如何在C ++中将UINT8转换为UINT32？

Question

I have a value of type UINT8 and I would like to make it UINT32 .

Would my following code be considered correct, valid, efficient and safe?

UINT32 convU8toU32(UINT8 *number) {
  UINT32 result = *number;
  return *result;
}

Please notice that I'm a new comer to C++ from the Java world.

Answer 1

The function is correct as is (the typo with the * in the return *result; aside), but you don't even need it. Integers (and other integral types) convert implicitly to one another, and as UINT32 can represent every value a UINT8 can have, you can simply write

UINT32 target = source;

for some UINT8 source .

Making the conversion explicit with a static_cast is optional; if the conversion was (potentially) narrowing, the cast would silence some compiler warnings.

Answer 2

You need

 UINT32 result = static_cast<UINT32>(*number);

to de-refernce the pointer and cast it to the right type

But would

UINT32 convU8toU32(UINT8 number) {
   return static_cast<UINT32>(number);
}

be better and avoid pointers in the first place

Event better avoid the function call and type in the static cast in the appropriate line of code.

Answer 3

 UINT32 convU8toU32(UINT8 *number) { UINT32 result = *number; return *result; } 

I assume in my answer that UINT32 and UINT8 are fancy aliases for fundamental integer types.

Would my [...] code be considered

• correct, valid

No, given my assumption that UINT32 is an integer. You cannot dereference an integer. Which is what you try to do on line return *result;

• efficient

Does not matter since it is not correct.

• safe

Well, it safely fails to compile.

---

This should be OK:

UINT32 convU8toU32(UINT8 number) {
    return number;
}

Of course, this is so simple that you may want to consider not calling the function, but assign directly in the first place:

// not UINT32 foo = convU8toU32(some_uint8);
// but instead:
UINT32 foo = some_uint8;

Answer 4

Well, no. Assuming UINT32 is a 32-bit unsigned integral type and UINT8 is an 8-bit unsigned integral type, your code

UINT32 convU8toU32(UINT8 *number) {
   UINT32 result = *number;
   return *result;
}

would not even compile. The reason is that an integral type cannot be dereferenced as if it is a pointer. The statement return *result will therefore not compile, let alone be executed.

In reality, converting an 8-bit unsigned integral value to a 32-bit unsigned integral type is perfectly simple.

UINT32 convU8toU32(UINT8 number)
{
   UINT32 result = number;
   return result;
}

or, even more simply,

UINT32 convU8toU32(UINT8 number)
{
   return number;
}

These rely on implicit conversions to 32-bit unsigned integral type. Since a 32-bit unsigned integral type can exactly represent every value that an 8-bit unsigned integral type can, the conversion preserves value. Conversion the other way (from 32-bit to 8-bit) potentially loses value.

If you want to avoid implicit conversions, simply do an explicit conversion, such as

UINT32 convU8toU32(UINT8 number)
{
    return (UINT32) number;    // C-style conversion - discouraged in C++
}

or

UINT32 convU8toU32(UINT8 number)
{
    return UINT32(number);
}

or (to really make it obvious to anyone looking, and easy to find when searching a source file)

UINT32 convU8toU32(UINT8 number)
{
    return static_cast<UINT32>(number);
}

Of course, a function isn't even needed

 UINT8 value8 = something();
 UINT32 value32 = value8;

will do instead of using this function.