How to get element from const map? [duplicate] - c++

Question

This question already has answers here:
Const map element access
(4 answers)
Closed 1 year ago.
I have some struct like bellow to get static const map:
struct SupportedPorts{
static std::map<std::string, std::string> init()
{
std::map<std::string, std::string> port;
port["COM1"] = "ttySO1";
port["COM2"] = "ttySO2";
port["USB"] = "ttyUSB0";
return port;
}
static const std::map<std::string, std::string> supported_ports;
};
But I have problem when I trying get some value by key. In cpp file of class Exmaple:
#include "Example.h"
const std::map<std::string, std::string> SupportedPorts::supported_ports = SupportedPorts::init();
Example::Example(){
std::cout << SupportedPorts::supported_ports['COM1'];
}
I get error like bellow:
note: initializing argument 1 of ‘std::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’
basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
error: conversion to non-const reference type ‘std::map<std::basic_string<char>, std::basic_string<char> >::key_type&& {aka class std::basic_string<char>&&}’ from rvalue of type ‘std::basic_string<char>’ [-fpermissive]
std::cout << SupportedPorts::supported_ports['COM1'];
^
(null):0: confused by earlier errors, bailing out

Answer 1

The operator [] is declared for the class template std::map the following way
T& operator[](const key_type& x);
T& operator[](key_type&& x);
That is the the operator returns a non-constant reference that may not be done for a constant object. And the operator may be called only for a non-constant object because it is declared without the qualifier const.
Instead use the function at declared like
T& at(const key_type& x);
const T& at(const key_type& x) const;
Also instead of a string literal you are using a multibyte character literal in this statement
std::cout << SupportedPorts::supported_ports['COM1'];
Write
std::cout << SupportedPorts::supported_ports.at( "COM1" );
Here is a demonstrative program.
#include <iostream>
#include <string>
#include <map>
int main()
{
const std::map<std::string, std::string> supported_ports =
{
{ "COM1", "ttySO1" }, { "COM2", "ttySO2" }, { "USB", "ttyUSB0" }
};
std::cout << supported_ports.at( "COM1" ) << '\n';
return 0;
}
The program output is
ttySO1