Error inserting string key,value in unordered_map

Question

I am writing my first STL program in C++, and I am facing this issue.

This is my program :

#include <iostream> #include <unordered_map> using namespace std; int main() { int n,m; // n : number of file extenstions // m : number of filenames to identify string extension, type, filename; cin >> n >> m; unordered_map<string,string> hashmap; while(n--) { cin >> extension >> type; hashmap.insert(make_pair<string,string>(extension,type)); } while(m--) { cin >> filename; extension = filename.substr(filename.find_last_of('.')+1); cout << extension << endl; } }

My input file is :

5 6 html text/html htm text/html png image/png svg image/svg+xml txt text/plain index.html this.file.has.lots.of.dots.txt nodotsatall virus.exe dont.let.the.png.fool.you case.matters.TXT

I am getting error : no matching function for call to ‘make_pair(std::string&, std::string&)’ . I cannot figure out the problem.

Is your input relevant? If not, remove it. Ditto for the loops, extension finding, lookup, output. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Oct 22, 2015 at 5:57

therainmaker · Accepted Answer · 2015-10-22 05:51:34Z

2

The error is in the line

make_pair<string,string>(extension,type)

It should instead be

make_pair(extension, type)

answered Oct 22, 2015 at 5:51

therainmaker

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3 Comments

skypjack Over a year ago

Or at least make_pair<string&,string&>(extension,type), but I like the auto deduction...

InsaneCoder Over a year ago

@skypjack : I was referencing from here : cplusplus.com/reference/unordered_map/unordered_map/insert . There also they have done the same way, <string,double> and not <string&,double>, but still that works. Please help me clear the doubt.

skypjack Over a year ago

First of all, I'd rather use make_pair(extension, type) and let the compiler to deduce the types in any case, to avoid any risks. That said, see en.cppreference.com/w/cpp/utility/pair/make_pair. Indeed, as far as I understand (I'm far from being an expert) the deduced types in your case would be T&, U&, thus using the couple T, U (as <string, string>) ends in a function prototype that doesn't accept references as arguments, while your are explicitly using references to initialize the pair (that's why you got the error). I simply suggested to specify the right type for them. :)

skypjack · Accepted Answer · 2015-10-22 16:38:52Z

First of all, I'd rather use make_pair(extension, type) and let the compiler to deduce the types in any case, to avoid any risks, as someone else suggested.

Anyway, I see that the details below are interesting from your point of view, thus it's worth to put them in a dedicated response, hoping they are also right.

That said, see the documentation.

As far as I understand (I'm far from being an expert) the deduced types in your case would be T&, U&, thus using the couple T, U (as <string, string>) ends in a function prototype that doesn't accept references as arguments (that's why you got the error and that one specifically).

You can use instead the following pattern in your case:

make_pair<string&,string&>(extension,type)

This one works for you actually use references to initialize the pair and you are going to require the sames as template arguments.

Again, for the make_pair can easily deduce the types of its arguments, you can freely omit them and I strongly suggest to do that.

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