c++:不能从double*到int*。

When I try to use a static_cast to cast a double* to an int*, I get the following error:

当我尝试使用static_cast将一个double*转换为int*时，我得到以下错误:

invalid static_cast from type ‘double*’ to type ‘int*’

Here is the code:

这是代码:

#include <iostream>
int main()
{
        double* p = new double(2);
        int* r;

        r=static_cast<int*>(p);

        std::cout << *r << std::endl;
}

I understand that there would be problems converting between a double and an int, but why is there a problem converting between a double* and an int*?

我知道在double和int之间转换会有问题，但是为什么在double*和int*之间转换会有问题呢?

5 个解决方案

#1

Aside from being pointers, double* and int* have nothing in common. You could say the same thing for Foo* and Bar* pointer types to any dissimilar structures.

除了是指针之外，double*和int*没有任何共同之处。对于Foo*和Bar*指针类型对于任何不同的结构来说都是一样的。

static_cast means that a pointer of the source type can be used as a pointer of the destination type, which requires a subtype relationship.

static_cast意味着可以使用源类型的指针作为目标类型的指针，这需要一个子类型关系。

#2

You should use reinterpret_cast for casting pointers, i.e.

您应该对转换指针使用reinterpret_cast，例如。

r = reinterpret_cast<int*>(p);

Of course this makes no sense,

unless you want take a int-level look at a double! You'll get some weird output and I don't think this is what you intended. If you want to cast the value pointed to by p to an int then,

除非你想要看一个双关!你会得到一些奇怪的输出，我不认为这是你想要的。如果你想将p指向的值转换为int，

*r = static_cast<int>(*p);

Also, r is not allocated so you can do one of the following:

而且，r没有被分配，所以你可以做以下其中的一个:

int *r = new int(0);
*r = static_cast<int>(*p);
std::cout << *r << std::endl;

Or

或

int r = 0;
r = static_cast<int>(*p);
std::cout << r << std::endl;

#3

Floating point-to-integer conversion is supported, so int a = static_cast<int>(5.2) is fine. However, it's a conversion - the underlying data types are completely incompatible. What you're asking is for the runtime to convert a pointer to an 8-byte structure to a pointer to a 4-byte structure, which it can't do in any meaningful way.

支持浮点到整数的转换，所以int a = static_cast (5.2)是可以的。但是，它是一个转换—底层数据类型完全不兼容。您所要求的是运行时将指针转换为8字节结构的指针，转换为指向4字节结构的指针，这是没有意义的。

That having been said, if you really want to interpret your double as an integer, int* r = reinterpret_cast<int*>(p) will work fine.

话虽如此，如果您真的想将double解释为一个整数，int* r = reinterpret_cast (p)将会工作得很好。 *>

#4

You can convert between a double and an int with static_cast<>, but not between pointers to different types. You can convert any pointer type to or from void * with static_cast<>.

您可以使用static_cast<>在double和int之间进行转换，但不能在指向不同类型的指针之间进行转换。您可以使用static_cast<>将任何指针类型转换为void *。

The rationale may be that int * and double * are often effectively arrays, and the implementation doesn't know how big the array is.

理由可能是int *和double *通常是有效的数组，而实现不知道数组的大小。

#5

Because you used double * instead of double

因为你用了double *而不是double *

The * after it means that you are declaring a pointer, which is vastly different from a regular double.

后面的*表示您正在声明一个指针，这与普通的double有很大的不同。

C++ can not safely static_cast a pointer to a different type of pointer like that.

c++不能安全地使用static_cast一个指针到另一种类型的指针。

If you are wanting to do this kinda thing, you must first dereference the variable.

如果您想要做这样的事情，您必须首先取消对变量的引用。

r=new int(static_cast<int>(*p));

You must use new because a double and an integer can not reside in the same memory space(sanely)

必须使用new，因为double和integer不能驻留在相同的内存空间中(明智地)