Casting of custom class vectors .

[tonnot]

I have the next :

struct one {
int a;
double b; };
vector<one> myvector(3);

I'd want to get the byte (char) data from myvector[0], myvector[1], etc.
I'd want to do :
1.- Save myvector[0] into my_char (a *char variable)
2.- Save the char data into a char buffer.
3.- Save the char data into a stream.
4.- Save the char data to a binary file.

What kind of casting (from the diferent castings) can I use ?
One of them are better, faster ?

I understand "reinterpret_cast<char*> (my_vector[0]);" but I have doubts about the other casts (I have read the cppreference but I dont understand nothing....)

Any help ? Thanks.

[coder777]

It does not make sense to cast an int to a char *. Use the ther conversion functions like itoa() sprintf()... for it or use streams

What kind of casting (from the diferent castings) can I use ?
One of them are better, faster ?

Don't cast it's not a panacea

[zb0S216C]

Don't cast vectors, or any other type for that matter. Casting is unsafe, and may result in buggy, and undefined behaving code. Some type-casting is safe. For example:

short Number( 10 );
static_cast < int > ( Number );       // Safe.
static_cast < long > ( Number );      // Safe.
static_cast < long long > ( Number ); // Safe.
( char )Number; // Safe. 

tonnot wrote:

Save myvector[0] into my_char (a *char variable)

Why do you want to cast a structure into a character? Are you trying to perform serialization?

_Wazzak

[tonnot]

I'm casting vectors to char buffer because I dont know how to write them as a whole object to a binary file. ( And I suposse that in the future I'm going to have send it over the network)


I'm storing data into a char buffer, to send later to a binary file using one instruction.
By now , it works. I'm almost finishing my method.
Thanks

[helios]

You could do ((char *)&vector[0])
That's safe if you pass that to a function along with vector.size()*sizeof(T), where T is the type of the elements of vector.

Note that if you're using this to send the vector to an I/O device, such as a file or the network, this will write the structures in a way only your computer and computers like yours can understand. Google endianness for more information.
If this is a problem, you should use a different method.

[tonnot]

Thanks helios.
I have spent a lot of time in this work....
I have view a lot of web pages and, yes I know the endianness problem.
Why is so difficult to find a good link talking about it and offering a solution?

Vector data are contiguous in memory ?

[helios]

Vector data are contiguous in memory ?

Yes.

Why is so difficult to find a good link talking about it and offering a solution?

Because there's no general solution to the problem. The closest to one is "find a consistent format for integers and use it whenever dealing with serial I/O". For example:

void write_little_dword(uint32_t integer,std::vector<byte> &buffer){
	for (int i=0;i<4;i++){
		buffer.push_back(integer&0xFF);
		integer>>=8;
	}
}