错误：从 &#39;const char&#39; 转换为非标量类型 &#39;std::string&#39; {aka &#39;std::__cxx11::basic_string<char> &#39;} 请求

Question

I want to count letters in const string&, and save result in map.
But compiler throws an error:

error: conversion from 'const char' to non-scalar type 'std::string' {aka 'std::__cxx11::basic_string'} requested

My code:

map<string, int>& MakeWordCounter (const string& word, map<string, int>& counter) {
    for (string i : word) {
        counter[i] = count(word.begin(), word.end(), i);
    }
}

Why this error is happening and how to fix it?

Answer 1

The dereferenced iterator of word has the type char , we can't convert it to string. And the function declaration can be more clear to directly return the map.

The key type here is char, we don't need to use a string type, its misleading and is a waste.

std::map<char, size_t> MakeWordCounter(const std::string& word) {
  std::map<char, size_t> counts;
  for (auto ch : word) {
    counts[ch]++;
  }
  return counts;
}

Or we can use STL algorithm instead of loop:

std::map<char, size_t> MakeWordCounter2(const std::string& word) {
  return std::accumulate(word.begin(), word.end(), std::map<char, size_t>{},
                         [](auto init, char cur) {
                           init[cur] += 1;
                           return init;
                         });
}

You may doubt the second version's performance, so I add the benchmark here, the two versions are generally the same.

https://quick-bench.com/q/OSzzp70rBSdlpivEMmMIj0aGJfU

Online demo