无法使用std :: string作为键在std :: map上进行迭代

Question

My question is almost identical to this one , but the solution there hasn't resolved my error.

In main.h I have:

#include <map>
#include <string>

std::map<std::string, int64_t> receive_times;

And in main.cpp :

std::map<std::string, int64_t>::const_iterator iter;
std::map<std::string, int64_t>::const_iterator eiter = receive_times.end();

for (iter = receive_times.begin(); iter < eiter; ++iter)
  printf("%s: %ld\n", iter->first.c_str(), iter->second);

However, when I try and compile I get the following error:

error: invalid operands to binary expression ('std::map<std::string, int64_t>::const_iterator' (aka '_Rb_tree_const_iterator<value_type>') and 'std::map<std::string, int64_t>::const_iterator'
  (aka '_Rb_tree_const_iterator<value_type>'))
  for (iter = receive_times.begin(); iter < eiter; ++iter)
                                     ~~~~ ^ ~~~~~

The solution in the question I linked to at the top is because there was a missing #include <string> , but obviously I have that included. Any hints?

Answer 1

Iterators are not relationally comparable, only for equality. So say iter != eiter .

A less noisy way to write the loop:

for (std::map<std::string, int64_t>::const_iterator iter = receive_times.begin(),
     end = receive_times.end(); iter != end; ++iter)
{
  // ...
}

(Usually best to typedef the map type!)

Or, in C++11:

for (auto it = receive_times.cbegin(), end = receive_timed.cend(); it != end; ++it)

Or even:

for (const auto & p : receive_times)
{
  // do something with p.first and p.second
}

Answer 2

容器迭代器的惯用循环结构为：

for (iter = receive_times.begin(); iter != eiter; ++iter)