I wanted to create a little macro to simulate for(auto item : container) in VC2010 which I can then replace with the real construct when it's release开发者_StackOverflow中文版d.
There is BOOST_FOREACH, however I would like auto support.
I've tried creating a macro. However it fails when the dereferenced iterator is a constant type.
#define _LIB_FOREACH_LINENAME_CAT(name, line) name##line
#define _LIB_FOREACH_LINENAME(name, line) _LIB_FOREACH_LINENAME_CAT(name, line)
#define LIB_AUTO_FOREACH(item, expr) \
decltype((expr))&& _LIB_FOREACH_LINENAME(con, __LINE__)((expr)); auto _LIB_FOREACH_LINENAME(it, __LINE__) = _LIB_FOREACH_LINENAME(con, __LINE__).begin(); for(auto (item) = *_LIB_FOREACH_LINENAME(con, __LINE__).begin(); _LIB_FOREACH_LINENAME(it, __LINE__) != _LIB_FOREACH_LINENAME(con, __LINE__).end(); ++_LIB_FOREACH_LINENAME(it, __LINE__), (item) = *_LIB_FOREACH_LINENAME(it, __LINE__))
Anyone up for the challenge to correct mine or find a working implementation?
EDIT:
Notice that (expr) should only be evaluated once.
There is BOOST_FOREACH, however I would like auto support.
BOOST_FOREACH
appears to support the C++0x auto
keyword alright.
As your macro is, boost's one is superior. It also works for arrays (and probably zero-terminated char arrays) and lets you use references for the loop variable, rather than making it appear from the thin air.
Instead of
decltype((expr))&&
you should probably just write
auto&&
A problem with your approach is that it doesn't work with ranges that don't provide begin/end member functions (such as arrays). The proposed C++0x for-range loop makes use of free begin/end functions and argument dependent lookup to solve this.
Perhaps it's a good idea to include the code block as macro parameter:
#include <cstddef>
namespace for_range_helpers {
template<class Container>
auto begin(Container& c) -> decltype(c.begin())
{ return c.begin(); }
template<class Container>
auto end(Container& c) -> decltype(c.end())
{ return c.end(); }
template<class T, std::size_t N>
T* begin(T (&array)[N])
{ return array+0; }
template<class T, std::size_t N>
T* end(T (&array)[N])
{ return array+N; }
} // namespace for_range_helpers
#define FOR_RANGE(init_,expression_,...) \
do { \
using ::for_range_helpers::begin; \
using ::for_range_helpers::end; \
auto&& range_ = expression_; \
for (auto iter_ = begin(range_), end_ = end(range_); \
iter_ != end_; ++iter_) \
{ \
init_ = *iter_; \
__VA_ARGS__ \
} \
} while (0)
#include <iostream>
#include <vector>
int main()
{
int arr[] = {2,3,5,7,11};
std::vector<int> vec;
FOR_RANGE(auto i, arr,
vec.push_back(i);
);
FOR_RANGE(auto i, vec,
std::cout << ' ' << i;
);
std::cout << '\n';
}
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