C++ unordered_map causing compile-time error

Question

I have the following:

#include<iostream>
#include<unordered_map>
#include<tuple>

using namespace std;

class CTest {
    // Properties
    public:
        unordered_map<const string, tuple<int, int> > Layout;
    // Methods
    public:
        CTest ();
        ~CTest ();
};

CTest::CTest () {
    Layout["XYZ"] = make_tuple (0, 1);
}

CTest::~CTest () {
  // Do nothing
}

int main (int argc, char *argv[]) {
    CTest Test;
    return 0;
}

Compiling this simple programme gives the following error:

error C2678: binary '==' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)

I'm using Visual Studio 2010 Professional in Windows 7.

hmjd · Accepted Answer · 2012-01-16 20:19:28Z

3

In addition to changing Layout to:

unordered_map<string, tuple<int, int> > Layout;

as stated by Johan and Benjamin you also need to #include <string>.

Note, I do not understand why the change to Layout is required, even if the const is superfluous.

edited Jan 16, 2012 at 20:19

answered Jan 16, 2012 at 20:14

hmjd

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3 Comments

Shredderroy Over a year ago

Thanks for the response. Sorry, I did indeed have #include<cstring> in my actual source code. Unfortunately, I get the same error even after the removal of const.

Seth Carnegie Over a year ago

@Shredderroy no, not cstring just string

hmjd Over a year ago

Could you update the post with your exact code in case there is some other discrepancy? After I made these changes the code compiles for me in VS2010 (with extensions disabled).

Community · Accepted Answer · 2017-05-23 10:28:27Z

2

You need to remove the const qualifier on the key.

unordered_map<const string, tuple<int, int> > Layout;

into

unordered_map<string, tuple<int, int> > Layout;

this is because keys are always const, according to this answer:

Using a const key for unordered_map

I figure the underlying reason is related to Duplicate const qualifier allowed in C but not in C++?

Also, as other posts pointed out you may need to include string (although with gcc it seems to come with iostream)

edited May 23, 2017 at 10:28

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answered Jan 16, 2012 at 20:09

Johan Lundberg

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2 Comments

Shredderroy Over a year ago

Thanks for the links. As noted above, the compilation fails even with const removed, but I will read your links for further hints.

Johan Lundberg Over a year ago

Shredderroy your code works and compiles fine with the const removed under gcc4.7 c++11.

mwigdahl · Accepted Answer · 2012-01-16 20:19:30Z

1

Visual Studio 2010 will compile your source code as is as long as you #include <string> so that it has the comparison tests for string available.

As the other posters mention, you should also make your key a regular string rather than a const string, as this is conformant to the STL standard, but this is not strictly necessary to make VS 2010 compile the source above.

answered Jan 16, 2012 at 20:19

mwigdahl

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2 Comments

hmjd Over a year ago

FYI, if you compile with extensions disabled compilation fails unless you make the change to Layout.

Shredderroy Over a year ago

Thanks for your response. As noted aobve, the compilation fails even with const removed.

Benjamin Lindley · Accepted Answer · 2012-01-16 20:43:18Z

1

As was pointed out, the reason for your initial error was that you needed to include <string>. However, you may have another problem with this:

unordered_map<const string, tuple<int, int> > Layout;

You (may) need to remove the const from that string:

unordered_map<string, tuple<int, int> > Layout;

This may not be necessary on your compiler, but it is on mine. First of all, the const is superfluous, map/unordered_map keys are const anyway, but that is not the problem. The problem has to do with the hash function template not working for const types.

The following simple program isolates the problem for me:

#include <functional>
int main (int argc, char *argv[])
{
    std::hash<const int> h;
    h(10);
}

http://ideone.com/k2vSy

undefined reference to `std::hash<int const>::operator()(int) const'

I cannot, at present time, explain this.

edited Jan 16, 2012 at 20:43

answered Jan 16, 2012 at 20:10

Benjamin Lindley

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5 Comments

Benjamin Lindley Over a year ago

@SethCarnegie: I cannot. I don't think it has anything to do with the reason that it's unnecessary, because that reason also applies to std::map. However, I get no compile error with map<const string, tuple<int, int>>. I know it has something to do with the hash function, but I cannot explain it.

Seth Carnegie Over a year ago

Ok, +1 and thanks for trying then. Edit: Yeah, I don't get any compile errors either. Weird.

Shredderroy Over a year ago

Thanks the response. The compilation still fails, even with const removed, but I am trying to use the discussions here to investigate further.

Shredderroy Over a year ago

Benjamin, your response also contains the solution to my problem. Seth's came first, so I accepted that. But thank you very much.

Chris Badger Over a year ago

I'm late to the party here, but only specific specializations of std::hash are provided by the library. std::hash<string> is one of them, std::hash<const string> is not.

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