Re: Bitset template error
On 10/3/2010 9:16 AM, Michal Sekletar wrote:
I am programming NetFlow collector and in module dealing with sorting ip
addresses into classes according do predefined rules I want to create
std::bitset to represent network mask. I have a method
void foo(struct in_addr addr, const size_t len).
I want to create bit set in this method std::bitset<len> b; but compiler
is complaining :
trie.cpp:22:17: error: ?len? cannot appear in a constant-expression
trie.cpp:22:20: error: template argument 1 is invalid.
It is fine to create bitset like this std::bitset<24> b; on the other
hand if I pass net mask length as an argument compilation ends up with
an error.
So, am I forgetting something important about std::bitset template class?
No, you're forgetting something important about constant-expressions. A
function argument is not a constant-expression, regardless of the
qualifications (in your case you qualified the argument 'const'). Since
the argument is initialized at run-time (when the function is actually
called), its value is not known until run-time, and the compiler cannot
use that argument to instantiate a template with a non-type template
argument of the type 'size_t' (like in the case of 'std::bitset').
V
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