关于使用一个长度为0的数组来实现变长结构体

Terryjia 2008-12-08 10:10:12
我听人家说这种用法不推荐在项目里使用,可能会出错,是这样吗?
还有我定义这个长度为0的数组的时候编译出错,定义一个长度为1的就OK,这是怎么回事?
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nizhaorong 2008-12-11
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顶楼上,直接往后加,若设为1,移植就有问题了
Terryjia 2008-12-10
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UP下
饶鹏 2008-12-10
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就是像iostream那样,直接把所有数据依次用<<运算符输入进内存里,所有数据的长度的和即为总数据长度。
使用的时候把内存首地址往上面那个类里一套,就可以直接用了。要转到下一个对象,就增加一个成员函数:

BYTE* Next() {return (char *)m_pA + sizeof(int) + Len();}

Terryjia 2008-12-10
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[Quote=引用 8 楼 fenghou1st 的回复:]
不要用结构体,用数据流形式就行了。实在想用结构体模式,可以这样:


C/C++ codeclass A
{
BYTE *m_pA;
public:
A(BYTE *pA) : m_pA(pA) {}

int Len() {return *(int *)m_pA;}
char Buf(size_t n = 0) {return *((char *)m_pA + sizeof(int) + n);}
};

int asdf(BYTE *pMem)
{
A aaa(pMem);
int nLen = aaa.Len();
char Buf0 = aaa.Buf();
char Buf100 = aa…
[/Quote]

能说下怎么用数据流吗?谢谢
饶鹏 2008-12-10
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不要用结构体,用数据流形式就行了。实在想用结构体模式,可以这样:

class A
{
BYTE *m_pA;
public:
A(BYTE *pA) : m_pA(pA) {}

int Len() {return *(int *)m_pA;}
char Buf(size_t n = 0) {return *((char *)m_pA + sizeof(int) + n);}
};

int asdf(BYTE *pMem)
{
A aaa(pMem);
int nLen = aaa.Len();
char Buf0 = aaa.Buf();
char Buf100 = aaa.Buf(100);
};
用户 昵称 2008-12-09
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从逻辑上讲,长度若为0,实体就不存在,虽然理论上也说的过去,比如“屋里有0个箱子”。俺倾向于长度至少为1.
Terryjia 2008-12-09
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那么还有什么别的方法实现类似的变长结构体的那?这种需求很多吧,比如通信协议之间的请求N个信息的命令
饶鹏 2008-12-08
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确实容易出错。
比如你现在要申请一个存放结构体实例(其中变长数组长度为N)的内存空间,你要申请多少字节的内存?


struct A
{
int nLen;
char pBuf[1];
};

用VC编译时,如果编译时内存最大对齐模数设为默认,则要申请的内存字节数为:

size_t nMaxAlign = sizeof(结构体中对齐要求最严格的类型);
size_t nVarAlign = sizeof(变长数组元素类型);
size_t nBufSize = sizeof(A) - nMaxAlign + (nVarAlign * N + nMaxAlign - 1) / nMaxAlign * nMaxAlign;

1. 这个结构体中对齐要求最严格的类型是需要人工确定的,因此正确性很难保证。
2. 如果改变最大对齐模数,计算方法又会不同。
3. 如果这个数据要在两个分别编译的模块里使用,那么就很难保证其最大对齐模数设置相同,即很难保证读取到的是正确位置的数据。
wocow3 2008-12-08
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确实有这种写法,不过不具有移植性
比较花哨
ilovedrv 2008-12-08
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这种做法在内核里面多了,有什么问题吗

还有我定义这个长度为0的数组的时候编译出错,定义一个长度为1的就OK,这是怎么回事?

