I've been trying to use c++17's std::reduce algorithm. Supposedly, it should support the same APIs supported by std::accumulate, but when compiling both in clang++-9.0 and gcc-9.2 with --std=c++17, the call to std::accumulate compiles successfully, and the call to std::reduce doesn't.
I tried a couple of things so far:
- Defining the
lambdaboth inside and outside of the call - Specifying explicitly the types with
std::reduce<...>syntax - Using the overloads with execution policy parameters
- Got errors on
#include <execution>
- Got errors on
The following is a sample code that uses std::reduce and std::accumulate to sum the lengths of strings in a std::vector:
#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <numeric>
int main() {
auto op = [](auto acc, auto val) {
return acc + val.size();
};
std::vector<std::string> v = {"a", "bb", "ccc", "dddd"};
int a = std::reduce(v.begin(), v.end(), 0, op);
int b = std::accumulate(v.begin(), v.end(), 0, op);
return 0;
}
Compilation fails, in the line calling to std::reduce with the following output from clang-9.0.0:
<source>:12:13: error: no matching function for call to 'reduce'
int a = std::reduce(v.begin(), v.end(), 0, op);
^~~~~~~~~~~
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/pstl/glue_numeric_defs.h:26:1: note: candidate template ignored: deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char> *, std::vector<std::__cxx11::basic_string<char>, std::allocator<std::__cxx11::basic_string<char> > > >' vs. 'int')
reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init);
^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/pstl/glue_numeric_defs.h:21:1: note: candidate function template not viable: requires 5 arguments, but 4 were provided
reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
^
/opt/compiler-explorer/gcc-9.2.0/lib/gcc/x86_64-linux-gnu/9.2.0/../../../../include/c++/9.2.0/pstl/glue_numeric_defs.h:31:1: note: candidate function template not viable: requires 3 arguments, but 4 were provided
reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last);
^
1 error generated.
and the following output from gcc-9.2:
<source>: In function 'int main()':
<source>:12:50: error: no matching function for call to 'reduce(std::vector<std::__cxx11::basic_string<char> >::iterator, std::vector<std::__cxx11::basic_string<char> >::iterator, int, main()::<lambda(auto:1, auto:2)>&)'
12 | int a = std::reduce(v.begin(), v.end(), 0, op);
| ^
In file included from /opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/numeric:229,
from <source>:5:
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:21:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp, class _BinaryOperation> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Tp, _BinaryOperation)'
21 | reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init,
| ^~~~~~
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:21:1: note: template argument deduction/substitution failed:
<source>:12:50: note: deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >' and 'int')
12 | int a = std::reduce(v.begin(), v.end(), 0, op);
| ^
In file included from /opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/numeric:229,
from <source>:5:
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:26:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, _ForwardIterator2> std::reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, _Tp)'
26 | reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, _Tp __init);
| ^~~~~~
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:26:1: note: template argument deduction/substitution failed:
<source>:12:50: note: deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >' and 'int')
12 | int a = std::reduce(v.begin(), v.end(), 0, op);
| ^
In file included from /opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/numeric:229,
from <source>:5:
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:31:1: note: candidate: 'template<class _ExecutionPolicy, class _ForwardIterator> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::value_type> std::reduce(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator)'
31 | reduce(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last);
| ^~~~~~
/opt/compiler-explorer/gcc-9.2.0/include/c++/9.2.0/pstl/glue_numeric_defs.h:31:1: note: template argument deduction/substitution failed:
<source>:12:50: note: deduced conflicting types for parameter '_ForwardIterator' ('__gnu_cxx::__normal_iterator<std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >' and 'int')
12 | int a = std::reduce(v.begin(), v.end(), 0, op);
| ^