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);