Question

I am trying to copy a vector of pair to another:

vector<pair<int,int>> vp = {pair<int,int>(1,1), pair<int,int>(2,2)};
vector<pair<int,int>> vp2;
for_each(vp.begin(), vp.end(), [vp2](pair<int,int> p){
                                               if(/*some condition*/){
                                                vp2.push_back(p);
                                               }
                                              });

I get this compiler error:

error: passing ‘const std::vector<std::pair<int, int> >’ as ‘this’ argument of ‘void std::vector<_Tp, _Alloc>::push_back(value_type&&) [with _Tp = std::pair<int, int>, _Alloc = std::allocator<std::pair<int, int> >, value_type = std::pair<int, int>]’ discards qualifiers

Using gcc 4.5.1 on ubuntu.

Answer 1

Copying is much easier than that:

vector<pair<int,int>> vp2(vp.begin(), vp.end());

or even:

vector<pair<int,int>> vp2 = vp; // or
vector<pair<int,int>> vp2(vp);

The error in your code is that you capture vp2 by value which effectively makes it const in your anonymous method, and you cannot call push_back on a const vector. The following should work:

for_each(vp.begin(), vp.end(), [&vp2](pair<int,int> p){vp2.push_back(p);});

But there’s no reason to use this instead of the simpler code.