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?
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();
为了改进Mathieu的c++ 11答案并避免通过使用std::function而产生的运行时成本,我建议使用以下方法
template<typename functor>
class deferred final
{
public:
template<typename functor2>
explicit deferred(functor2&& f) : f(std::forward<functor2>(f)) {}
~deferred() { this->f(); }
private:
functor f;
};
template<typename functor>
auto defer(functor&& f) -> deferred<typename std::decay<functor>::type>
{
return deferred<typename std::decay<functor>::type>(std::forward<functor>(f));
}
这个简单的模板类将接受任何不需要任何参数就可以调用的函子,并且这样做不需要任何动态内存分配,因此更好地符合c++的抽象目标,没有不必要的开销。附加的函数模板用于简化模板参数推断的使用(类模板参数不可用)
使用的例子:
auto guard = defer(executeThisFunctionInAnyCase);
bool conditionA = executeStepA();
if (!conditionA) return;
bool conditionB = executeStepB();
if (!conditionB) return;
bool conditionC = executeStepC();
if (!conditionC) return;
正如Mathieu的答案,这个解决方案是完全异常安全的,在所有情况下都将调用executeThisFunctionInAnyCase。如果executeThisFunctionInAnyCase本身被抛出,析构函数被隐式地标记为noexcept,因此将发出对std::terminate的调用,而不是在堆栈展开期间引发异常。
As @Jefffrey said, you can use the conditional short-circuit feature in almost every language, I personally dislike conditional statements with more than 2 condition (more than a single && or ||), just a matter of style. This code does the same (and probably would compile the same) and it looks a bit cleaner to me. You don't need curly braces, breaks, returns, functions, lambdas (only c++11), objects, etc. as long as every function in executeStepX() returns a value that can be cast to true if the next statement is to be executed or false otherwise.
if (executeStepA())
if (executeStepB())
if (executeStepC())
//...
if (executeStepN()); // <-- note the ';'
executeThisFunctionInAnyCase();
任何时候,任何函数返回false,都不会调用下一个函数。
我喜欢@Mayerz的答案,因为你可以在运行时改变要调用的函数(以及它们的顺序)。这有点像观察者模式,其中有一组订阅者(函数、对象等),只要满足给定的任意条件,就会调用和执行这些订阅者。在许多情况下,这可能是一个过度杀戮,所以明智地使用它:)