointer.html
