在clang和gcc中，是否存在可视C ++ __declspec（属性声明属性）的替代方法？ [英] Is there an alternative for visual C++ __declspec (property declaration attribute) in clang and gcc?

问题描述

有一个Microsoft特定的扩展名，它使得可以这样定义属性获取器和设置器：

There is a Microsoft specific extension, which makes it possible to define property getters and setters like this:

// declspec_property.cpp
struct S {
   int i;
   void putprop(int j) {
      i = j;
   }

   int getprop() {
      return i;
   }

   __declspec(property(get = getprop, put = putprop)) int the_prop;
};

int main() {
   S s;
   s.the_prop = 5;
   return s.the_prop;
}

是否可以用clang或gcc定义属性声明属性？
如果我搜索 __ declspec ，我发现的只是 __ declspec（dllexport），但我没有看

Is there any way to define property declaration attribute with clang or gcc? If I search for __declspec, all I find is __declspec(dllexport), but I am not looking for that.

推荐答案

尽管C ++不提供对智能可重写运算符的支持（并且没有gcc扩展），但是语言允许您使用其现有功能来实现它。

下面的示例（并不假定涵盖所有情况！）显示了使用本机C ++ 11或更高版本的可能解决方案。

我们可以使用虚拟覆盖来覆盖属性，但这不是现代智能属性在其他语言（如swift，C＃等）中的工作方式，因此-我正在使用lambda注入用于setter和getter的覆盖代码。

While C++ does not offer support for smart overridable operators (and there are no gcc extensions for that), the language allows you to implement it using it's existing features.
The following example (which does not assume to cover all cases!) shows a possible solution using native C++ 11 or higher.
We could use virtual overrides to override the properties, but that's not how modern smart properties work in other languages such as swift, C# etc, so instead - I'm using lambdas to inject overriding code for setters and getters.

// The following is by no means a FULL solution!
#include <functional>
#include <iostream>
#include <cassert>

template<typename T> 
class Property {
public:
    Property(){}
    operator const T& () const {
        // Call override getter if we have it
        if (getter) return getter();
        return get();
    }
    const T& operator = (const T& other) {
        // Call override setter if we have it
        if (setter) return setter(other);
        return set(other);
    }
    bool operator == (const T& other) const {
        // Static cast makes sure our getter operator is called, so we could use overrides if those are in place
        return static_cast<const T&>(*this) == other;
    }
    // Use this to always get without overrides, useful for use with overriding implementations
    const T& get() const {
        return t;
    } 
    // Use this to always set without overrides, useful for use with overriding implementations
    const T& set(const T& other) {
        return t = other;
    }
    // Assign getter and setter to these properties
    std::function<const T&()> getter;
    std::function<const T&(const T&)> setter;
private:
    T t;
};

// Basic usage, no override
struct Test {
    Property<int> prop;
};

// Override getter and setter
struct TestWithOverride {
    TestWithOverride(){
        prop.setter = [&](const int& other){
            std::cout << "Custom setter called" << std::endl;
            return prop.set(other);
        };
        prop.setter = std::bind(&TestWithOverride::setProp,this,std::placeholders::_1);
        prop.getter = std::bind(&TestWithOverride::getProp,this);
    }
    Property<int> prop;
private:
    const int& getProp() const {
        std::cout << "Custom getter called" << std::endl;
        return prop.get();
    }
    const int& setProp(const int& other){
        std::cout << "Custom setter called" << std::endl;
        return prop.set(other);
    }
};

int main(int,char**){
    Test t;
    TestWithOverride t1;
    t.prop = 1;
    assert(t.prop == 1);
    t1.prop = 1;
    assert(t1.prop == 1);
    /*
    Expected output:
    1. No aborts on assertions
    2. Text:
    Custom setter called
    Custom getter called
    */
    return 0;
}

编译如下：

c ++ -std = c ++ 11 test.cpp -o test

运行：

./ test

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