Can't compile lambda without "const" in the parameter list

Question

#include <stack> #include <list> #include <string> #include <iostream> #include <algorithm> #include <vector> #include <numeric> using namespace std; void biggies(vector<string>& str,vector<string>::size_type sz) { sort(str.begin(),str.end()); auto end_unique=unique(str.begin(), str.end()); str.erase(end_unique,str.end()); //When I remove the "const" in the parameter list, the code can't compile stable_sort(str.begin(), str.end(), [](const string&a,const string&b){return a.size()<b.size();}); auto wc=find_if(str.begin(), str.end(), [sz](string& a){return a.size()>=sz;}); for_each(wc, str.end(), [](string& s){cout<<s<<endl;}); } int main() { vector<string>vec{"11","22","1","1111","2222","2","111","222"}; biggies(vec, 2); }

I test the code in Xcode 6.4 and Visual Studio 2015 and it turns out that both cannot compile without the "const" in the parameter list. I wonder why the lack of "const" would disrupt the compilation? I'll be very thankful with your answers.

It wouldn't make sense for a comparator to change the things it compares, so I at least the compilation error is a good thing. — juanchopanza
– juanchopanza, Commented Aug 5, 2015 at 6:39
@MKII there're errors in <algorithm >: No matching function for call to object of type... — Greg
– Greg, Commented Aug 5, 2015 at 6:43

Jared Mulconry · Accepted Answer · 2015-08-05 07:14:06Z

I can't find anything in the standard (N3337) that puts any specific requirements on the parameter types for a comparator passed to sorting-related algorithms. All I can find that kind of hints at why you're having this issue is this:

25.4.2: It is assumed that comp will not apply any non-constant function through the dereferenced iterator.

It is somewhat indirect, but since it is 'assumed' that your comparator won't apply any non-const function to what's given to you by the algorithm, I guess it's valid that the algorithm pass const objects to it; this is likely the source of your problem.

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