使用指针C ++进行转换

Question

Say I have an object:

void *tmpValue;

and say I know that tmpValue points to a double. A way to cast this into a double is to do the following:

double* dblPtr = (double*) tmpValue;
double dbl = *dblPtr;

But why does a direct casting from void* to double not work?

double dbl = (double) tmpValue; //error: "cannot convert from 'void*' to 'double'

Thanks in advance.

Answer 1

Interpreting a pointer (a memory address) as a floating-point value is not a sensible operation, and it probably fails on your platform because void * and double are not even the same size.

What you want to do is interpret the pointer as double * and dereference that double * pointer, as in your second code snippet.

You can do this in one line:

double dbl = *(double *)tmpValue;

But, hey, this is C++. Better to do

double dbl = *static_cast<double *>(tmpValue);

Answer 2

You can't cast from a pointer type to double. Recall that pointer type is essentially a memory address (typically 4 bytes containing location information of the data on the memory). Hence you can cast pointer type to integer, but casting to double wouldn't make sense.

When you cast the void* pointer into double* pointer type, you are essentially saying "this is now an address of a double rather than an address of void*"

If you want to get the value as double in one liner nevertheless, you can do

double dbl = *( (double*) tmpValue );

Answer 3

double dbl = *(double*)tmpValue

Answer 4

取消引用指针并强制转换值