Question

Just ran into this:

#include <iostream>

using namespace std;

int main(int argc, char** argv)
{
    float *a = new float[10];
    void **b;
    b = static_cast<void**>(&a);
    delete(a); 
    return 0;
}

macbook:C nils$ g++ -Wall -g -o static_cast static_cast.cpp 
static_cast.cpp: In function ‘int main(int, char**)’:
static_cast.cpp:9: error: invalid static_cast from type ‘float**’ to type ‘void**’
macbook:C nils$ clang++ -Wall -g -o static_cast static_cast.cpp 
static_cast.cpp:9:9: error: static_cast from 'float **' to 'void **' is not
      allowed
    b = static_cast<void**>(&a);
        ^~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
macbook:C nils$ 

Why is it not allowed? While b = (void**)(&a); works.

Answer 1

$5.2.9/2 –

“An expression e can be explicitly
converted to a type T using a
static_cast of the form
static_cast(e) if the declaration
“T t(e);” is well-formed, for some
invented temporary variable t (8.5).
The effect of such an explicit
conversion is the same as performing
the declaration and initialization and
then using the temporary variable as
the result of the conversion. The
result is an lvalue if T is a
reference type (8.3.2), and an rvalue
otherwise. The expression e is used as
an lvalue if and only if the
initialization uses it as an lvalue.”

Let us take the case of Code snippet 1 as below

float *f;

void *p = f;

Here initialization of ‘p’ is well-formed. This is in accordance with $4.2

An rvalue of type “pointer to cv T,” where T is an object type, can be converted to an rvalue of type “pointer to cv void.”

Now let us take the code in OP

In our case, ‘E’ is 'float **' and ‘T’ is 'void **'

So, whether static_cast will work for the attempted conversion if ‘p’ can be initialized as shown below

float **f;

void **p = f;

The initialization of ‘p’ is ill-formed as it is not a valid condition listed under $4.10

Now, coming to why b = (void**)(&a); works?

This is the case where an explicit cast is used ($5.4). In this case, this explicit cast is equivalent of reinterpret_cast ($5.4/5). In this particular case, this conversion is allowed ($5.2.1/7).

Does this help?