Question

I have code as following

vector<unique_ptr<int>> v;
v.insert(v.end(), new int(1)); // this is okay
v.push_back(new int(1)); // this is wrong, cannot convert int* to unique_ptr<int>&&

Why does the compilation (vc2010) show the differences? Thanks.

Answer 1

That’s because in VS2010, v.insert(v.end(), new int(1)); is optimized to callstd::vector::emplace_back which construct object in place while std::vector::push_back will try to copy/convert int* to std::unique_ptr<int> then it failed. To make push smart pointers into STL container you can specify exact type:

v.insert(v.end(), unique_ptr<int>(new int(1))); 
v.push_back(unique_ptr<int>(new int(1))); 

Or simply call

v.emplace_back(new int(1));

I’ve tested your code on VS2010 and VS2012, howerver VS2012 disallows v.insert(v.end(), new int(1));, but emplace_back works in both cases.