开关案例编程实践

Question

enum SQLErrorCode{
      OK = 0,
      PARTIAL_OK = 1,
      SOMEWHAT_OK = 2,
      NOT_OK = 3,
};

代码1:

int error = getErrorCode();
if((error == SQLErrorCode.PARTIAL_OK) ||
  (error == SQLErrorCode.SOMEWHAT_OK) ||
  (error == SQLErrorCode.NOT_OK) ||
  (error < 0))
   callFunction1();
else
    callFunction2();

代码2:

switch(error){
       case SQLErrorCode.PARTIAL_OK: 
                                    callFunction1();
                                    break;
        case SQLErrorCode.SOMEWHAT_OK:
                                    callFunction1();
                                    break;
        case SQLErrorCode.NOT_OK: 
                                    callFunction1();
                                    break;
        default:
                                    callFunction2();
                                    break;
}

我应该选择哪种方法.就性能而言,应该没有太大差异.如何在switch case中处理错误<0条件.

编辑:乔尔的解决方案:

switch(error) {
     case SQLErrorCode.PARTIAL_OK: 
     case SQLErrorCode.SOMEWHAT_OK:
     case SQLErrorCode.NOT_OK: 
         callFunction1();
         break;
     case SQLErrorCode.OK:
         callFunction2();
         break;
     default:     // error < 0 is handled
         callFunction1();
         break;
}

问:错误<0被处理.如果我必须处理错误的其他数字,这些数字不属于此处包括默认的任何情况.

Answer 1

如果没有表达对最佳选择的偏好,这是另一种可能性:

switch(error){
    case SQLErrorCode.PARTIAL_OK: 
    case SQLErrorCode.SOMEWHAT_OK:
    case SQLErrorCode.NOT_OK: 
                                callFunction1();
                                break;
    default:
                                callFunction2();
                                break;
}

Answer 2

对于这么少的情况switch 并不重要,但事实上整数更快:它可以并且经常被实现为跳转表而不是一系列条件检查.

作为比较,不同案例的数量为10:

enum SQLErrorCode{
    CODE0 = 0,
    CODE1 = 1,
    CODE2 = 2,
    CODE3 = 3,
    CODE4 = 4,
    CODE5 = 5,
    CODE6 = 6,
    CODE7 = 7,
    CODE8 = 8,
    CODE9 = 9
};

enum SQLErrorCode getErrorCode();

void run()
{
    int error = getErrorCode();
#ifdef CASE1
    if((error == CODE0) ||       
       (error == CODE1) ||
       (error == CODE2) ||
       (error == CODE3) ||
       (error == CODE4) ||
       (error == CODE5) ||
       (error == CODE6) ||
       (error == CODE7) ||
       (error == CODE8) ||
       (error == CODE9) ||
       (error < 0))
        callFunction1();
    else
        callFunction2();
#endif
#ifdef CASE2
    switch(error)
    {
        case CODE0:
            callFunction1();
            break;
    case CODE1:
        callFunction1();
        break;
    case CODE2:
        callFunction1();
        break;
    case CODE3:
        callFunction1();
        break;
    case CODE4:
        callFunction1();
        break;
    case CODE5:
        callFunction1();
        break;
    case CODE6:
        callFunction1();
        break;
    case CODE7:
        callFunction1();
        break;
    case CODE8:
        callFunction1();
        break;
    case CODE9:
        callFunction1();
        break;
    default:
        callFunction2();
        break;
}
#endif

}

现在看看使用GCC在Linux上构建时,通过执行第一个案例而不是第二个案例生成的程序集.

如果你看一下程序集,你会看到一个显着的区别(对于较大的语句):||s 系列(或if/ else如果你那样做)是一次一个分支.将switch变成一个大表:需要更多的代码,但可能意味着它可以在一个跳跃来处理.

(顺便说一下,我们在这里谈论C？不是C#？你的代码将无法编译:在C枚举器中不使用枚举名称作为前缀.所以它PARTIAL_OK没有了SQLErrorCode.)

代码1:cc -DCASE1 -s switch.s switch.c

        .file   "1241256.c"
        .text
.globl run
        .type   run, @function
run:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        call    getErrorCode
        movl    %eax, -4(%ebp)
        cmpl    $0, -4(%ebp)
        je      .L2
        cmpl    $1, -4(%ebp)
        je      .L2
        cmpl    $2, -4(%ebp)
        je      .L2
        cmpl    $3, -4(%ebp)
        je      .L2
        cmpl    $4, -4(%ebp)
        je      .L2
        cmpl    $5, -4(%ebp)
        je      .L2
        cmpl    $6, -4(%ebp)
        je      .L2
        cmpl    $7, -4(%ebp)
        je      .L2
        cmpl    $8, -4(%ebp)
        je      .L2
        cmpl    $9, -4(%ebp)
        je      .L2
        cmpl    $0, -4(%ebp)
        jns     .L13
.L2:
        call    callFunction1
        jmp     .L15
.L13:
        call    callFunction2
.L15:
        leave
        ret
        .size   run, .-run
        .ident  "GCC: (GNU) 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"
        .section        .note.GNU-stack,"",@progbits

代码2:cc -DCASE2 -s switch.s switch.c

        .text
.globl run
        .type   run, @function
run:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $24, %esp
        call    getErrorCode
        movl    %eax, -4(%ebp)
        cmpl    $9, -4(%ebp)
        ja      .L2
        movl    -4(%ebp), %eax
        sall    $2, %eax
        movl    .L13(%eax), %eax
        jmp     *%eax
        .section        .rodata
        .align 4
        .align 4
.L13:
        .long   .L3
        .long   .L4
        .long   .L5
        .long   .L6
        .long   .L7
        .long   .L8
        .long   .L9
        .long   .L10
        .long   .L11
        .long   .L12
        .text
.L3:
        call    callFunction1
        jmp     .L15
.L4:
        call    callFunction1
        jmp     .L15
.L5:
        call    callFunction1
        jmp     .L15
.L6:
        call    callFunction1
        jmp     .L15
.L7:
        call    callFunction1
        jmp     .L15
.L8:
        call    callFunction1
        jmp     .L15
.L9:
        call    callFunction1
        jmp     .L15
.L10:
        call    callFunction1
        jmp     .L15
.L11:
        call    callFunction1
        jmp     .L15
.L12:
        call    callFunction1
        jmp     .L15
.L2:
        call    callFunction2
.L15:
        leave
        ret
        .size   run, .-run
        .ident  "GCC: (GNU) 4.2.4 (Ubuntu 4.2.4-1ubuntu4)"
        .section        .note.GNU-stack,"",@progbits