这个跟编译器有关,vc是要用1的
zaodt 2008-12-08
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你说的是不是 C++ 中的 new 和 delete 吧?
The C programming Language 第二版英文版 內容列表 Table of Contents Preface.......................................................... Preface to the first edition..................................... Introduction..................................................... Chapter 1 - A Tutorial Introduction.............................. 1.1 Getting Started................................ 1.2 Variables and Arithmetic Expressions........... 1.3 The for statement.............................. 1.4 Symbolic Constants............................. 1.5 Character Input and Output..................... 1.5.1 File Copying.......................... 1.5.2 Character Counting.................... 1.5.3 Line Counting......................... 1.5.4 Word Counting......................... 1.6 Arrays......................................... 1.7 Functions...................................... 1.8 Arguments - Call by Value...................... 1.9 Character Arrays............................... 1.10 External Variables and Scope.................. Chapter 2 - Types, Operators and Expressions..................... 2.1 Variable Names................................. 2.2 Data Types and Sizes........................... 2.3 Constants...................................... 2.4 Declarations................................... 2.5 Arithmetic Operators........................... 2.6 Relational and Logical Operators............... 2.7 Type Conversions............................... 2.8 Increment and Decrement Operators.............. 2.9 Bitwise Operators.............................. 2.10 Assignment Operators and Expressions.......... 2.11 Conditional Expressions....................... 2.12 Precedence and Order of Evaluation............ Chapter 3 - Control Flow......................................... 3.1 Statements and Blocks.......................... 3.2 If-Else........................................ 3.3 Else-If........................................ 3.4 Switch......................................... 3.5 Loops - While and For.......................... 3.6 Loops - Do-While............................... 3.7 Break and Continue............................. 3.8 Goto and labels................................ Chapter 4 - Functions and Program Structure...................... 4.1 Basics of Functions............................ 4.2 Functions Returning Non-integers............... 4.3 External Variables............................. 4.4 Scope Rules.................................... 4.5 Header Files................................... 4.6 Static Variables................................ 4.7 Register Variables.............................. 4.8 Block Structure................................. 4.9 Initialization.................................. 4.10 Recursion...................................... 4.11 The C Preprocessor............................. 4.11.1 File Inclusion........................ 4.11.2 Macro Substitution.................... 4.11.3 Conditional Inclusion................. Chapter 5 - Pointers and Arrays.................................. 5.1 Pointers and Addresses......................... 5.2 Pointers and Function Arguments................ 5.3 Pointers and Arrays............................ 5.4 Address Arithmetic............................. 5.5 Character Pointers and Functions............... 5.6 Pointer Arrays; Pointers to Pointers........... 5.7 Multi-dimensional Arrays....................... 5.8 Initialization of Pointer Arrays............... 5.9 Pointers vs. Multi-dimensional Arrays.......... 5.10 Command-line Arguments........................ 5.11 Pointers to Functions......................... 5.12 Complicated Declarations...................... Chapter 6 - Structures........................................... 6.1 Basics of Structures........................... 6.2 Structures and Functions....................... 6.3 Arrays of Structures........................... 6.4 Pointers to Structures......................... 6.5 Self-referential Structures.................... 6.6 Table Lookup................................... 6.7 Typedef........................................ 6.8 Unions......................................... 6.9 Bit-fields..................................... Chapter 7 - Input and Output..................................... 7.1 Standard Input and Output....................... 7.2 Formatted Output - printf....................... 7.3 Variable-length Argument Lists.................. 7.4 Formatted Input - Scanf......................... 7.5 File Access..................................... 7.6 Error Handling - Stderr and Exit................ 7.7 Line Input and Output........................... 7.8 Miscellaneous Functions......................... 7.8.1 String Operations...................... 7.8.2 Character Class Testing and Conversion. 7.8.3 Ungetc................................. 7.8.4 Command Execution...................... 7.8.5 Storage Management..................... 7.8.6 Mathematical Functions................. 7.8.7 Random Number generation............... Chapter 8 - The UNIX System Interface............................ 8.1 File Descriptors............................... 8.2 Low Level I/O - Read and Write................. 8.3 Open, Creat, Close, Unlink..................... 8.4 Random Access - Lseek.......................... 8.5 Example - An implementation of Fopen and Getc.. 8.6 Example - Listing Directories.................. 8.7 Example - A Storage Allocator.................. Appendix A - Reference Manual.................................... A.1 Introduction................................... A.2 Lexical Conventions............................ A.2.1 Tokens................................ A.2.2 Comments.............................. A.2.3 Identifiers........................... A.2.4 Keywords.............................. A.2.5 Constants............................. A.2.6 String Literals....................... A.3 Syntax Notation................................ A.4 Meaning of Identifiers......................... A.4.1 Storage Class......................... A.4.2 Basic Types........................... A.4.3 Derived types......................... A.4.4 Type Qualifiers....................... A.5 Objects and Lvalues............................ A.6 Conversions.................................... A.6.1 Integral Promotion.................... A.6.2 Integral Conversions.................. A.6.3 Integer and Floating.................. A.6.4 Floating Types........................ A.6.5 Arithmetic Conversions................ A.6.6 Pointers and Integers................. A.6.7 Void.................................. A.6.8 Pointers to Void...................... A.7 Expressions.................................... A.7.1 Pointer Conversion.................... A.7.2 Primary Expressions................... A.7.3 Postfix Expressions................... A.7.4 Unary Operators....................... A.7.5 Casts................................. A.7.6 Multiplicative Operators.............. A.7.7 Additive Operators.................... A.7.8 Shift Operators....................... A.7.9 Relational Operators.................. A.7.10 Equality Operators................... A.7.11 Bitwise AND Operator................. A.7.12 Bitwise Exclusive OR Operator........ A.7.13 Bitwise Inclusive OR Operator........ A.7.14 Logical AND Operator................. A.7.15 Logical OR Operator.................. A.7.16 Conditional Operator................. A.7.17 Assignment Expressions............... A.7.18 Comma Operator.......................... A.7.19 Constant Expressions.................... A.8 Declarations..................................... A.8.1 Storage Class Specifiers................. A.8.2 Type Specifiers.......................... A.8.3 Structure and Union Declarations......... A.8.4 Enumerations............................. A.8.5 Declarators.............................. A.8.6 Meaning of Declarators................... A.8.7 Initialization........................... A.8.8 Type names............................... A.8.9 Typedef.................................. A.8.10 Type Equivalence........................ A.9 Statements....................................... A.9.1 Labeled Statements....................... A.9.2 Expression Statement..................... A.9.3 Compound Statement....................... A.9.4 Selection Statements..................... A.9.5 Iteration Statements..................... A.9.6 Jump statements.......................... A.10 External Declarations........................... A.10.1 Function Definitions.................... A.10.2 External Declarations................... A.11 Scope and Linkage............................... A.11.1 Lexical Scope........................... A.11.2 Linkage................................. A.12 Preprocessing................................... A.12.1 Trigraph Sequences...................... A.12.2 Line Splicing........................... A.12.3 Macro Definition and Expansion.......... A.12.4 File Inclusion.......................... A.12.5 Conditional Compilation................. A.12.6 Line Control............................ A.12.7 Error Generation........................ A.12.8 Pragmas................................. A.12.9 Null directive.......................... A.12.10 Predefined names....................... A.13 Grammar......................................... Appendix B - Standard Library.................................... B.1.1 File Operations................................ B.1.2 Formatted Output......................... B.1.3 Formatted Input.......................... B.1.4 Character Input and Output Functions..... B.1.5 Direct Input and Output Functions........ B.1.6 File Positioning Functions............... B.1.7 Error Functions.......................... B.2 Character Class Tests: ................. B.3 String Functions: ..................... B.4 Mathematical Functions: ................. B.5 Utility Functions: ....................

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