How can I create new style for creating loop? - c++

Question

I want to use macros to create my source code simpler
This is my code
constexpr auto make_index_sequence_array(size_t i ,std::index_sequence<arg...>) {
return std::array<size_t, sizeof...(arg)> {i+arg...};
}
#define RANGE(start , end) make_index_sequence_array(start,std::make_index_sequence<end-start>{})
#define ZeroTo(end) RANGE(0 , end)
#define Repeat(end) for(auto itr : RANGE(0 , end))
void main() {
vector<int> a = { 1,2,3 };
for (auto row : RANGE(0,a.size()))
cout << std::setw(4) << a[row];
cout << '\n';
}
I know I can use :
void main() {
vector<int> a = { 1,2,3 };
for (auto itr: a)
cout << std::setw(4) << itr;
cout << '\n';
}
But it is a simple example and I want to use this style in more situations.
The error while compiling is:
Error C2975 '_Size': invalid template argument for 'std::make_index_sequence', expected compile-time constant expression
How can I use my macros? Or even is it possible?

Answer 1

My recommendation is that you use iterators, I wrote an example below. Something like this would do it in C++ < C++17:
#include <iostream>
template <class T>
struct RangeIter {
RangeIter(T from, T to, T curr ) :
_from(from), _to(to), _curr(curr) {
}
T operator*() const {
return _curr;
}
T operator++() {
++_curr;
return _curr;
}
bool operator==(const RangeIter & other) {
assert(_from == other._from && _to == other._to);
return _curr == other._curr;
}
bool operator!=(const RangeIter & other) {
return !(_curr == other._curr);
}
T _from, _to, _curr;
};
template <class T>
struct Range {
Range(T from, T to) : _from(from), _to(to) {}
RangeIter<T> begin() { return RangeIter<T>(_from, _to, _from); }
RangeIter<T> end() {
return RangeIter<T>(_from, _to, _to);
}
T _from, _to;
};
template <class T>
Range<T> makeRange(T to, T from) {
return Range<T>(to, from);
}
int main() {
for (auto i : makeRange(0, 10)) {
std::cout << i << std::endl;
}
}
For C++17 you can use different types for the begin and end iterator and improve on this. You can use sentinels. You could take a look here: How the new range-based for loop in C++17 helps Ranges TS?
A C++-17 only solution here:
#include <iostream>
template <class T>
struct RangeSentinel {
RangeSentinel(T stopVal) : _stopVal(stopVal) {}
T _stopVal;
};
template <class T>
struct RangeIter {
RangeIter(T from, T to, T curr) :
_from(from), _to(to), _curr(curr) {
}
T operator*() const {
return _curr;
}
T operator++() {
++_curr;
return _curr;
}
bool operator==(const RangeSentinel<T> & other) {
assert(_from == other._from && _to == other._to);
return _curr == other._stopVal;
}
bool operator!=(const RangeSentinel<T> & other) {
return !(_curr == other._stopVal);
}
T _from, _to, _curr;
};
template <class T>
struct Range {
Range(T from, T to) : _from(from), _to(to) {}
RangeIter<T> begin() { return RangeIter<T>(_from, _to, _from); }
RangeSentinel<T> end() {
return RangeSentinel<T>(_to);
}
T _from, _to;
};
template <class T>
Range<T> makeRange(T to, T from) {
return Range<T>(to, from);
}
int main() {
for (auto i : makeRange(0, 10)) {
std::cout << i << std::endl;
}
}
As you can see, in the C++17 solution I do not need to store again _from and _to variables, since the sentinel is a different type.