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olson_ord@yahoo.it
Guest
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 Posted: Fri Jan 27, 2006 12:14 pm Post subject: Subscript operator overloading with vectors |
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Hi,
I am not exactly new to C++, but I have never done operator
overloading before.
I have some old code that tries to implement a Shift Register - but I
cannot seem to get it to work. Here's a simpler version of it.
-------------------- main.cpp---------------------------
# include <iostream>
# include <vector>
# include <cassert>
class ShiftRegister {
public:
ShiftRegister(unsigned size) : _reg(size) {
}
bool& operator[](unsigned ix) {
return _reg[ix];
}
const bool& operator[](unsigned ix) const {
return _reg[ix];
}
private:
std::vector<bool> _reg;
};
int main(int argc, char* argv[]) {
ShiftRegister sftreg(2);
}
------------------ end of main.cpp ----------------
The above contains two definitions of the subscript operator one
ordinary and one constant. Compiling this I get
1. For the ordinary or variable subscript operator
error: could not convert `std::vector<bool,
_Alloc>::operator[](unsigned int) [with _Alloc =
std::allocator<bool>](ix)'
to `bool&'
2. For the constant subscript operator
warning: returning reference to temporary
I think I am missing something here, but looking in books or on the
internet does not make me understand what I am doing wrong i.e. they
seem to be doing the same thing (but with arrays or pointers - not with
vectors.)
I hope someone has some ideas on how to get this to work.
Thanks a lot.
O.O.
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Ben Radford
Guest
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| Posted: Fri Jan 27, 2006 12:22 pm Post subject: Re: Subscript operator overloading with vectors |
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[email]olson_ord (AT) yahoo (DOT) it[/email] wrote:
| Quote: | Hi,
I am not exactly new to C++, but I have never done operator
overloading before.
I have some old code that tries to implement a Shift Register - but I
cannot seem to get it to work. Here's a simpler version of it.
-------------------- main.cpp---------------------------
# include <iostream
# include
# include
class ShiftRegister {
public:
ShiftRegister(unsigned size) : _reg(size) {
}
bool& operator[](unsigned ix) {
return _reg[ix];
}
const bool& operator[](unsigned ix) const {
return _reg[ix];
}
private:
std::vector
};
int main(int argc, char* argv[]) {
ShiftRegister sftreg(2);
}
------------------ end of main.cpp ----------------
The above contains two definitions of the subscript operator one
ordinary and one constant. Compiling this I get
1. For the ordinary or variable subscript operator
error: could not convert `std::vector<bool,
_Alloc>::operator[](unsigned int) [with _Alloc =
std::allocator<bool>](ix)'
to `bool&'
2. For the constant subscript operator
warning: returning reference to temporary
I think I am missing something here, but looking in books or on the
internet does not make me understand what I am doing wrong i.e. they
seem to be doing the same thing (but with arrays or pointers - not with
vectors.)
I hope someone has some ideas on how to get this to work.
Thanks a lot.
O.O.
|
My first thoughts on this are that it's to do with the specialisation of
the std::vector template for bools (ie std::vector<bool> is a vector of
bits not bools which the standard says must be at least 1 byte). Does
the code work if you use another primitive type?
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Ben Radford
Guest
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| Posted: Fri Jan 27, 2006 12:32 pm Post subject: Re: Subscript operator overloading with vectors |
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|
Ben Radford wrote:
| Quote: | olson_ord (AT) yahoo (DOT) it wrote:
Hi,
I am not exactly new to C++, but I have never done operator
overloading before.
I have some old code that tries to implement a Shift Register - but I
cannot seem to get it to work. Here's a simpler version of it.
