将const void *转换为const int * [英] Casting const void* to const int*

问题描述

我之前没有使用过void *和const_correctness，所以我不理解下面的代码在做什么.我想要的只是将由const对象的成员函数返回的void *转换为int *.请提出更好的方法.谢谢.

I haven't used void* and const_correctness before so I am not understanding what I am doing wrong in the below code. All I want is to cast a void* returned by a member function of a const object to int*. Please suggest better approaches. Thank you.

我收到以下错误

passing 'const MyClass' as 'this' argument of 'void* MyClass::getArr()' discards qualifiers

这是我遇到的实际程序

 class MyClassImpl{
    CvMat* arr;
    public:
        MyClassImpl(){arr = new CvMat[10];}
        CvMat *getArr(){return arr;}
};
class MyClass{
    MyClassImpl *d;
    public:
        const void *getArr()const{ return (void*)d->getArr(); }

};
void print(const MyClass& obj){
    const int* ptr = static_cast<const int *>(obj.getArr());
}
int main(){
    MyClass obj1;
    print(obj1);
}

在这种情况下，只有诸如'print()'之类的方法才知道'getData'返回的数据类型.我不能使用模板，因为用户不知道MyClass是如何实现的.谢谢你.随时提出替代方案.

Only the methods such as 'print()' in this case know the datatype returned by 'getData'. I can't use templates because the user doesn't know how MyClass is implemented. Thank you. Feel free to suggest alternatives.

推荐答案

我认为问题不在于从数组到 void * 的转换，而在于尝试调用 obj.当 obj 被标记为 const 并且 MyClass :: getArr()不是 const >成员函数.如果您将该成员函数的定义更改为

I think the problem is not in the cast from your array to a void * but in trying to call obj.getArr() when obj is marked const and MyClass::getArr() is not a const member function. If you change your definition of that member function to

 const void *getArr() const { return static_cast<const void*>(arr); }

然后，该错误应自行解决.您可能还想做一个const-overload:

Then this error should resolve itself. You might want to do a const-overload as well:

 const void *getArr() const { return static_cast<const void*>(arr); }
       void *getArr()       { return static_cast<      void*>(arr); }

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