[英]Check if the input is a whole number in C++ in simple way using isdigit
[英]Is there a simple way to check unsafe expression in C++?
我正在嘗試找到一種 [更好的] 方法來運行/檢查可能不安全的表達式或以更優雅的方式執行多個 null 檢查。
這是我想改進的代碼示例:
if (myObjectPointer &&
myObjectPointer->getSubObject() &&
myObjectPointer->getSubObject()->getSubSubObject() &&
myObjectPointer->getSubObject()->getSubSubObject()->getTarget()) {
// Use safely target
... *(myObjectPointer->getSubObject()->getSubSubObject()->getTarget()) ...
}
我試圖找到一種更優雅的方法來實現這一點(而不是上面冗長的 null 檢查)。 這是我的第一個想法:
template<typename T>
bool isSafe(T && function) {
try {
function();
// Just running the func above, but we could e.g. think about returning the actual value instead of true/fase - not that important.
return true;
}
catch (...) {
return false;
}
}
...
// And use the above as follow :
if(isSafe([&](){ myObjectPointer->getSubObject()->getSubSubObject()->getTarget(); })) {
// Use safely target
}
...
上面的問題是我們無法捕獲信號(Segmentation fault,...)。 而且我顯然不想處理程序中的所有信號,而只是在這個非常具體的檢查/評估 function 中。
我是不是以錯誤的方式解決問題? 還有其他建議嗎? 還是冗長的 if 是不可避免的?
提前謝謝了。
我在想這個,就像 Jarod42 說的,一定有一些可變參數模板的東西。 我在這方面不是最好的,但想出了這個:
#include <memory>
#include <functional>
#include <iostream>
template <typename T, typename MemFn, typename... Params>
void safeExecute(T* ptr, MemFn memFn, Params&&... params) {
if (ptr != nullptr)
safeExecute(std::invoke(memFn, ptr), std::forward<Params>(params)...);
}
template <typename T, typename MemFn>
void safeExecute(T* ptr, MemFn memFn) {
if (ptr != nullptr) std::invoke(memFn, ptr);
}
struct Target {
void Bar() { std::cout << "tada!\n"; };
};
template<typename T>
class Object {
private:
std::unique_ptr<T> ptr;
public:
Object() : ptr(std::make_unique<T>()) {}
T* Get() { return ptr.get(); }
};
using SubSubObject = Object<Target>;
using SubObject = Object<SubSubObject>;
using MyObject = Object<SubObject>;
int main() {
auto myObjectPtr = std::make_unique<MyObject>();
safeExecute(myObjectPtr.get(),
&MyObject::Get,
&SubObject::Get,
&SubSubObject::Get,
&Target::Bar);
}
編輯:我一直在嘗試使用更通用的返回類型,所以我嘗試了不調用成員 function 的選項,而是返回指向 object 的 std::optional 指針。 這導致我使用以下代碼:
#include <memory>
#include <functional>
#include <iostream>
#include <optional>
template <typename T, typename MemFn, typename... Params>
auto safeGetObject(T* ptr, MemFn memFn, Params&&... params)
-> decltype(safeGetObject(std::invoke(memFn, std::declval<T>()), std::forward<Params>(params)...))
{
if (ptr != nullptr) return safeGetObject(std::invoke(memFn, ptr), std::forward<Params>(params)...);
return {};
}
template <typename T, typename MemFn>
auto safeGetObject(T* ptr, MemFn memFn) -> std::optional<decltype(std::invoke(memFn, std::declval<T>()))> {
if (ptr != nullptr) return std::invoke(memFn, ptr);
return {};
}
struct Target {
int Bar(int a, int b) const noexcept {
return a+b;
};
};
template<typename T>
class Object {
private:
std::unique_ptr<T> ptr;
public:
Object() noexcept : ptr(std::make_unique<T>()) {}
T* Get() const noexcept { return ptr.get(); }
};
using SubSubObject = Object<Target>;
using SubObject = Object<SubSubObject>;
using MyObject = Object<SubObject>;
int main() {
auto myObjectPtr = std::make_unique<MyObject>();
auto optionalTarget = safeGetObject(
myObjectPtr.get(),
&MyObject::Get,
&SubObject::Get,
&SubSubObject::Get);
auto result = optionalTarget ? optionalTarget.value()->Bar(3, 4) : -1;
std::cout << " result " << result << '\n';
}
拋開可能的設計問題,您可以使用std::optional
的擴展版本。 由於並非所有接口都在您的控制之下,因此您必須將必要的函數包裝到自由函數中。 假設您可以更改myObjectPointer
的 class MyClass
,但不能更改子對象的類。
class MyClass {
public:
optional<std::reference_wrapper<SubObjectClass>> getSubObject();
};
optional<std::reference_wrapper<SubSubObjectClass>> getSubSubObject(SubObjectClass& s) {
SubSubObjectClass* ptr = s.getSubSubObject();
if (ptr) {
return std::ref(s.getSubSubObject());
} else {
return {};
}
}
optional<std::reference_wrapper<Target>> getTarget(SubSubObjectCLass& s) {
...
