Wikipedia:Reference desk/Archives/Computing/2016 February 3#more STL questions

First of all, thank you to everyone who helped me yesterday; I really appreciate it. Now, why doesn't the following code compile?

1. include
2. include
3. include
4. include

typedef std::vector myvec;

typedef std::unordered_map > > uom;

int f(){

myvec v;

uom S;

v.clear();

v.push_back(3);

v.push_back(-3);

S[v] = 4.4;

return (0);

}

The compiler gives a whole bunch of error messages starting with

Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_map:386:17: error: no matching function for

call to object of type 'const std::__1::equal_to > >'

{return static_cast(*this)(__x);}

^19:59, 3 February 2016 (UTC)19:59, 3 February 2016 (UTC)19:59, 3 February 2016 (UTC)19:59, 3 February 2016 (UTC)19:59, 3 February 2016 (UTC)19:59, 3 February 2016 (UTC)~

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:2014:21: note: in instantiation of member

function 'std::__1::__unordered_map_hasher >, std::__1::__hash_value_type

std::__1::allocator >, double>, std::__1::equal_to > >, true>::operator()' requested here

size_t __hash = hash_function()(__k);

I don't understand these messages, can anyone advise? Thanks, Robinh (talk) 19:59, 3 February 2016 (UTC)

:Don't use Nimur's code; it is wrong (and causing compilation errors) for reasons unrelated to your problem. To use unordered_map, just replace map with unordered_map in your code and provide a hash function for vector<int>. In the previous thread I linked to a couple of possible hash functions and an explanation of how to use custom hash functions with unordered_map. -- BenRG (talk) 03:06, 4 February 2016 (UTC)