Is this c++ template param deduction incorrect? - c++

Question

#include <iostream>
#include <regex>
int main(void)
{
std::cmatch cm;
std::regex_match("subject", cm, std::regex("(sub)(.*)"));
//std::for_each(cm.begin(), cm.end(), [](const std::sub_match<const char *> &s){ <---- Working statement
std::for_each(cm.begin(), cm.end(), [](const std::cmatch &s){ /*<--- Non-working statement*/
std::cout << "match:" << s.str() <<std::endl;
});
return 0;
}
The error is as following:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/algorithm:853:9: error: no matching function for call to object of type '(lambda at main.cpp:73:41)'
__f(*__first);
^~~
main.cpp:73:10: note: in instantiation of function template specialization 'std::__1::for_each<std::__1::__wrap_iter<const std::__1::sub_match<const char *> *>, (lambda at main.cpp:73:41)>' requested here
std::for_each(cm.begin(), cm.end(), [](const std::match_results<const char*> &s){
^
main.cpp:73:41: note: candidate function not viable: no known conversion from 'const std::__1::sub_match<const char *>' to 'const std::match_results<const char *>' for 1st argument
std::for_each(cm.begin(), cm.end(), [](const std::match_results<const char*> &s){
^
maintool.cpp:73:41: note: conversion candidate of type 'void (*)(const std::match_results<const char *> &)'
1 error generated.
In non-working example why is template deduced as std::__1::for_each<std::__1::__wrap_iter<const std::__1::sub_match<const char *> *> ?
I was expecting param will be deduced to be std:::cmatch
Can you please explain how param deduction is working here?

Answer 1

std::cmatch is an alias for std::match_results<char const*>; you want std::sub_match<char const*>, whose alias is std::csub_match.
std::for_each(cm.begin(), cm.end(), [](const std::csub_match &s) { ... }