Why doesn't this reinterpret_cast compile?

Question

I understand that reinterpret_cast is dangerous, I'm just doing this to test it. I have the following code:

int x = 0;
double y = reinterpret_cast<double>(x);

When I try to compile the program, it gives me an error saying

invalid cast from type 'float' to type 'double

What's going on? I thought reinterpret_cast was the rogue cast that you could use to convert apples to submarines, why won't this simple cast compile?

Solution 1

Perhaps a better way of thinking of reinterpret_cast is the rouge operator that can "convert" pointers to apples as pointers to submarines.

By assigning y to the value returned by the cast you're not really casting the value x, you're converting it. That is, y doesn't point to x and pretend that it points to a float. Conversion constructs a new value of type float and assigns it the value from x. There are several ways to do this conversion in C++, among them:

int main()
{
    int x = 42;
    float f = static_cast<float>(x);
    float f2 = (float)x;
    float f3 = float(x);
    float f4 = x;
    return 0;
}

The only real difference being the last one (an implicit conversion) will generate a compiler diagnostic on higher warning levels. But they all do functionally the same thing -- and in many case actually the same thing, as in the same machine code.

Now if you really do want to pretend that x is a float, then you really do want to cast x, by doing this:

#include <iostream>
using namespace std;

int main()
{
    int x = 42;
    float* pf = reinterpret_cast<float*>(&x);
    (*pf)++;
    cout << *pf;
    return 0;
}

You can see how dangerous this is. In fact, the output when I run this on my machine is 1, which is decidedly not 42+1.

Solution 2

In C++ reinterpret_cast can only perform a specific set of conversions, explicitly listed in the language specification. In short, reinterpret_cast can only perform pointer-to-pointer conversions and reference-to-reference conversions (plus pointer-to-integer and integer-to-pointer conversions). This is consistent with the intent expressed in the very name of the cast: it is intended to be used for pointer/reference reinterpretation.

What you are trying to do is not reinterpretation. If you want to reinterpret an int as a double you'd have to convert it to a reference type

double y = reinterpret_cast<double&>(x); 

although the equivalent pointer-based reinterpretation is probably more explicit

double y = *reinterpret_cast<double*>(&x); // same as above

Note though, that while reinterpret_cast can convert the reference/pointer types, the actual attempt to read the data through the resultant reference/pointer produces undefined behavior.

And in any case this, of course, can't make much sense on a platform with int and double of different size (since in case of larger double you will read beyond the memory occupied by x).

So, in the end it all boils down to what you were trying to achieve. Memory reinterpretation? See above. Some kind of more meaningful int to double conversion? If so, reinterpret_cast won't help you here.

Solution 3

If you are trying to convert the bits of your int to a the representation of a double, you need to cast the address not the value. You must also make sure the sizes match:

uint64_t x = 0x4045000000000000;
double y = *reinterpret_cast<double *>(&x);

Solution 4

reinterpret_cast is not a general cast. According to the C++03 spec section 5.2.10.1:

Conversions that can be performed explicitly using reinterpret_cast are listed below. No other conversion can be performed explicitly using reinterpret_cast.

And there is nothing listed that describes converting between integral and floating point types (or between integral types, even this is illegal reinterpret_cast<long>(int(3));)

Solution 5

The compiler rejects what you wrote as nonsense because int and double may be objects with different sizes. You could achieve the same effect this way, although it is certainly dangerous:

int x = 0;
double y = *reinterpret_cast<double*>(&x);

This is potentially dangerous because if x and y are diffrent sizes (let's say int is four bytes and double is eight bytes) then when you dereference the eight bytes of memory at &x to fill in y you will access four bytes of x and four bytes of ... whatever comes next in memory (possibly the start of y, or garbage, or something else entirely.)

If you want to convert a integer to a double, use a static_cast and it will perform conversion.

If you want to access the bit-pattern of x, cast to some convenient pointer type (say, byte*) and access up to sizeof(int) / sizeof(byte):

byte* p = reinterpret_cast<byte*>(&x);
for (size_t i = 0; i < sizeof(int); i++) {
  // do something with p[i]
}

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Comments

[David Rodríguez - dribeas]

Not the reason why the compiler rejects it, but the discussion on type sizes is valuable.

[user4951]

will (float)x return 42 or some binary representation of some unrelated double. I think this guy means reinterpret and that's what he wants.

[John Dibling]

(float)x will perform a conversion, not a cast.

[RaGa__M]

reinterpret_cast can only perform pointer-to-pointer conversions and reference-to-reference conversions (plus pointer-to-integer and integer-to-pointer conversions) this flattened the question and could be accepted as an answer.

[Maciej Piechotka]

Your code below is strict aliasing violation and therefore has an undefined behavior.

[Technophile]

When I do this sort of thing in low-level code, frequently it's to display the bits without ANY sort of conversion applied (except to printable hex). E.g. when (de-)serializing data structures swapped with a microcontroller. Think "debugger".

[fcdt]

No, using an union this way is implementation-specific and therefore not the least error-prone way (see Purpose of Unions in C and C++) For example: If i is 32 bits and f is 64 bits, does i correspond to the upper or lower 32 bits of f?

[MrRickDean]

If the sizes mismatch, reinterpreting pointers leads to even bigger problems, but your link is very good. I concede that unions are not well grounded for this purpose, and likely to have portability problems, but those are inherent in the task.

[François Andrieux]

This approach doesn't perform any conversion. It doesn't achieve what the question is asking.

[FrankHB]

@JohnDibling A cast is an explicit conversion, by definition.