Re: std::vector of const values
On 21 Lis, 23:52, Dave <thedaverud...@gmail.com> wrote:
Hi all,
I have run into this problem a few times and was hoping there was a
more elegant solution than what I was doing. Suppose I have the
following code:
#include <vector>
#include <iostream>
void doStuff( const std::vector< const int* >& ints )
{
for( size_t i = 0; i != ints.size(); ++i )
std::cout << *ints[ i ] << std::endl;
}
int main()
{
int i1, i2, i3;
std::vector< int* > ints;
ints.push_back( &i1 );
ints.push_back( &i2 );
ints.push_back( &i3 );
doStuff( ints );
return 0;
}
Now, if I try to compile this (with gcc 4.3.2, though I get similar
problems with MSVC++ 2003), I get the following error:
constvector.cc: In function 'int main()':
constvector.cc:19: error: invalid initialization of reference of type
'const std::vector<const int*, std::allocator<const int*> >&' from
expression of type 'std::vector<int*, std::allocator<int*> >'
constvector.cc:5: error: in passing argument 1 of 'void doStuff(const
std::vector<const int*, std::allocator<const int*> >&)'
Which is really just saying that the parameter to doStuff() is vector<
const int* > but the argument I am passing it is vector< int* >
(without the const). To get around this, I have to create a copy of
the vector with the right template param type and then pass that to my
function.
However, it seems to me that, if the compiler wanted to, it could just
treat the argument as vector< const int* > and just proceed. So does
anyone know of a way to do that? Or am I stuck with making copies. Is
there some fundamental reason why a vector of non-const values could
not be treated as a vector of const values?
I wouldn't worry about const correctness in this particular case. It
could
be an issue if you would write an interface for other programmer to
implement
and you wouldn't want him to mess data you pass to him. In this case I
can
think of two solutions:
1. Easy way - provide doStuff virtual method for one parameter:
virtual void doStuff(const int& val) = 0;
and non-virtual method for iterating virtual one
typedef std::vector<int*>::const_iterator Iter;
void doStuff(Iter begin, Iter end) {
for(Iter i = begin; i != end; ++i)
std::cout << **i << std::endl;
}
2. Hard (?) way - you could write iterator adapter
(e.g. ConstPtrIter<I> - I for iterator class with value_type T*)
that for iterator with value_type T* would convert it to const T*.
Method in your iterface would look sth like this:
typedef ConstPtrIter<std::vector<int*>::const_iterator> Iter
virtual void doStuff(Iter begin, Iter end) = 0;
However if you are just trying to be const correct at all costs
without a reason, just don't, drop the const from the vector
template parameter and get on with your program ;)
Cheers
Sfider
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