fixed: error C2784 from compiling adding an entry to a std::map

Got the above error, when compiling something that looked like this:

std::map<std::string, myType> myMap;
myMap[“Test”] = myType();

Looked around on the net, didn’t find the answer, thought about it for a bit and then realised I was missing #include <string> which fixed the problem. The error was a bit cryptic. Look for yourself at this dump from the compiler output:

1>C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(143) : error C2784: ‘bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)’ : could not deduce template argument for ‘const std::_Tree<_Traits> &’ from ‘const std::string’

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xtree(1372) : see declaration of ‘std::operator <‘

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(142) : while compiling class template member function ‘bool std::less<_Ty>::operator ()(const _Ty &,const _Ty &) const’

1> with

1> [

1> _Ty=std::string

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\map(72) : see reference to class template instantiation ‘std::less<_Ty>’ being compiled

1> with

1> [

1> _Ty=std::string

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xtree(26) : see reference to class template instantiation ‘std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,_Mfl>’ being compiled

1> with

1> [

1> _Kty=std::string,

1> _Ty=std::string,

1> _Pr=std::less<std::string>,

1> _Alloc=std::allocator<std::pair<const std::string,std::string>>,

1> _Mfl=false

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xtree(68) : see reference to class template instantiation ‘std::_Tree_nod<_Traits>’ being compiled

1> with

1> [

1> _Traits=std::_Tmap_traits<std::string,std::string,std::less<std::string>,std::allocator<std::pair<const std::string,std::string>>,false>

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xtree(94) : see reference to class template instantiation ‘std::_Tree_ptr<_Traits>’ being compiled

1> with

1> [

1> _Traits=std::_Tmap_traits<std::string,std::string,std::less<std::string>,std::allocator<std::pair<const std::string,std::string>>,false>

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xtree(112) : see reference to class template instantiation ‘std::_Tree_val<_Traits>’ being compiled

1> with

1> [

1> _Traits=std::_Tmap_traits<std::string,std::string,std::less<std::string>,std::allocator<std::pair<const std::string,std::string>>,false>

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\map(82) : see reference to class template instantiation ‘std::_Tree<_Traits>’ being compiled

1> with

1> [

1> _Traits=std::_Tmap_traits<std::string,std::string,std::less<std::string>,std::allocator<std::pair<const std::string,std::string>>,false>

1> ]

1> c:\projects\flewse\source\x-planeflewse\VisualSettings.h(16) : see reference to class template instantiation ‘std::map<_Kty,_Ty>’ being compiled

1> with

1> [

1> _Kty=std::string,

1> _Ty=std::string

1> ]

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xtree(1372) : see declaration of ‘std::operator <‘

1>C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(143) : error C2784: ‘bool std::operator <(const std::vector<_Ty,_Alloc> &,const std::vector<_Ty,_Alloc> &)’ : could not deduce template argument for ‘const std::vector<_Ty,_Alloc> &’ from ‘const std::string’

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\vector(1276) : see declaration of ‘std::operator <‘

1>C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(143) : error C2784: ‘bool std::operator <(const std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2> &)’ : could not deduce template argument for ‘const std::reverse_iterator<_RanIt> &’ from ‘const std::string’

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\xutility(1880) : see declaration of ‘std::operator <‘

1>C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(143) : error C2784: ‘bool std::operator <(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)’ : could not deduce template argument for ‘const std::pair<_Ty1,_Ty2> &’ from ‘const std::string’

1> C:\Program Files\Microsoft Visual Studio 8\VC\include\utility(76) : see declaration of ‘std::operator <‘

1>C:\Program Files\Microsoft Visual Studio 8\VC\include\functional(143) : error C2676: binary ‘<‘ : ‘const std::string’ does not define this operator or a conversion to a type acceptable to the predefined operator

21 Responses to fixed: error C2784 from compiling adding an entry to a std::map

Felix says:

April 19, 2010 at 5:57 pm

Thanks a lot, I was stuck with exactly the same problem.. and the same fix. I guess it was using another string definition somewhere.. THANK YOU!

Gaurav says:

October 28, 2010 at 5:39 pm

Nice catch… i had exactly the same error which wasted my 2 hrs….
Thanks a lot

Flo says:

November 18, 2010 at 12:42 am

Thanks, good trick, fix my current problem (error messages are, like usually, very explicit…) !

Dan says:

February 5, 2011 at 10:10 am

WOOW, thank you so much! Very cryptic error message!

Hongqing says:

April 19, 2011 at 4:48 pm

Thanks… With the same problem cannt #include work…until i changed it to #include…

Hongqing says:

April 19, 2011 at 4:48 pm

Thanks… With the same problem cannt #include string.h work…until i changed it to #include string …

Stephane says:

June 3, 2011 at 11:16 pm

Thanks, it was exactly what I needed.

Ashwin Kumar says:

June 8, 2011 at 7:17 am

Thank you! Its am awesome catch !!

Rayniery says:

July 27, 2011 at 11:06 am

Thanks a lot for posting this.

Nebullus says:

September 6, 2011 at 1:41 am

Yes, this solved my headache too.
Thank you, guys! 😉

phamthang says:

November 12, 2011 at 12:28 am

so much thanks

peter says:

December 11, 2011 at 11:00 pm

rock on! 🙂 saved my bacon

Alastair says:

April 5, 2012 at 12:44 am

THANKS !! 🙂 Very grateful for your answer.

Dev says:

April 18, 2012 at 8:27 am

Thanks much! Now it’s number 1 on Google/Bing with “c2784 map” 🙂

koby meir says:

September 8, 2012 at 5:32 pm

Yes, this solved my headache too.

Thanks a lot 🙂

Meteotrix says:

October 3, 2012 at 1:54 am

Aw yeah, thanks :3

nwish says:

October 16, 2012 at 11:19 am

lots thanks 🙂 you rock

Sakthipriyan Vairamani says:

November 21, 2012 at 6:07 pm

Did you find why it solved the problem? Probably string would have overloaded the ‘<' operator?

Mr.Venkatesh Pai says:

October 15, 2013 at 8:20 pm

This can happen when map which involves a user defined datatype as the key. To resolve this issue compiler is expecting a Overloaded version for operator “<" from the user who defined that datatype. When you write overloaded operator function user must keep in mind that he must not modify the left or right hand side operands of the call(By mentioning "const" keyword) .So versions protoytpes could be as follows:
1. bool operator < (const datatype& arg) const
{
//….body
}
2. bool operator < (datatype arg) const
{
//….body
}

- Mr.Venkatesh Pai says:
  
  October 15, 2013 at 8:28 pm
  
  If my explanation has helped you please reply me with Thank you…I want see to how many people it helps
  
- johnstewien says:
  
  October 16, 2013 at 7:53 am
  
  Thank you for your explanation, I only just got a chance to check my e-mails so I was a bit slow in approving your comment. I put up this post 4 years ago but it still seems popular. Good to have a full explanation for what is going on.

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