treating set of member data as an array

[mojmir]

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you
whether it is a good and safe programming practice or not.

many thanks in advance,
mojmir

[Zeppe]

mojmir wrote:

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you
whether it is a good and safe programming practice or not.

many thanks in advance,
mojmir

the only fact that you don't know if it's a good practice, it means that
is not a good practice (i.e., adds complexity where it can be avoided).
Anyway, afaik is not safe due to the fact that there cen be alignement
requirements for x, y, z.

Regards,

Zeppe

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

On 2007-10-25 14:23, mojmir wrote:

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you whether it is a good and safe programming practice or not.

The important word is *can*. The above code can work due to placement in
memory, but it can also not work due to placement in memory. A safer
version could look like this:

struct vector
{
float x, y, z;
float& operator[](size_t i)
{
assert(0 <= i && i <3);
return i < 2 ? i == 0 ? x : y : z;
}
};

The body of the [] operator can be replaced with if-statements if you
like that better.

--
Erik Wikström

[terminator]

On Oct 25, 4:24 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-25 14:23, mojmir wrote:

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you whether it is a good and safe programming practice or not.

The important word is *can*. The above code can work due to placement in
memory, but it can also not work due to placement in memory. A safer
version could look like this:

struct vector
{
float x, y, z;
float& operator[](size_t i)
{
assert(0 <= i && i <3);
return i < 2 ? i == 0 ? x : y : z;
}

};

The body of the [] operator can be replaced with if-statements if you
like that better.

--
Erik Wikström- Hide quoted text -

- Show quoted text -

Wait a sec!!! why not just a simple array?

enum{x=0,y=1,z= 2};
typedef float vec3d[z+1];

vec3d v={1,2,3};
{//in some function
v[x]=....
v[y]=...
v[z]=...
}

thanks,
FM.

[terminator]

On Oct 25, 7:19 pm, terminator <farid.mehr...@ gmail.comwrote:

On Oct 25, 4:24 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-25 14:23, mojmir wrote:

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you whether it is a good and safe programming practice or not.

The important word is *can*. The above code can work due to placement in
memory, but it can also not work due to placement in memory. A safer
version could look like this:

struct vector
{
float x, y, z;
float& operator[](size_t i)
{
assert(0 <= i && i <3);
return i < 2 ? i == 0 ? x : y : z;
}

};

The body of the [] operator can be replaced with if-statements if you
like that better.

--
Erik Wikström- Hide quoted text -

- Show quoted text -

Wait a sec!!! why not just a simple array?

enum{x=0,y=1,z= 2};
typedef float vec3d[z+1];

vec3d v={1,2,3};
{//in some function
v[x]=....
v[y]=...
v[z]=...

}

thanks,
FM.- Hide quoted text -

because it is unsecure on array bounds.

[hodges.r]

On Oct 25, 5:26 pm, terminator <farid.mehr...@ gmail.comwrote:

On Oct 25, 7:19 pm, terminator <farid.mehr...@ gmail.comwrote: On Oct 25, 4:24 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-25 14:23, mojmir wrote:

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you whether it is a good and safe programming practice or not.

The important word is *can*. The above code can work due to placementin
memory, but it can also not work due to placement in memory. A safer
version could look like this:

struct vector
{
float x, y, z;
float& operator[](size_t i)
{
assert(0 <= i && i <3);
return i < 2 ? i == 0 ? x : y : z;
}

};

The body of the [] operator can be replaced with if-statements if you
like that better.

--
Erik Wikström- Hide quoted text -

- Show quoted text -

Wait a sec!!! why not just a simple array?

enum{x=0,y=1,z= 2};
typedef float vec3d[z+1];

vec3d v={1,2,3};
{//in some function
v[x]=....
v[y]=...
v[z]=...

}

thanks,
FM.- Hide quoted text -

because it is unsecure on array bounds.

and "return *(&x + i);" isn't ???

[=?UTF-8?B?RXJpayBXaWtzdHLDtm0=?=]

On 2007-10-25 18:56, ho******@gmail. com wrote:

On Oct 25, 5:26 pm, terminator <farid.mehr...@ gmail.comwrote:

>On Oct 25, 7:19 pm, terminator <farid.mehr...@ gmail.comwrote: On Oct 25, 4:24 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-25 14:23, mojmir wrote:

hello,

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you whether it is a good and safe programming practice or not.

The important word is *can*. The above code can work due to placement in
memory, but it can also not work due to placement in memory. A safer
version could look like this:

struct vector
{
float x, y, z;
float& operator[](size_t i)
{
assert(0 <= i && i <3);
return i < 2 ? i == 0 ? x : y : z;
}

};

The body of the [] operator can be replaced with if-statements if you
like that better.

--
Erik Wikström- Hide quoted text -

- Show quoted text -

Wait a sec!!! why not just a simple array?

enum{x=0,y=1,z= 2};
typedef float vec3d[z+1];

vec3d v={1,2,3};
{//in some function
v[x]=....
v[y]=...
v[z]=...

}

thanks,
FM.- Hide quoted text -

because it is unsecure on array bounds.

and "return *(&x + i);" isn't ???

Not with a proper assert on the value of i.

--
Erik Wikström

[mojmir]

because it is unsecure on array bounds.

>

and "return *(&x + i);" isn't ???

Not with a proper assert on the value of i.

yes, the assert is there.

so in my case it's working because sizeof(float)=4 , sizeof(Vector)= 12
and the
machine word is 32b. but if i move to 64bit platform for example, the
&x+1 will
eventually point to 4 padding bytes inserted by compiler, right?

regards,
mojmir

[terminator]

On Oct 26, 10:33 am, mojmir <svobod...@gmai l.comwrote:

>because it is unsecure on array bounds.

and "return *(&x + i);" isn't ???

Not with a proper assert on the value of i.

yes, the assert is there.

so in my case it's working because sizeof(float)=4 , sizeof(Vector)= 12
and the
machine word is 32b. but if i move to 64bit platform for example, the
&x+1 will
eventually point to 4 padding bytes inserted by compiler, right?

consider:

class A a;
size_t n;

then following is always true:

&A+n == static_cast<A*> (static_cast<si ze_t>(&A)+n*siz eof(A))

So your code works fine provided that overflow does not happen (that
is you do not index the vector with n 2).But Erik`s code never
overflows even if the assertion is omitted.

regards,
FM.

[James Kanze]

On Oct 25, 2:24 pm, Erik Wikström <Erik-wikst...@telia. comwrote:

On 2007-10-25 14:23, mojmir wrote:

i've just encountered following piece of code:

struct Vector
{
float x, y, z;

inline float & operator[] (size_t i)
{
assert(i<3);
return *(&x + i);
}
};

i think it can work due to placement in memory, but i'd like to ask
you whether it is a good and safe programming practice or not.

The important word is *can*. The above code can work due to placement in
memory, but it can also not work due to placement in memory. A safer
version could look like this:

struct vector
{
float x, y, z;
float& operator[](size_t i)
{
assert(0 <= i && i <3);
return i < 2 ? i == 0 ? x : y : z;
}
};

The body of the [] operator can be replaced with if-statements
if you like that better.

Or a switch. Alternatively, you could declare an array, and
provide functions "x() { return values[ 0 ] ; }", etc.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
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