假设我有这样的伪代码:
bool conditionA = executeStepA();
if (conditionA){
bool conditionB = executeStepB();
if (conditionB){
bool conditionC = executeStepC();
if (conditionC){
...
}
}
}
executeThisFunctionInAnyCase();
函数executeStepX当且仅当前一个成功时执行。
在任何情况下,executeThisFunctionInAnyCase函数都应该在最后被调用。
我在编程方面是一个新手,所以很抱歉提出一个非常基本的问题:有没有一种方法(例如在C/ c++中)以代码易读性为代价,避免长if链产生那种“金字塔式代码”?
我知道如果我们可以跳过executeThisFunctionInAnyCase函数调用,代码可以简化为:
bool conditionA = executeStepA();
if (!conditionA) return;
bool conditionB = executeStepB();
if (!conditionB) return;
bool conditionC = executeStepC();
if (!conditionC) return;
但是约束是executeThisFunctionInAnyCase函数调用。
break语句可以以某种方式使用吗?
为什么没有人给出最简单的解决方案?: D
如果你所有的函数都有相同的签名,那么你可以这样做(对于C语言):
bool (*step[])() = {
&executeStepA,
&executeStepB,
&executeStepC,
...
};
for (int i = 0; i < numberOfSteps; i++) {
bool condition = step[i]();
if (!condition) {
break;
}
}
executeThisFunctionInAnyCase();
对于简洁的c++解决方案,您应该创建一个接口类,其中包含一个执行方法,并将步骤包装在对象中。
然后,上面的解看起来像这样:
Step *steps[] = {
stepA,
stepB,
stepC,
...
};
for (int i = 0; i < numberOfSteps; i++) {
Step *step = steps[i];
if (!step->execute()) {
break;
}
}
executeThisFunctionInAnyCase();
在c++中,实际上有一种方法可以延迟操作:使用对象的析构函数。
假设你可以访问c++ 11:
class Defer {
public:
Defer(std::function<void()> f): f_(std::move(f)) {}
~Defer() { if (f_) { f_(); } }
void cancel() { f_ = std::function<void()>(); }
private:
Defer(Defer const&) = delete;
Defer& operator=(Defer const&) = delete;
std::function<void()> f_;
}; // class Defer
然后使用这个工具:
int foo() {
Defer const defer{&executeThisFunctionInAnyCase}; // or a lambda
// ...
if (!executeA()) { return 1; }
// ...
if (!executeB()) { return 2; }
// ...
if (!executeC()) { return 3; }
// ...
return 4;
} // foo
在c++中(这个问题在C和c++中都有标记),如果你不能改变函数来使用异常,你仍然可以使用异常机制,如果你写一个像这样的小助手函数
struct function_failed {};
void attempt(bool retval)
{
if (!retval)
throw function_failed(); // or a more specific exception class
}
然后你的代码可以如下所示:
try
{
attempt(executeStepA());
attempt(executeStepB());
attempt(executeStepC());
}
catch (function_failed)
{
// -- this block intentionally left empty --
}
executeThisFunctionInAnyCase();
如果你喜欢花哨的语法,你可以通过显式强制转换来让它工作:
struct function_failed {};
struct attempt
{
attempt(bool retval)
{
if (!retval)
throw function_failed();
}
};
然后您可以将代码编写为
try
{
(attempt) executeStepA();
(attempt) executeStepB();
(attempt) executeStepC();
}
catch (function_failed)
{
// -- this block intentionally left empty --
}
executeThisFunctionInAnyCase();
正如Rommik提到的,您可以为此应用设计模式,但我将使用Decorator模式而不是Strategy,因为您想要链式调用。如果代码很简单,那么我会选择一个结构良好的答案来防止嵌套。但是,如果它很复杂或者需要动态链接,那么Decorator模式是一个很好的选择。这是一个yUML类图:
下面是一个示例LinqPad c#程序:
void Main()
{
IOperation step = new StepC();
step = new StepB(step);
step = new StepA(step);
step.Next();
}
public interface IOperation
{
bool Next();
}
public class StepA : IOperation
{
private IOperation _chain;
public StepA(IOperation chain=null)
{
_chain = chain;
}
public bool Next()
{
bool localResult = false;
//do work
//...
// set localResult to success of this work
// just for this example, hard coding to true
localResult = true;
Console.WriteLine("Step A success={0}", localResult);
//then call next in chain and return
return (localResult && _chain != null)
? _chain.Next()
: true;
}
}
public class StepB : IOperation
{
private IOperation _chain;
public StepB(IOperation chain=null)
{
_chain = chain;
}
public bool Next()
{
bool localResult = false;
//do work
//...
// set localResult to success of this work
// just for this example, hard coding to false,
// to show breaking out of the chain
localResult = false;
Console.WriteLine("Step B success={0}", localResult);
//then call next in chain and return
return (localResult && _chain != null)
? _chain.Next()
: true;
}
}
public class StepC : IOperation
{
private IOperation _chain;
public StepC(IOperation chain=null)
{
_chain = chain;
}
public bool Next()
{
bool localResult = false;
//do work
//...
// set localResult to success of this work
// just for this example, hard coding to true
localResult = true;
Console.WriteLine("Step C success={0}", localResult);
//then call next in chain and return
return (localResult && _chain != null)
? _chain.Next()
: true;
}
}
恕我直言,关于设计模式最好的书是《Head First design patterns》。
假设你不需要单独的条件变量,反转测试并使用else-falthrough作为“ok”路径,将允许你获得更垂直的if/else语句集:
bool failed = false;
// keep going if we don't fail
if (failed = !executeStepA()) {}
else if (failed = !executeStepB()) {}
else if (failed = !executeStepC()) {}
else if (failed = !executeStepD()) {}
runThisFunctionInAnyCase();
省略失败的变量使代码在我看来有点太晦涩了。
在里面声明变量就可以了,不用担心= vs ==。
// keep going if we don't fail
if (bool failA = !executeStepA()) {}
else if (bool failB = !executeStepB()) {}
else if (bool failC = !executeStepC()) {}
else if (bool failD = !executeStepD()) {}
else {
// success !
}
runThisFunctionInAnyCase();
这很模糊,但很紧凑:
// keep going if we don't fail
if (!executeStepA()) {}
else if (!executeStepB()) {}
else if (!executeStepC()) {}
else if (!executeStepD()) {}
else { /* success */ }
runThisFunctionInAnyCase();