std::map does not automatically deduct types in C++17

[omair-khalid]

When I compile the example code for std::map, the following line gives an error:

std::map mutable_map = {std::pair{42, 42.42}, std::pair{23, 23.23}};

The error is:

class template argument deduction failed:
   std::map mutable_map = {std::pair{42, 42.42}, std::pair{23, 23.23}};

I am compiling using c++17, but there still is this error. It gets resolved only when I specify the template types of std::map i.e. std::map<int, float> mutable_map = {std::pair{42, 42.42}, std::pair{23, 23.23}};

Any idea why this is happening to me?

[jayabrata97]

Hi @omair-khalid, it is working fine for me. Can you please mention how you are compiling it?

[omair-khalid]

Hi Jay, I am compiling it using the following command:
c++ -std=c++17 -o map map.cpp

map.cpp is the name of my cpp file.

[jayabrata97]

Ok, I am also using the same. Can you please share the complete code? I will also try to compile that.

[omair-khalid]

Please find the code below:

#include <iostream>
#include <map>
#include <utility>

using std::cout;
using std::endl;

int main() {
  const std::map<int, double> const_map = {{42, 42.42}, {23, 23.23}};
  // Work with a const map
  cout << "Map is empty: " << const_map.empty() << endl;
  cout << "Map size: " << const_map.size() << endl;
  cout << const_map.at(23) << endl;
  // Work with a mutable map
  std::map mutable_map = {std::pair{42, 42.42}, std::pair{23, 23.23}};
  cout << "Map size: " << mutable_map.size() << endl;
  mutable_map[4242] = 23.42; // Creates a new key-value pair
  mutable_map[12] = 15.12;
  cout << "New map size: " << mutable_map.size() << endl;
  cout << mutable_map.at(4242) << endl;
  mutable_map.at(4242) = 42.42; // Ref to an existing value
  cout << mutable_map.at(4242) << endl;
  mutable_map.erase(4242);
  cout << mutable_map.size() << endl;
  return 0;
}

Thanks!

[jayabrata97]

Hi @omair-khalid, It is working fine for me. I am attaching a screenshot of the code compilation. The code is in transparent background.

[niosus]

Hey @omair-khalid, sorry for a late response. For whatever reason, I was not notified of this discussion. Could you also provide some info about the system you're using? Also what does the c++ --version print?

[niosus]

For completeness, it might be that your compiler is too old and does not fully support all the C++17 features. For example, if I play with your code in the Compiler Explorer, I see that the earliest version of the GCC compiler that can compile this code is 8.1:

• The code compiles with GCC 8.1: https://godbolt.org/z/6bqYb7Trr
• And does not with GCC 7.5: https://godbolt.org/z/q8q5Ynvnv

The same story with clang, version 7.0 fails to compile the code, but 8.0 compiles it without issues.

[jayabrata97]

Hi @niosus, I have one question: the output on the right-hand side of https://godbolt.org/z/6bqYb7Trr. Is that assembly code?

[niosus]

Yep, it is. But it's not important for us for now. We only care if it compiles or not 😉

[omair-khalid]

Hey @omair-khalid, sorry for a late response. For whatever reason, I was not notified of this discussion. Could you also provide some info about the system you're using? Also what does the c++ --version print?

Hi Igor, sorry for the delay from my side as well.
I am using Ubuntu 18. c++ --version gives me c++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0

[omair-khalid]

For completeness, it might be that your compiler is too old and does not fully support all the C++17 features. For example, if I play with your code in the Compiler Explorer, I see that the earliest version of the GCC compiler that can compile this code is 8.1:

• The code compiles with GCC 8.1: https://godbolt.org/z/6bqYb7Trr
• And does not with GCC 7.5: https://godbolt.org/z/q8q5Ynvnv

The same story with clang, version 7.0 fails to compile the code, but 8.0 compiles it without issues.

Right! Ill update the compiler and clang to sort it out! Thanks for the help!