Thread: Initialize struct vector

Hi,

How to set the size of a struct vector?

I got until this:

Code:

typedef struct DES_ks
{
    union
    {
        unsigned char cblock[8];
        unsigned long deslong[2];
    } ks[16];
} DES_key;

vector<DES_key> v =
 { // start of initializer list for struct example
 { // start of initializer-list for ks
    {0}, // initializes first element of the union
    {0} // initializes second element of the union
 } };

Originally Posted by Ducky

Hi,

How to set the size of a struct vector?

This _compiles_ for me. No idea whether it works as expected.

Code:

#include <vector>


using namespace std;
int main(int argc, char *argv[])
{
    struct data 
    {
        int data[2];
        int value[3];
    };


    vector<data> v (30, 
            { 
             {0, 0},
             {3, 4, 5},
            }
            );


    return 0;
}

Edit: Yeah, works as expected.

Thanks!
So the 30 goes for the union array ks[16]; ?

So how do you initialize a union inside a struct because this one wont compile for me.
I get "error C2143: syntax error: missing ';' before '{'"

Code:

typedef struct DES_ks
{
    union
    {
        unsigned char cblock[8];
        unsigned long deslong[2];
    } ks[16];
} Data;


 vector<Data> v = ( 200,
 { { 
    {0}, 
    {0}, 
 } } );

I think your misunderstanding lies here:

Code:

{0}, // initializes first element of the union
{0} // initializes second element of the union

Here is what the C++ standard has to say about unions:

Originally Posted by C++11 Clause 9.5 Paragraph 1

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time. (...) The size of a union is sufficient to contain the largest of its non-static data members. Each non-static data member is allocated as if it were the sole member of a struct.

So, your attempt to initialise the first element and the second element is bogus: at any given time, it is as if the union only has one element, even if in fact it has more than one element.

From what I see, you only need to write:

Code:

auto v = std::vector<Data>(200);

This will already value initialise the Data elements.

Originally Posted by Ducky

So how do you initialize a union inside a struct because this one wont compile for me.
I get "error C2143: syntax error: missing ';' before '{'"

Code:

typedef struct DES_ks
{
    union
    {
        unsigned char cblock[8];
        unsigned long deslong[2];
    } ks[16];
} Data;


 vector<Data> v = ( 200,
 { { 
    {0}, 
    {0}, 
 } } );

This syntax is bogus. You should know that to call the constructor of a class, you should write

Type NameOfVar(ArgumentsToConstructor);

Hence, you should write

Code:

vector<Data> v(200,
 { { 
    {0}, 
    {0}, 
 } } );

(Assuming everything else is right.)
Also there's no need for typedef whatsoever. This isn't C, so let's simplify this. Instead, we can write:

Code:

struct Data
{
    union
    {
        unsigned char cblock[8];
        unsigned long deslong[2];
    } ks[16];
};

Awesome, now its working!

Thanks Laserlight and Elysia for the detailed explanations!

Its a large C program that I transformed into c++.

Replaced mallocs with vectors.

Originally Posted by Aslaville

I meant how did you solve the above issue ?

Just like Laserlight showed me:

Code:

vector<DATA> ks1(200);