C++：不能 static_cast 从 double* 到 int*
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C++: can't static_cast from double* to int*
提问by samoz
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 double* 转换为 int* 时，出现以下错误：
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 之间转换会出现问题，但是为什么在 double* 和 int* 之间转换会出现问题？
回答by Jacob
You should use reinterpret_castfor casting pointers, i.e.
您应该reinterpret_cast用于铸造指针，即
r = reinterpret_cast<int*>(p);
Of course this makes no sense,
这当然没有意义，
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 valuepointed to by pto an intthen,
除非您想int对double! 你会得到一些奇怪的输出，我认为这不是你想要的。如果你想投的价值指向p到int那时，
*r = static_cast<int>(*p);
Also, ris not allocatedso you can do one of the following:
此外，r未分配，因此您可以执行以下操作之一：
int *r = new int(0);
*r = static_cast<int>(*p);
std::cout << *r << std::endl;
Or
或者
int r = 0;
r = static_cast<int>(*p);
std::cout << r << std::endl;
回答by Potatoswatter
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.
除了作为指针，double*并int*没有什么共同点。你可能会说同样的事情Foo*和Bar*指针类型的任何不同的结构。
static_castmeans that a pointer of the source type can be usedas a pointer of the destination type, which requires a subtype relationship.
static_cast意味着源类型的指针可以被用来作为目的地类型，这需要一个亚型关系的指针。
回答by Dathan
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.
支持浮点到整数转换，所以int a = static_cast<int>(5.2)很好。然而，这是一种转换——底层数据类型完全不兼容。您要求运行时将指向 8 字节结构的指针转换为指向 4 字节结构的指针，但它无法以任何有意义的方式执行此操作。
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 解释为整数，则int* r = reinterpret_cast<int*>(p)可以正常工作。
回答by David Thornley
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<>.
您可以在 double 和 int 之间进行转换static_cast<>，但不能在指向不同类型的指针之间进行转换。您可以将任何指针类型void *与static_cast<>.
The rationale may be that int *and double *are often effectively arrays, and the implementation doesn't know how big the array is.
基本原理可能是int *并且double *通常是有效的数组，并且实现不知道数组有多大。
回答by Earlz
Because you used double *instead of double
因为你用double *而不是double
The *after it means that you are declaring a pointer, which is vastly different from a regular double.
在*经过这意味着你将指针声明，这是从一个普通的双大不相同。
C++ can not safely static_cast a pointer to a different type of pointer like that.
C++ 不能安全地将指针 static_cast 指向不同类型的指针。
If you are wanting to do this kinda thing, you must first dereference the variable.
如果你想做这种事情，你必须首先取消对变量的引用。
r=new int(static_cast<int>(*p));
You must use newbecause a double and an integer can not reside in the same memory space(sanely)
您必须使用，new因为双精度和整数不能驻留在同一个内存空间中（理智地）
