[英]Casting from void* to an object array in c++
I'm having problems getting this to work, 我有问题让这个工作,
class A {
public:
A(int n) {
a = n;
}
int getA() {
return a;
}
private:
int a;
};
int main(){
A* a[3];
A* b[3];
for (int i = 0; i < 3; ++i) {
a[i] = new A(i + 1);
}
void * pointer = a;
b = (A* [])pointer; // DOESNT WORK Apparently ISO C++ forbids casting to an array type ‘A* []’.
b = static_cast<A*[]>(pointer); // DOESN'T WORK invalid static_cast from type ‘void*’ to type ‘A* []’
return 0;
}
And i can't use generic types for what i need. 我不能使用通用类型来满足我的需求。
Thanks in advance. 提前致谢。
Arrays are second-class citizen in C (and thus in C++). 数组是C中的二等公民(因此在C ++中)。 For example, you can't assign them. 例如,您无法分配它们。 And it's hard to pass them to a function without them degrading to a pointer to their first element. 并且很难将它们传递给函数,而不会将它们降级为指向其第一个元素的指针。
A pointer to an array's first element can for most purposes be used like the array - except you cannot use it to get the array's size. 对于大多数用途,指向数组的第一个元素的指针可以像数组一样使用 - 除非您不能使用它来获取数组的大小。
When you write 当你写作
void * pointer = a;
a
is implicitly converted to a pointer to its first element, and that is then casted to void*
. a
被隐式转换为指向其第一个元素的指针,然后将其转换为void*
。
From that, you cannot have the array back, but you can get the pointer to the first element: 从那里,您不能返回数组,但您可以获得指向第一个元素的指针:
A* b = static_cast<A*>(pointer);
(Note: casting between pointers to unrelated types requires a reinterpret_cast
, except for casts to void*
which are implicit and from void*
to any other pointer, which can be done using a static_cast
.) (注意:在指向不相关类型的指针之间进行转换需要reinterpret_cast
,除了转换为void*
,它们是隐式的,从void*
为任何其他指针,这可以使用static_cast
来完成。)
Perhaps you mean to do 也许你的意思是做
memcpy(b, (A**)pointer, sizeof b);
? ?
A static_cast
version is also possible. static_cast
版本也是可能的。
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