Passing std algorithm. Template deducing [duplicate] - c++

Question

This question already has answers here:
Why can't "transform(s.begin(),s.end(),s.begin(),tolower)" be complied successfully?
(5 answers)
Closed 6 years ago.
I was trying to make this small example work
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<char> v{'a', 'b', 'c', 'd'};
std::transform(v.begin(), v.end(), v.begin(), std::toupper);
std::for_each(v.begin(), v.end(), [](char & c) {
std::cout << c << " ";
});
}
I think it's obvious, what i'm trying to achive. However, I get these errors
||=== Build file: "no target" in "no project" (compiler: unknown) ===|
C:\Users\vitaly.bushaev\Documents\vk cup\1.cpp||In function 'int main()':|
C:\Users\vitaly.bushaev\Documents\vk cup\1.cpp|8|error: no matching function for call to 'transform(std::vector<char>::iterator, std::vector<char>::iterator, std::vector<char>::iterator, <unresolved overloaded function type>)'|
C:\Users\vitaly.bushaev\Documents\vk cup\1.cpp|8|note: candidates are:|
C:\Program Files (x86)\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_algo.h|4152|note: template<class _IIter, class _OIter, class _UnaryOperation> _OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation)|
C:\Program Files (x86)\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_algo.h|4152|note: template argument deduction/substitution failed:|
C:\Users\vitaly.bushaev\Documents\vk cup\1.cpp|8|note: couldn't deduce template parameter '_UnaryOperation'|
C:\Program Files (x86)\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_algo.h|4189|note: template<class _IIter1, class _IIter2, class _OIter, class _BinaryOperation> _OIter std::transform(_IIter1, _IIter1, _IIter2, _OIter, _BinaryOperation)|
C:\Program Files (x86)\CodeBlocks\MinGW\lib\gcc\mingw32\4.9.2\include\c++\bits\stl_algo.h|4189|note: template argument deduction/substitution failed:|
C:\Users\vitaly.bushaev\Documents\vk cup\1.cpp|8|note: candidate expects 5 arguments, 4 provided|
||=== Build failed: 1 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|
So I'm reading this and I understand the error, but i can't understand why is it happening. Sure this works fine:
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<char> v{'a', 'b', 'c', 'd'};
using it = std::vector<char>::iterator;
std::transform<it, it, int (int)>(v.begin(), v.end(), v.begin(), std::toupper);
std::for_each(v.begin(), v.end(), [](char & c) {
std::cout << c << " ";
});
}
So here's the question:
1) Why c++ can't deduce template parameter here ?
2) Is there a way around that, to not specify types ?

Answer 1

There is more than one overload for std::toupper.
Template deduction happens before overload selection, so you have to either be specific or wrap the call in a lambda or function object.
#include <iostream>
#include <vector>
#include <algorithm>
int main() {
std::vector<char> v{'a', 'b', 'c', 'd'};
// method 1
struct make_upper
{
int operator()(int ch) const { return std::toupper(ch); }
};
std::transform(v.begin(), v.end(), v.begin(), make_upper());
// method 2
std::transform(v.begin(), v.end(), v.begin(), [](auto&& ch) { return std::toupper(ch); });
}
references:
http://en.cppreference.com/w/cpp/locale/toupper
http://en.cppreference.com/w/cpp/string/byte/toupper