-------------------- main.cpp---------------------------
# include <iostream
# include
# include
class ShiftRegister {
public:
ShiftRegister(unsigned size) : _reg(size) {
}
bool& operator[](unsigned ix) {
return _reg[ix];
}
const bool& operator[](unsigned ix) const {
return _reg[ix];
}
private:
std::vector
};
int main(int argc, char* argv[]) {
ShiftRegister sftreg(2);
}
------------------ end of main.cpp ----------------
The above contains two definitions of the subscript operator one
ordinary and one constant. Compiling this I get
1. For the ordinary or variable subscript operator
error: could not convert `std::vector<bool,
_Alloc>::operator[](unsigned int) [with _Alloc =
std::allocator<bool>](ix)'
to `bool&'
2. For the constant subscript operator
warning: returning reference to temporary
I think I am missing something here, but looking in books or on the
internet does not make me understand what I am doing wrong i.e. they
seem to be doing the same thing (but with arrays or pointers - not with
vectors.)
I hope someone has some ideas on how to get this to work.
Thanks a lot.
O.O.
My first thoughts on this are that it's to do with the specialisation of
the std::vector template for bools (ie std::vector<bool> is a vector of
bits not bools which the standard says must be at least 1 byte). Does
the code work if you use another primitive type?
|
To elaborate, the bools in std::vector<bool> don't exist as bools. When
you access the vector you get given a temporary bool created by
examining whether the relative bit is on or off. Since it is only
temporary you can't return a reference to it. I hope that's a bit
clearer than my initial reply =)
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olson_ord@yahoo.it
Guest
|
| Posted: Fri Jan 27, 2006 2:32 pm Post subject: Re: Subscript operator overloading with vectors |
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Very, very good guess Ben. I never realised or even thought about it.
Yes I tried to compile my program with this bool replaced by int - and
it works.
However I actually need bool 's. (If you have any idea about what a
Shift Register is - you would know why I don't really require
integers.) I also wanted to optimize this for speed. I would think
about this problem and see what can be done - now that I at least know
what the problem is.
Thanks once again.
O.O.
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Ben Pope
Guest
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| Posted: Fri Jan 27, 2006 2:37 pm Post subject: Re: Subscript operator overloading with vectors |
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[email]olson_ord (AT) yahoo (DOT) it[/email] wrote:
| Quote: | Hi,
I am not exactly new to C++, but I have never done operator
overloading before.
I have some old code that tries to implement a Shift Register - but I
cannot seem to get it to work. Here's a simpler version of it.
|
<snip std::vector
| Quote: | I hope someone has some ideas on how to get this to work.
Thanks a lot.
|
Are you aware of std::bitset?
Ben Pope
--
I'm not just a number. To many, I'm known as a string...
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roberts.noah@gmail.com
Guest
|
| Posted: Fri Jan 27, 2006 2:43 pm Post subject: Re: Subscript operator overloading with vectors |
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[email]olson_ord (AT) yahoo (DOT) it[/email] wrote:
| Quote: | Very, very good guess Ben. I never realised or even thought about it.
Yes I tried to compile my program with this bool replaced by int - and
it works.
However I actually need bool 's. (If you have any idea about what a
Shift Register is - you would know why I don't really require
integers.)
|
Use chars...there really isn't a diff anyway.
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Marcus Kwok
Guest
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| Posted: Fri Jan 27, 2006 3:02 pm Post subject: Re: Subscript operator overloading with vectors |
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Ben Radford <benjamin.radford (AT) new (DOT) ox.ac.uk> wrote:
| Quote: | Ben Radford wrote:
My first thoughts on this are that it's to do with the specialisation of
the std::vector template for bools (ie std::vector<bool> is a vector of
bits not bools which the standard says must be at least 1 byte). Does
the code work if you use another primitive type?
To elaborate, the bools in std::vector<bool> don't exist as bools. When
you access the vector you get given a temporary bool created by
examining whether the relative bit is on or off. Since it is only
temporary you can't return a reference to it. I hope that's a bit
clearer than my initial reply =)
|
Yes, for more information:
When Is a Container Not a Container?
http://www.gotw.ca/publications/mill09.htm
vector<bool> Is Nonconforming, and Forces Optimization Choice
http://www.gotw.ca/publications/N1185.pdf
vector<bool>: More Problems, Better Solutions
http://www.gotw.ca/publications/N1211.pdf
--
Marcus Kwok
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