如何避免基于数据类型的运行时切换/案例代码？ [英] How to avoid runtime switch/case code based on data type?

问题描述

小组，我有一个问题，我找不到一个好的解决方案。我是使用netCDF格式的科学数据文件的
。 netCDF数据的

属性之一是在运行时只知道实际的数据类型

。因此很多基于模板的技巧都不太有用。

考虑


数据文件

浮动x（3）3.5 ，2.5,8.9

double y（3）2.7，-2.3,1.2

int z（3）5,2,3


假设在我的代码中，我需要写一个求和函数并返回

结果，我可以做


模板< class T> < br $>
T sum（T * data，size_t n）;


这一切都很好，但是在调用子程序中我被卡住了

因为在编译时我没有实际数据类型的先验知识

包含在数据文件中，因此我必须依赖基于开关/案例代码的

在运行时包含在数据文件中。考虑：


void sum_caller（）{

mytype type = datafile.getline（i）.type

switch（type） {

case float_type：

float ret = sum（datafile.getline（i）.data）;

do_something（ret）;

case double_type：

float ret = sum（datafile.getline（i）.data）;

...

....

}


我不能声明''模板< class T> void sum_caller（）;''因为sum_caller中只有

，所以类型信息可用。 （你可以争辩说

我可以声明sum_caller模板，但这不是重点。

某些点，类型信息变得可用且适当

可采取行动）。是否有设计模式或实施

策略可以帮助我解决这个难题，从而避免交换机/案例基于
的代码？谢谢！

Hi Group, I''ve got a problem I couldn''t find a good solution. I am
working with scientific data files in netCDF format. One of the
properties of netCDF data is that the actual type of data is only known
at run time. Therefore a lot of template based trick isn''t too useful.
Considering

datafile
float x(3) 3.5, 2.5, 8.9
double y(3) 2.7, -2.3, 1.2
int z(3) 5, 2, 3

Suppose in my code, I need to write a summation function and return the
result, I could do

template <class T>
T sum(T * data, size_t n);

This is all good and dandy, but in the calling subroutine I am stuck
because at compile I have no priori knowledge of actual data type
contained in the data file, thus I must rely on switch/case code based
on what may be contained in the data file during run time. Consider:

void sum_caller(){
mytype type = datafile.getline(i).type
switch(type){
case float_type:
float ret = sum(datafile.getline(i).data);
do_something(ret);
case double_type:
float ret = sum(datafile.getline(i).data);
...
....
}

I cannot declare ''template <class T> void sum_caller();'' because only
within sum_caller, the type information is available. (you could argue
I can declare sum_caller template based, but that''s not the point. At
some point, the type information becomes available and appropriate
action can be taken). Is there a design pattern or implementation
strategy that can help me with this dilemma and thus avoid switch/case
based code altogether? Thanks!

推荐答案

刘飞写道：

嗨小组，我有问题我找不到一个好的解决方案。我正在使用netCDF格式的科学数据文件。 netCDF数据的一个属性是实际的数据类型只在运行时才知道。因此很多基于模板的技巧都不太有用。
考虑数据文件
浮动x（3）3.5,2.5,8.9
双y（3 ）2.7，-2.3,1.2
int z（3）5,2,3

假设在我的代码中，我需要编写一个求和函数并返回
结果，我可以做

模板< class T>
T sum（T * data，size_t n）;

这一切都很好，花花公子，但在调用子程序我被卡住了
因为在编译时我对数据文件中包含的实际数据类型没有先验知识，因此我必须依赖基于开关/案例代码的
来处理可能包含的内容在运行时的数据文件中。考虑：

void sum_caller（）{
mytype type = datafile.getline（i）.type
switch（type）{
case float_type：
float ret = sum（datafile.getline（i）.data）;
do_something（ret）;
case double_type：
float ret = sum（datafile.getline（i）.data）;
...
...


我无法声明''template< class T> void sum_caller（）;''因为只有sum_caller中的
，所以类型信息可用。 （你可以争辩说
我可以声明sum_caller模板，但这不是重点。在某些时候，类型信息变得可用并且可以采取适当的行动）。是否有一种设计模式或实施策略可以帮助我解决这个难题，从而完全避免基于开关/案例的代码？谢谢！

Hi Group, I''ve got a problem I couldn''t find a good solution. I am
working with scientific data files in netCDF format. One of the
properties of netCDF data is that the actual type of data is only known
at run time. Therefore a lot of template based trick isn''t too useful.
Considering

datafile
float x(3) 3.5, 2.5, 8.9
double y(3) 2.7, -2.3, 1.2
int z(3) 5, 2, 3

Suppose in my code, I need to write a summation function and return the
result, I could do

template <class T>
T sum(T * data, size_t n);

