从带有 std::tuple 的 map 中输出

Question

I have made a small map that I call BMW . It contains the keys Usage and Diesel , as shown below.

#include <iostream>
#include <bits/stdc++.h>
#include <map>
#include <vector>
using namespace std;

int main()
{

    // initialize container
    std::map<string, std::tuple<string, string>> BMW;

    // insert elements
    BMW.insert({"Usage", {"1", "2"}});
    BMW.insert({"Disel", {"2", "3"}});

    std::cout << "Usage => " << BMW.find('Usage')->second << '\n';

    return 0;
}

What I want to do is to find the key Usage in the map and then print out the strings containing the values for Usage (1, 2). The code I tried with does not work and I am not able to find a good answer why here on Stackoverflow. Here is the error I get:

error: no matching function for call to 'std::map<std::__cxx11::basic_string<char>, std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::find(int)'|

It would be great if I could be able to get only one of the strings, like the first one, if I want to do that.

(the strings will later be converted to int when this is appropriate, but because of technical reasons I want to read them as strings for now)

Answer 1

BMW.find('Usage')->second
//       ^     ^

These are single quotes .

Single quotes delimit char literals.

You wanted double quotes .

That's because double quotes delimit string literals.

---

The reason the error message says you're trying to use the non-existent find(int) overload, is that a char literal that actually has more than one character in it is a special thing called a "multi-character literal", of int type and implementation-defined value. You usually don't intend to use these.

---

Next, you're going to run into the problem that there is no built-in cout formatting for a tuple. You will have to take BMW.find("Usage")->second and give it to some function that prints the contents in whatever way you want.

Answer 2

For starters if you are using the member function find then you should check whether the data is indeed found.

In this expression

BMW.find('Usage')

you are using a multibyte character literal instead of a string literal. The expression must be written like

BMW.find( "Usage" )

There is no operator << for an object of the type std::tuple . You have to output individual data members of the tuple. Here is a demonstrative program.

#include <iostream>
#include <string>
#include <tuple>
#include <map>
#include <iterator>

int main()
{
    std::map<std::string, std::tuple<std::string, std::string>> BMW;

    // insert elements
    BMW.insert( { "Usage", { "1", "2" } } );
    BMW.insert( { "Disel", { "2", "3" } } );

    auto it = BMW.find( "Usage" );

    if ( it != std::end( BMW ) )
    {
        std::cout << "Usage => ( " << std::get<0>( it->second ) << ", " << std::get<1>( it->second ) << " )\n";
    }        
}

Its output is

Usage => ( 1, 2 )