}
你現在可以寫類似
optional<MyClass*> myObjectPointer = ...;
myObjectPointer.and_then(MyClass::getSubObject)
.and_then(getSubSubObject)
.and_then(getTarget)
.map( doSomethingWithTarget ):
好的,我可能會刪除我之前的答案,因為我一直在重新考慮這一點,現在考慮使用 std::optional 和鏈接。 你的原創
myObjectPointer->getSubObject()->getSubSubObject()->getTarget()
不是真正可重現的,因為operator->()
不能是 static。 但是我們可以使用另一個運算符,例如operator>>()
。 因此:
#include <memory>
#include <iostream>
#include <optional>
#include <functional>
struct Target {
int Bar(int a, int b) const noexcept { return a+b; };
};
template<typename T>
class Object {
private:
T* const ptr;
public:
Object(T* ptr) noexcept : ptr(ptr) {}
T* Get() const noexcept { return ptr; }
};
using SubSubObject = Object<Target>;
using SubObject = Object<SubSubObject>;
using MyObject = Object<SubObject>;
template <typename T>
auto makeOptional(T* ptr) -> std::optional< std::reference_wrapper<T>> {
if (ptr) return std::ref(*ptr);
return {};
}
template <typename T, typename MemFn>
auto operator>> (std::optional<std::reference_wrapper<T>> optObj, MemFn memFn)
-> std::optional< std::reference_wrapper<std::remove_pointer_t<decltype(std::invoke(memFn, std::declval<T>()))>>> {
if (optObj) return makeOptional(std::invoke(memFn, *optObj));
return {};
}
int main() {
{
//complete
auto TargetPtr = std::make_unique<Target>();
auto subSubObjectPtr = std::make_unique<SubSubObject>(TargetPtr.get());
auto subObjectPtr = std::make_unique<SubObject>(subSubObjectPtr.get());
auto myObjectPtr = std::make_unique<MyObject>(subObjectPtr.get());
auto optionalMyObject = makeOptional(myObjectPtr.get());
auto optionalTarget = optionalMyObject >> &MyObject::Get >> &SubObject::Get >> &SubSubObject::Get;
auto result = (optionalTarget) ? optionalTarget->get().Bar(3, 4) : -1;
std::cout << "result is " << result << '\n';
}
{
// incomplete
auto subObjectPtr = std::make_unique<SubObject>(nullptr);
auto myObjectPtr = std::make_unique<MyObject>(subObjectPtr.get());
auto optionalMyObject = makeOptional(myObjectPtr.get());
auto optionalTarget = optionalMyObject >> &MyObject::Get >> &SubObject::Get >> &SubSubObject::Get;
auto result = (optionalTarget) ? optionalTarget->get().Bar(3, 4) : -1;
std::cout << "result is " << result << '\n';
}
}
會工作......讓我知道這是否是你要找的。
編輯:我也試過把它放在包裝器 class
#include <memory>
#include <iostream>
#include <functional>
#include <optional>
struct Target {
constexpr int Bar(int a, int b) const noexcept { return a + b; };
};
template<typename T>
class Object {
private:
T* const ptr;
public:
constexpr Object(T* const ptr) noexcept : ptr(ptr) {}
constexpr T* Get() const noexcept { return ptr; }
};
using SubSubObject = Object<Target>;
using SubObject = Object<SubSubObject>;
using MyObject = Object<SubObject>;
template<typename T>
class ObjectWrapper {
private:
std::optional<std::reference_wrapper<T>> optRefObj{};
public:
constexpr ObjectWrapper(T* ptr) noexcept
: optRefObj(ptr ? std::make_optional(std::ref(*ptr)) : std::nullopt)
{}
template<typename MemFn>
constexpr auto operator>>(MemFn memFn) const noexcept {
return ObjectWrapper<std::remove_pointer_t<decltype(std::invoke(memFn, std::declval<T>()))>>
(optRefObj ? std::invoke(memFn, *optRefObj) : nullptr);
}
constexpr operator bool() const noexcept { return optRefObj.has_value(); }
constexpr T* Get() noexcept { return optRefObj ? &optRefObj->get() : nullptr; }
};
int main() {
{
//complete
auto const TargetPtr = std::make_unique<Target>();
auto const subSubObjectPtr = std::make_unique<SubSubObject>(TargetPtr.get());
auto const subObjectPtr = std::make_unique<SubObject>(subSubObjectPtr.get());
auto const myObjectPtr = std::make_unique<MyObject>(subObjectPtr.get());
auto const myObjWrp = ObjectWrapper(myObjectPtr.get());
auto optionalTarget = myObjWrp >> &MyObject::Get >> &SubObject::Get >> &SubSubObject::Get;
auto const result = optionalTarget ? optionalTarget.Get()->Bar(3, 4) : -1;
std::cout << "result is " << result << '\n';
}
{
// incomplete
auto const subObjectPtr = std::make_unique<SubObject>(nullptr);
auto const myObjectPtr = std::make_unique<MyObject>(subObjectPtr.get());
auto const myObjWrp = ObjectWrapper(myObjectPtr.get());
auto optionalTarget = myObjWrp >> &MyObject::Get >> &SubObject::Get >> &SubSubObject::Get;
auto const result = optionalTarget ? optionalTarget.Get()->Bar(3, 4) : -1;
std::cout << "result is " << result << '\n';
}
}
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