This is all good and dandy, but in the calling subroutine I am stuck
because at compile I have no priori knowledge of actual data type
contained in the data file, thus I must rely on switch/case code based
on what may be contained in the data file during run time. Consider:

void sum_caller(){
mytype type = datafile.getline(i).type
switch(type){
case float_type:
float ret = sum(datafile.getline(i).data);
do_something(ret);
case double_type:
float ret = sum(datafile.getline(i).data);
...
...
}

I cannot declare ''template <class T> void sum_caller();'' because only
within sum_caller, the type information is available. (you could argue
I can declare sum_caller template based, but that''s not the point. At
some point, the type information becomes available and appropriate
action can be taken). Is there a design pattern or implementation
strategy that can help me with this dilemma and thus avoid switch/case
based code altogether? Thanks!

查看Andrei Alexandrescu撰写的_Modern C ++ Design_的第8章。一个

Loki工厂的目的是为了填补这样的空白，并且很可能会为您提供所需的产品（好吧，只需付出一点努力）。您可以从


的最新源代码.href =http://sourceforge.net/projects/loki-lib/ target =_ blank> http://sourceforge.net/projects/loki-lib/


干杯！ --M

Check out chapter 8 of _Modern C++ Design_ by Andrei Alexandrescu. A
Loki factory is intended to fill just such a gap and will likely do
what you need (well, with a little effort on your part). You can get
the latest source code for it from

http://sourceforge.net/projects/loki-lib/

Cheers! --M

2006-03-03，刘飞< fe ***** @ gmail.com>写道：

On 2006-03-03, Fei Liu <fe*****@gmail.com> wrote:

嗨组，我有一个问题，我找不到一个好的解决方案。我正在使用netCDF格式的科学数据文件。 netCDF数据的一个属性是实际的数据类型仅在运行时已知。因此，很多基于模板的技巧都不是很有用。考虑数据文件
浮点x（3）3.5,2.5,8.9
double y（3）2.7，-2.3,1.2
int z（3）5， 2,3，

Hi Group, I''ve got a problem I couldn''t find a good solution. I am
working with scientific data files in netCDF format. One of the
properties of netCDF data is that the actual type of data is only
known at run time. Therefore a lot of template based trick isn''t too
useful. Considering

datafile
float x(3) 3.5, 2.5, 8.9
double y(3) 2.7, -2.3, 1.2
int z(3) 5, 2, 3

看起来你需要一个解析器，而且它已经写了

。访问NCO的CVS存储库似乎是最好的开始。


-

Neil Cerutti

我从发动机罩下面冒出一股烟雾。

我意识到这辆车着火所以带走了我的狗并闷死它

用毯子。 --Insurance Claim Blooper

It looks like you need a parser, rather, and it''s already been
written. A visit to the CVS repository for NCO seems like the best
place to start.

--
Neil Cerutti
I pulled into a lay-by with smoke coming from under the bonnet.
I realized the car was on fire so took my dog and smothered it
with a blanket. --Insurance Claim Blooper

Neil Cerutti写道：

Neil Cerutti wrote:

2006-03-03，刘飞< ; FE ***** @ gmail.com>写道：

On 2006-03-03, Fei Liu <fe*****@gmail.com> wrote:

嗨组，我有一个问题，我找不到一个好的解决方案。我正在使用netCDF格式的科学数据文件。 netCDF数据的一个属性是实际的数据类型仅在运行时已知。因此，很多基于模板的技巧都不是很有用。考虑数据文件
浮点x（3）3.5,2.5,8.9
double y（3）2.7，-2.3,1.2
int z（3）5， 2,3，

Hi Group, I''ve got a problem I couldn''t find a good solution. I am
working with scientific data files in netCDF format. One of the
properties of netCDF data is that the actual type of data is only
known at run time. Therefore a lot of template based trick isn''t too
useful. Considering

datafile
float x(3) 3.5, 2.5, 8.9
double y(3) 2.7, -2.3, 1.2
int z(3) 5, 2, 3

看起来你需要一个解析器，而且它已经被编写了。参观NCO的CVS资料库似乎是最好的起点。

-
Neil Cerutti
我拉着烟雾来了从发动机罩下面。
我意识到汽车着火了，所以带着我的狗用毯子闷死它。 --Insurance Claim Blooper

It looks like you need a parser, rather, and it''s already been
written. A visit to the CVS repository for NCO seems like the best
place to start.

--
Neil Cerutti
I pulled into a lay-by with smoke coming from under the bonnet.
I realized the car was on fire so took my dog and smothered it
with a blanket. --Insurance Claim Blooper

解析器如何解决我的问题？

How does a parser solve my problem?

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