Auto deduction of non-type template char parameters from a string in c++11

Let's say we have the following structure defined

template <char T, char S>
struct chars_trait {}

Moreover, we have a bunch of two-char strings, for instance

const std::string ab = "ab";
const std::string cd = "cd";
const std::string ef = "ef";

My goal is to create chars_trait object for each of those strings, so

ab -> chars_trait<'a','b'>{}
cd -> chars_trait<'c','d'>{}
ef -> chars_trait<'e','f'>{}

and use them, for instance, like this

std::for_each(vecOfStrs.begin(), vecOfStrs.end(),
              [&](const std::string& str){doSthWithCharsTrait(getCharTrait(str));});

I have tried to employ a map but got stuck with two problems - first that this approach is possible from c++14, and second, much more significant - it does not work. So my question is - can I somehow improve my code? Or there is no chance of making it as I proposed?

All code here

#include <iostream>
#include <string>
#include <map>

const std::string ab = "ab";
const std::string cd = "cd";
const std::string ef = "ef";

template <char T, char S>
struct chars_trait {};

template <char T, char S>
std::map<std::string, chars_trait<T, S>> MAPPER
{
    {ab, chars_trait<'a', 'b'>{}},
    {cd, chars_trait<'c', 'd'>{}},
    {ef, chars_trait<'e', 'f'>{}}  
};

template <char T, char S>
void print(const chars_trait<T, S>&)
{
    std::cout << T << S << std::endl;
}

void print(const std::string& str)
{
    print(MAPPER.at(str));
}


int main()
{
    print(ab);
    return 0;
}